整体框架图

vfs系统类关系图

vfs层次关系通

初始化过程

在内核启动的主函数中

start_kernel()
{
	vfs_caches_init_early();
	vfs_caches_init();
	rest_init();
}

rest_init() 
{
	kernel_init();
}

kernel_init() 
{  
	kernel_init_freeable();
}

在启动过程中相关的函数差不多就这样了。

vfs_caches_init_early 进行早期的初始化

vfs_caches_init 这里面主要是初始化inode 缓存 dentry的缓存等等。

主要关键的很难理解的是rest_init

而rest_init 又调用了kernel_init 然后又调用了kernel_init_freeable，这里面做什么事情呢？

做一件非常关键的事情，就是第一次开机在什么都没有的情况下，相当于硬盘是空的。
OK，描述下这个过程：

1、硬盘是空的，怎么办

2、从flash中加载initramfs 到内存，在内存中临时建立一个 “/” 根文件系统，将初次需要的一些操作，在这个内存的fs里面做。比如sys设备。

3、完成内核的最后阶段要去执行第一个进程/init 的时候，把这个内存里的 fs，送到硬盘。OK，完成第一次启动

4、第二次或者以后还要启动的时候，检查硬盘是不是空的，不是空的，检查硬盘里面有没有init，有就直接启动init完成后续启动。

文件read流程分析

fs/read_write.c

ssize_t vfs_read(struct file *file, char __user *buf, size_t count, loff_t *pos)
{
  ssize_t ret;

  if (!(file->f_mode & FMODE_READ))
    return -EBADF;
  if (!file->f_op || (!file->f_op->read && !file->f_op->aio_read))
    return -EINVAL;
  if (unlikely(!access_ok(VERIFY_WRITE, buf, count)))
    return -EFAULT;

  ret = rw_verify_area(READ, file, pos, count);
  if (ret >= 0) {
    count = ret;
    if (file->f_op->read)
      ret = file->f_op->read(file, buf, count, pos);
    else
      ret = do_sync_read(file, buf, count, pos);
    if (ret > 0) {
      fsnotify_access(file);
      add_rchar(current, ret);
    }
    inc_syscr(current);
  }

  return ret;
}

ssize_t do_sync_read(struct file *filp, char __user *buf, size_t len, loff_t *ppos)
{
  struct iovec iov = { .iov_base = buf, .iov_len = len };
  struct kiocb kiocb;
  ssize_t ret;

  init_sync_kiocb(&kiocb, filp);
  kiocb.ki_pos = *ppos;
  kiocb.ki_left = len;
  kiocb.ki_nbytes = len;

  ret = filp->f_op->aio_read(&kiocb, &iov, 1, kiocb.ki_pos);
  if (-EIOCBQUEUED == ret)
    ret = wait_on_sync_kiocb(&kiocb);
  *ppos = kiocb.ki_pos;
  return ret;
}

fs/ext4/files.c

const struct file_operations ext4_file_operations = {
  .llseek    = ext4_llseek,
  .read    = do_sync_read,
  .write    = do_sync_write,
  .aio_read  = generic_file_aio_read,
  .aio_write  = ext4_file_write,
  .unlocked_ioctl = ext4_ioctl,
#ifdef CONFIG_COMPAT
  .compat_ioctl  = ext4_compat_ioctl,
#endif
  .mmap    = ext4_file_mmap,
  .open    = ext4_file_open,
  .release  = ext4_release_file,
  .fsync    = ext4_sync_file,
  .splice_read  = generic_file_splice_read,
  .splice_write  = generic_file_splice_write,
  .fallocate  = ext4_fallocate,
};

generic_file_aio_read
./mm/filemap.c

ssize_t
generic_file_aio_read(struct kiocb *iocb, const struct iovec *iov,
    unsigned long nr_segs, loff_t pos)
{
  struct file *filp = iocb->ki_filp;
  ssize_t retval;
  unsigned long seg = 0;
  size_t count;
  loff_t *ppos = &iocb->ki_pos;

  count = 0;
  retval = generic_segment_checks(iov, &nr_segs, &count, VERIFY_WRITE);
  if (retval)
    return retval;

  /* coalesce the iovecs and go direct-to-BIO for O_DIRECT */
  if (filp->f_flags & O_DIRECT) {
    loff_t size;
    struct address_space *mapping;
    struct inode *inode;

    mapping = filp->f_mapping;
    inode = mapping->host;
    if (!count)
      goto out; /* skip atime */
    size = i_size_read(inode);
    if (pos < size) {
      retval = filemap_write_and_wait_range(mapping, pos,
          pos + iov_length(iov, nr_segs) - 1);
      if (!retval) {
        retval = mapping->a_ops->direct_IO(READ, iocb,
              iov, pos, nr_segs);
      }
      if (retval > 0) {
        *ppos = pos + retval;
        count -= retval;
      }
...
      if (retval < 0 || !count || *ppos >= size) {
        file_accessed(filp);
        goto out;
      }
    }
  }

  count = retval;
  for (seg = 0; seg < nr_segs; seg++) {
    read_descriptor_t desc;
    loff_t offset = 0;
...
    desc.written = 0;
    desc.arg.buf = iov[seg].iov_base + offset;
    desc.count = iov[seg].iov_len - offset;
    if (desc.count == 0)
      continue;
    desc.error = 0;
    do_generic_file_read(filp, ppos, &desc, file_read_actor);
    retval += desc.written;
    if (desc.error) {
      retval = retval ?: desc.error;
      break;
    }
    if (desc.count > 0)
      break;
  }
out:
  return retval;
}

do_generic_file_read
./mm/filemap.c

static void do_generic_file_read(struct file *filp, loff_t *ppos,
    read_descriptor_t *desc, read_actor_t actor)
{
  struct address_space *mapping = filp->f_mapping;
  struct inode *inode = mapping->host;
  struct file_ra_state *ra = &filp->f_ra;
  pgoff_t index;
  pgoff_t last_index;
  pgoff_t prev_index;
  unsigned long offset;   
  unsigned int prev_offset;
  int error;

  index = *ppos >> PAGE_CACHE_SHIFT;
  prev_index = ra->prev_pos >> PAGE_CACHE_SHIFT;
  prev_offset = ra->prev_pos & (PAGE_CACHE_SIZE-1);
  last_index = (*ppos + desc->count + PAGE_CACHE_SIZE-1) >> PAGE_CACHE_SHIFT;
  offset = *ppos & ~PAGE_CACHE_MASK;

  for (;;) {
    struct page *page;
    pgoff_t end_index;
    loff_t isize;
    unsigned long nr, ret;

    cond_resched();
find_page:
    page = find_get_page(mapping, index);
    if (!page) {
      page_cache_sync_readahead(mapping,
          ra, filp,
          index, last_index - index);
      page = find_get_page(mapping, index);
      if (unlikely(page == NULL))
        goto no_cached_page;
    }
    if (PageReadahead(page)) {
      page_cache_async_readahead(mapping,
          ra, filp, page,
          index, last_index - index);
    }
    if (!PageUptodate(page)) {
      if (inode->i_blkbits == PAGE_CACHE_SHIFT ||
          !mapping->a_ops->is_partially_uptodate)
        goto page_not_up_to_date;
      if (!trylock_page(page))
        goto page_not_up_to_date;
      /* Did it get truncated before we got the lock? */
      if (!page->mapping)
        goto page_not_up_to_date_locked;
      if (!mapping->a_ops->is_partially_uptodate(page,
                desc, offset))
        goto page_not_up_to_date_locked;
      unlock_page(page);
    }
page_ok:
...

    isize = i_size_read(inode);
    end_index = (isize - 1) >> PAGE_CACHE_SHIFT;
    if (unlikely(!isize || index > end_index)) {
      page_cache_release(page);
      goto out;
    }

    /* nr is the maximum number of bytes to copy from this page */
    nr = PAGE_CACHE_SIZE;
    if (index == end_index) {
      nr = ((isize - 1) & ~PAGE_CACHE_MASK) + 1;
      if (nr <= offset) {
        page_cache_release(page);
        goto out;
      }
    }
    nr = nr - offset;

...
    if (mapping_writably_mapped(mapping))
      flush_dcache_page(page);
...
    if (prev_index != index || offset != prev_offset)
      mark_page_accessed(page);
    prev_index = index;

....
    ret = actor(desc, page, offset, nr);
    offset += ret;
    index += offset >> PAGE_CACHE_SHIFT;
    offset &= ~PAGE_CACHE_MASK;
    prev_offset = offset;

    page_cache_release(page);
    if (ret == nr && desc->count)
      continue;
    goto out;

page_not_up_to_date:
    error = lock_page_killable(page);
    if (unlikely(error))
      goto readpage_error;

page_not_up_to_date_locked:
    /* Did it get truncated before we got the lock? */
    if (!page->mapping) {
      unlock_page(page);
      page_cache_release(page);
      continue;
    }

    /* Did somebody else fill it already? */
    if (PageUptodate(page)) {
      unlock_page(page);
      goto page_ok;
    }

readpage:
    ClearPageError(page);

    error = mapping->a_ops->readpage(filp, page);

    if (unlikely(error)) {
      if (error == AOP_TRUNCATED_PAGE) {
        page_cache_release(page);
        goto find_page;
      }
      goto readpage_error;
    }

    if (!PageUptodate(page)) {
      error = lock_page_killable(page);
      if (unlikely(error))
        goto readpage_error;
      if (!PageUptodate(page)) {
        if (page->mapping == NULL) {
          /*
           * invalidate_mapping_pages got it
           */
          unlock_page(page);
          page_cache_release(page);
          goto find_page;
        }
        unlock_page(page);
        shrink_readahead_size_eio(filp, ra);
        error = -EIO;
        goto readpage_error;
      }
      unlock_page(page);
    }

    goto page_ok;

readpage_error:
    /* UHHUH! A synchronous read error occurred. Report it */
    desc->error = error;
    page_cache_release(page);
    goto out;

no_cached_page:

    page = page_cache_alloc_cold(mapping);
    if (!page) {
      desc->error = -ENOMEM;
      goto out;
    }
    error = add_to_page_cache_lru(page, mapping,
            index, GFP_KERNEL);
    if (error) {
      page_cache_release(page);
      if (error == -EEXIST)
        goto find_page;
      desc->error = error;
      goto out;
    }
    goto readpage;
  }

out:
  ra->prev_pos = prev_index;
  ra->prev_pos <<= PAGE_CACHE_SHIFT;
  ra->prev_pos |= prev_offset;

  *ppos = ((loff_t)index << PAGE_CACHE_SHIFT) + offset;
  file_accessed(filp);
}

文件read和BIO调度分析

int ext4_mpage_readpages(struct address_space *mapping,
       struct list_head *pages, struct page *page,
       unsigned nr_pages)
{
  struct bio *bio = NULL;
  unsigned page_idx;
  sector_t last_block_in_bio = 0;

  struct inode *inode = mapping->host;
  const unsigned blkbits = inode->i_blkbits;
  const unsigned blocks_per_page = PAGE_CACHE_SIZE >> blkbits;
  const unsigned blocksize = 1 << blkbits;
  sector_t block_in_file;
  sector_t last_block;
  sector_t last_block_in_file;
  sector_t blocks[MAX_BUF_PER_PAGE];
  unsigned page_block;
  struct block_device *bdev = inode->i_sb->s_bdev;
  int length;
  unsigned relative_block = 0;
  struct ext4_map_blocks map;

  map.m_pblk = 0;
  map.m_lblk = 0;
  map.m_len = 0;
  map.m_flags = 0;

  for (page_idx = 0; nr_pages; page_idx++, nr_pages--) {
    int fully_mapped = 1;
    unsigned first_hole = blocks_per_page;

    prefetchw(&page->flags);
    if (pages) {
      page = list_entry(pages->prev, struct page, lru);
      list_del(&page->lru);
      if (add_to_page_cache_lru(page, mapping, page->index,
          mapping_gfp_constraint(mapping, GFP_KERNEL)))
        goto next_page;
    }

    if (page_has_buffers(page))
      goto confused;

    block_in_file = (sector_t)page->index << (PAGE_CACHE_SHIFT - blkbits);
    last_block = block_in_file + nr_pages * blocks_per_page;
    last_block_in_file = (i_size_read(inode) + blocksize - 1) >> blkbits;
    if (last_block > last_block_in_file)
      last_block = last_block_in_file;
    page_block = 0;

    /*
     * Map blocks using the previous result first.
     */
    if ((map.m_flags & EXT4_MAP_MAPPED) &&
        block_in_file > map.m_lblk &&
        block_in_file < (map.m_lblk + map.m_len)) {
      unsigned map_offset = block_in_file - map.m_lblk;
      unsigned last = map.m_len - map_offset;

      for (relative_block = 0; ; relative_block++) {
        if (relative_block == last) {
          /* needed? */
          map.m_flags &= ~EXT4_MAP_MAPPED;
          break;
        }
        if (page_block == blocks_per_page)
          break;
        blocks[page_block] = map.m_pblk + map_offset +
          relative_block;
        page_block++;
        block_in_file++;
      }
    }

    /*
     * Then do more ext4_map_blocks() calls until we are
     * done with this page.
     */
    while (page_block < blocks_per_page) {
      if (block_in_file < last_block) {
        map.m_lblk = block_in_file;
        map.m_len = last_block - block_in_file;

        if (ext4_map_blocks(NULL, inode, &map, 0) < 0) {
        set_error_page:
          SetPageError(page);
          zero_user_segment(page, 0,
                PAGE_CACHE_SIZE);
          unlock_page(page);
          goto next_page;
        }
      }
      if ((map.m_flags & EXT4_MAP_MAPPED) == 0) {
        fully_mapped = 0;
        if (first_hole == blocks_per_page)
          first_hole = page_block;
        page_block++;
        block_in_file++;
        continue;
      }
      if (first_hole != blocks_per_page)
        goto confused;    /* hole -> non-hole */

      /* Contiguous blocks? */
      if (page_block && blocks[page_block-1] != map.m_pblk-1)
        goto confused;
      for (relative_block = 0; ; relative_block++) {
        if (relative_block == map.m_len) {
          /* needed? */
          map.m_flags &= ~EXT4_MAP_MAPPED;
          break;
        } else if (page_block == blocks_per_page)
          break;
        blocks[page_block] = map.m_pblk+relative_block;
        page_block++;
        block_in_file++;
      }
    }
    if (first_hole != blocks_per_page) {
      zero_user_segment(page, first_hole << blkbits,
            PAGE_CACHE_SIZE);
      if (first_hole == 0) {
        SetPageUptodate(page);
        unlock_page(page);
        goto next_page;
      }
    } else if (fully_mapped) {
      SetPageMappedToDisk(page);
    }
    if (fully_mapped && blocks_per_page == 1 &&
        !PageUptodate(page) && cleancache_get_page(page) == 0) {
      SetPageUptodate(page);
      goto confused;
    }

    /*
     * This page will go to BIO.  Do we need to send this
     * BIO off first?
     */
    if (bio && (last_block_in_bio != blocks[0] - 1)) {
    submit_and_realloc:
      ext4_submit_bio_read(bio);
      bio = NULL;
    }
    if (bio == NULL) {
      struct ext4_crypto_ctx *ctx = NULL;

      if (ext4_encrypted_inode(inode) &&
          S_ISREG(inode->i_mode)) {
        ctx = ext4_get_crypto_ctx(inode);
        if (IS_ERR(ctx))
          goto set_error_page;
      }
      bio = bio_alloc(GFP_KERNEL,
        min_t(int, nr_pages, BIO_MAX_PAGES));
      if (!bio) {
        if (ctx)
          ext4_release_crypto_ctx(ctx);
        goto set_error_page;
      }
      bio->bi_bdev = bdev;
      bio->bi_iter.bi_sector = blocks[0] << (blkbits - 9);
      bio->bi_end_io = mpage_end_io;
      bio->bi_private = ctx;
    }

    length = first_hole << blkbits;
    if (bio_add_page(bio, page, length, 0) < length)
      goto submit_and_realloc;

    if (((map.m_flags & EXT4_MAP_BOUNDARY) &&
         (relative_block == map.m_len)) ||
        (first_hole != blocks_per_page)) {
      ext4_submit_bio_read(bio);
      bio = NULL;
    } else
      last_block_in_bio = blocks[blocks_per_page - 1];
    goto next_page;
  confused:
    if (bio) {
      ext4_submit_bio_read(bio);
      bio = NULL;
    }
    if (!PageUptodate(page))
      block_read_full_page(page, ext4_get_block);
    else
      unlock_page(page);
  next_page:
    if (pages)
      page_cache_release(page);
  }
  BUG_ON(pages && !list_empty(pages));
  if (bio)
    ext4_submit_bio_read(bio);
  return 0;
}


int bio_add_page(struct bio *bio, struct page *page,
     unsigned int len, unsigned int offset)
{
  struct bio_vec *bv;

  /*
   * cloned bio must not modify vec list
   */
  if (WARN_ON_ONCE(bio_flagged(bio, BIO_CLONED)))
    return 0;

  /*
   * For filesystems with a blocksize smaller than the pagesize
   * we will often be called with the same page as last time and
   * a consecutive offset.  Optimize this special case.
   */
  if (bio->bi_vcnt > 0) {
    bv = &bio->bi_io_vec[bio->bi_vcnt - 1];

    if (page == bv->bv_page &&
        offset == bv->bv_offset + bv->bv_len) {
      bv->bv_len += len;
      goto done;
    }
  }

  if (bio->bi_vcnt >= bio->bi_max_vecs)
    return 0;

  bv    = &bio->bi_io_vec[bio->bi_vcnt];
  bv->bv_page  = page;
  bv->bv_len  = len;
  bv->bv_offset  = offset;

  bio->bi_vcnt++;
done:
  bio->bi_iter.bi_size += len;
  return len;
}

//
sumbit (bio)

/****************************************驱动程序****************************************************************/
getbio();
 struct bio_vec* bvec; 
 pRHdata = pdev->data + (bio->bi_sector * RAMHD_SECTOR_SIZE); 
  bio_for_each_segment(bvec, bio, i){ 
    pBuffer = kmap(bvec->bv_page) + bvec->bv_offset; 
    switch(bio_data_dir(bio)){ 
      case READ: 
        memcpy(pBuffer, pRHdata, bvec->bv_len); 
        flush_dcache_page(bvec->bv_page); 
        break; 
          
      case WRITE: 
        flush_dcache_page(bvec->bv_page); 
        memcpy(pRHdata, pBuffer, bvec->bv_len); 
        break; 
          
      default: 
        kunmap(bvec->bv_page); 
        goto out; 
    } 
      
    kunmap(bvec->bv_page); 
    pRHdata += bvec->bv_len; 
  } 

• 首先是疑问，就是在准备探索cfq和红黑树的时候，知道红黑树应该是根据磁盘的bi_sector进行树的组织的，那么这个bi_setctor是哪里来的，所以找回去readpage流程。

• 第二个疑问：然后之前也一直疑问bio，应该是好几个page组成的，之前没有看到，所以就用单个bio去探索也不影响分析代码，今天顺便一起看下

第一问解答：
从上面代码就知道了，第一个标黑体的地方和第二个标记黑体的地方就是bi_setctor的由来

const unsigned blkbits = inode->i_blkbits;

bio->bi_iter.bi_sector = blocks[0] << (blkbits - 9);

虽然还有些模糊，比如blkbits 是inode节点，相当于节点编号，相当于硬盘位置，但是这个iNode的又是最终是怎么来的呢？还需要探索。
还有减去9，还有blocks 左移是什么意思。

第二问解答：

bio_alloc
bio_add_page
如果bio还是空的，就用bio_alloc开辟一个，然后后面的page用bio_add_page。
然后在bio_add_page函数中我们对比bvec的小节点的数据流动，很清楚的看到，每个page的数据指针进行的数据传递，其实是个数组，并没有组织成链表。

文件write过程分析

从vfs_write 开始分析

ssize_t vfs_write(struct file *file, const char __user *buf, size_t count, loff_t *pos)
    {
      ssize_t ret;
    
      if (!(file->f_mode & FMODE_WRITE))
        return -EBADF;
      if (!(file->f_mode & FMODE_CAN_WRITE))
        return -EINVAL;
      if (unlikely(!access_ok(VERIFY_READ, buf, count)))
        return -EFAULT;
    
      ret = rw_verify_area(WRITE, file, pos, count);
      if (ret >= 0) {
        count = ret;
        file_start_write(file);
        ret = __vfs_write(file, buf, count, pos);
        if (ret > 0) {
          fsnotify_modify(file);
          add_wchar(current, ret);
        }
        inc_syscw(current);
        file_end_write(file);
      }
    
      return ret;
    }

• access_ok
• __vfs_write
• fsnotify_modify

ssize_t __vfs_write(struct file *file, const char __user *p, size_t count,
        loff_t *pos)
{
  if (file->f_op->write)
    return file->f_op->write(file, p, count, pos);
  else if (file->f_op->write_iter)
    return new_sync_write(file, p, count, pos);
  else
    return -EINVAL;
}

这里检查f_op 的write接口，我们以ext4为例

const struct file_operations ext4_file_operations = {
  .llseek    = ext4_llseek,
  .read_iter  = generic_file_read_iter,
  .write_iter  = ext4_file_write_iter,
  .unlocked_ioctl = ext4_ioctl,
#ifdef CONFIG_COMPAT
  .compat_ioctl  = ext4_compat_ioctl,
#endif
  .mmap    = ext4_file_mmap,
  .open    = ext4_file_open,
  .release  = ext4_release_file,
  .fsync    = ext4_sync_file,
  .splice_read  = generic_file_splice_read,
  .splice_write  = iter_file_splice_write,
  .fallocate  = ext4_fallocate,
};

那么它会走write_iter接口，

static ssize_t new_sync_write(struct file *filp, const char __user *buf, size_t len, loff_t *ppos)
{
  struct iovec iov = { .iov_base = (void __user *)buf, .iov_len = len };
  struct kiocb kiocb;
  struct iov_iter iter;
  ssize_t ret;

  init_sync_kiocb(&kiocb, filp);
  kiocb.ki_pos = *ppos;
  iov_iter_init(&iter, WRITE, &iov, 1, len);

  ret = filp->f_op->write_iter(&kiocb, &iter);
  BUG_ON(ret == -EIOCBQUEUED);
  if (ret > 0)
    *ppos = kiocb.ki_pos;
  return ret;
}

这里只是转换数据结构。

static ssize_t
ext4_file_write_iter(struct kiocb *iocb, struct iov_iter *from)
{
  struct file *file = iocb->ki_filp;
  struct inode *inode = file_inode(iocb->ki_filp);
  struct mutex *aio_mutex = NULL;
  struct blk_plug plug;
  int o_direct = iocb->ki_flags & IOCB_DIRECT;
  int overwrite = 0;
  ssize_t ret;


  if (o_direct) {
    size_t length = iov_iter_count(from);
    loff_t pos = iocb->ki_pos;
    blk_start_plug(&plug);
    if (ext4_should_dioread_nolock(inode) && !aio_mutex &&
        !file->f_mapping->nrpages && pos + length <= i_size_read(inode)) {
      struct ext4_map_blocks map;
      unsigned int blkbits = inode->i_blkbits;
      int err, len;

      map.m_lblk = pos >> blkbits;
      map.m_len = (EXT4_BLOCK_ALIGN(pos + length, blkbits) >> blkbits)
        - map.m_lblk;
      len = map.m_len;

      err = ext4_map_blocks(NULL, inode, &map, 0);
  
      if (err == len && (map.m_flags & EXT4_MAP_MAPPED))
        overwrite = 1;
    }
  }

  ret = __generic_file_write_iter(iocb, from);
  mutex_unlock(&inode->i_mutex);

  if (ret > 0) {
    ssize_t err;

    err = generic_write_sync(file, iocb->ki_pos - ret, ret);
    if (err < 0)
      ret = err;
  }
  if (o_direct)
    blk_finish_plug(&plug);

  if (aio_mutex)
    mutex_unlock(aio_mutex);
  return ret;

out:
  mutex_unlock(&inode->i_mutex);
  if (aio_mutex)
    mutex_unlock(aio_mutex);
  return ret;
}

这边检查o_direct，应该就是直接写入的标签，前面会做一些工作，我先假设没有这个东东。

ssize_t __generic_file_write_iter(struct kiocb *iocb, struct iov_iter *from)
{
  struct file *file = iocb->ki_filp;
  struct address_space * mapping = file->f_mapping;
  struct inode   *inode = mapping->host;
  ssize_t    written = 0;
  ssize_t    err;
  ssize_t    status;

  current->backing_dev_info = inode_to_bdi(inode);
  err = file_remove_privs(file);
  if (err)
    goto out;

  err = file_update_time(file);
  if (err)
    goto out;

  if (iocb->ki_flags & IOCB_DIRECT) {
    loff_t pos, endbyte;

    written = generic_file_direct_write(iocb, from, iocb->ki_pos);
    if (written < 0 || !iov_iter_count(from) || IS_DAX(inode))
      goto out;

    status = generic_perform_write(file, from, pos = iocb->ki_pos);

    if (unlikely(status < 0)) {
      err = status;
      goto out;
    }

    endbyte = pos + status - 1;
    err = filemap_write_and_wait_range(mapping, pos, endbyte);
    if (err == 0) {
      iocb->ki_pos = endbyte + 1;
      written += status;
      invalidate_mapping_pages(mapping,
             pos >> PAGE_CACHE_SHIFT,
             endbyte >> PAGE_CACHE_SHIFT);
    } else {

    }
  } else {
    written = generic_perform_write(file, from, iocb->ki_pos);
    if (likely(written > 0))
      iocb->ki_pos += written;
  }
out:
  current->backing_dev_info = NULL;
  return written ? written : err;
}

这里还是会检查IOCB_DIRECT，直接写流程，我们先不看了。

ssize_t generic_perform_write(struct file *file,
        struct iov_iter *i, loff_t pos)
{
  struct address_space *mapping = file->f_mapping;
  const struct address_space_operations *a_ops = mapping->a_ops;
  long status = 0;
  ssize_t written = 0;
  unsigned int flags = 0;

  if (!iter_is_iovec(i))
    flags |= AOP_FLAG_UNINTERRUPTIBLE;

  do {
    struct page *page;
    unsigned long offset;  /* Offset into pagecache page */
    unsigned long bytes;  /* Bytes to write to page */
    size_t copied;    /* Bytes copied from user */
    void *fsdata;

    offset = (pos & (PAGE_CACHE_SIZE - 1));
    bytes = min_t(unsigned long, PAGE_CACHE_SIZE - offset,
            iov_iter_count(i));

again:

    if (unlikely(iov_iter_fault_in_readable(i, bytes))) {
      status = -EFAULT;
      break;
    }

    if (fatal_signal_pending(current)) {
      status = -EINTR;
      break;
    }

    status = a_ops->write_begin(file, mapping, pos, bytes, flags,
            &page, &fsdata);
    if (unlikely(status < 0))
      break;

    if (mapping_writably_mapped(mapping))
      flush_dcache_page(page);

    copied = iov_iter_copy_from_user_atomic(page, i, offset, bytes);
    flush_dcache_page(page);

    status = a_ops->write_end(file, mapping, pos, bytes, copied,
            page, fsdata);
    if (unlikely(status < 0))
      break;
    copied = status;

    cond_resched();

    iov_iter_advance(i, copied);
    if (unlikely(copied == 0)) {

      bytes = min_t(unsigned long, PAGE_CACHE_SIZE - offset,
            iov_iter_single_seg_count(i));
      goto again;
    }
    pos += copied;
    written += copied;

    balance_dirty_pages_ratelimited(mapping);
  } while (iov_iter_count(i));

  return written ? written : status;
}

这里就是循环一页一页的写。当然是写到缓存中，

1、a_ops->write_begin 和a_ops交互准备开始写

2、iov_iter_copy_from_user_atomic 拷贝用户数据到 page

3、a_ops->write_end 写入

4、balance_dirty_pages_ratelimited 做限制检查

那么准备cache的过程在write_begin中。做限制的后面再看。我们的目标就是这两个。

static const struct address_space_operations ext4_aops = {
  .readpage    = ext4_readpage,
  .readpages    = ext4_readpages,
  .writepage    = ext4_writepage,
  .writepages    = ext4_writepages,
  .write_begin    = ext4_write_begin,
  .write_end    = ext4_write_end,
  .bmap      = ext4_bmap,
  .invalidatepage    = ext4_invalidatepage,
  .releasepage    = ext4_releasepage,
  .direct_IO    = ext4_direct_IO,
  .migratepage    = buffer_migrate_page,
  .is_partially_uptodate  = block_is_partially_uptodate,
  .error_remove_page  = generic_error_remove_page,
};

static int ext4_write_begin(struct file *file, struct address_space *mapping,
          loff_t pos, unsigned len, unsigned flags,
          struct page **pagep, void **fsdata)
{
  struct inode *inode = mapping->host;
  int ret, needed_blocks;
  handle_t *handle;
  int retries = 0;
  struct page *page;
  pgoff_t index;
  unsigned from, to;

  trace_android_fs_datawrite_start(inode, pos, len,
           current->pid, current->comm);
  trace_ext4_write_begin(inode, pos, len, flags);

  needed_blocks = ext4_writepage_trans_blocks(inode) + 1;
  index = pos >> PAGE_CACHE_SHIFT;
  from = pos & (PAGE_CACHE_SIZE - 1);
  to = from + len;

  if (ext4_test_inode_state(inode, EXT4_STATE_MAY_INLINE_DATA)) {
    ret = ext4_try_to_write_inline_data(mapping, inode, pos, len,
                flags, pagep);
    if (ret < 0)
      return ret;
    if (ret == 1)
      return 0;
  }


retry_grab:
  page = grab_cache_page_write_begin(mapping, index, flags);
  if (!page)
    return -ENOMEM;
  unlock_page(page);

retry_journal:
  handle = ext4_journal_start(inode, EXT4_HT_WRITE_PAGE, needed_blocks);
  if (IS_ERR(handle)) {
    page_cache_release(page);
    return PTR_ERR(handle);
  }

  lock_page(page);
  if (page->mapping != mapping) {
    /* The page got truncated from under us */
    unlock_page(page);
    page_cache_release(page);
    ext4_journal_stop(handle);
    goto retry_grab;
  }

  wait_for_stable_page(page);

#ifdef CONFIG_EXT4_FS_ENCRYPTION
  if (ext4_should_dioread_nolock(inode))
    ret = ext4_block_write_begin(page, pos, len,
               ext4_get_block_write);
  else
    ret = ext4_block_write_begin(page, pos, len,
               ext4_get_block);
#else
  if (ext4_should_dioread_nolock(inode))
    ret = __block_write_begin(page, pos, len, ext4_get_block_write);
  else
    ret = __block_write_begin(page, pos, len, ext4_get_block);
#endif
  if (!ret && ext4_should_journal_data(inode)) {
    ret = ext4_walk_page_buffers(handle, page_buffers(page),
               from, to, NULL,
               do_journal_get_write_access);
  }

  if (ret) {
    unlock_page(page);

    if (pos + len > inode->i_size && ext4_can_truncate(inode))
      ext4_orphan_add(handle, inode);

    ext4_journal_stop(handle);
    if (pos + len > inode->i_size) {
      ext4_truncate_failed_write(inode);

      if (inode->i_nlink)
        ext4_orphan_del(NULL, inode);
    }

    if (ret == -ENOSPC &&
        ext4_should_retry_alloc(inode->i_sb, &retries))
      goto retry_journal;
    page_cache_release(page);
    return ret;
  }
  *pagep = page;
  mtk_btag_pidlog_write_begin(*pagep);

  return ret;
}

• 前面做一些检查
• 后面的handle是和block buffers进行交互的
• 主要的过程是中间的grab_cache_page_write_begin

struct page *grab_cache_page_write_begin(struct address_space *mapping,
          pgoff_t index, unsigned flags)
{
  struct page *page;
  int fgp_flags = FGP_LOCK|FGP_ACCESSED|FGP_WRITE|FGP_CREAT;

  if (flags & AOP_FLAG_NOFS)
    fgp_flags |= FGP_NOFS;

  page = pagecache_get_page(mapping, index, fgp_flags,
      mapping_gfp_mask(mapping));
  if (page)
    wait_for_stable_page(page);

  return page;
}

pagecache_get_page 从radix中获取一页。

struct page *pagecache_get_page(struct address_space *mapping, pgoff_t offset,
  int fgp_flags, gfp_t gfp_mask)
{
  struct page *page;

repeat:
  page = find_get_entry(mapping, offset);
  if (radix_tree_exceptional_entry(page))
    page = NULL;
  if (!page)
    goto no_page;

  if (fgp_flags & FGP_LOCK) {
    if (fgp_flags & FGP_NOWAIT) {
      if (!trylock_page(page)) {
        page_cache_release(page);
        return NULL;
      }
    } else {
      lock_page(page);
    }

    /* Has the page been truncated? */
    if (unlikely(page->mapping != mapping)) {
      unlock_page(page);
      page_cache_release(page);
      goto repeat;
    }
    VM_BUG_ON_PAGE(page->index != offset, page);
  }

  if (page && (fgp_flags & FGP_ACCESSED))
    mark_page_accessed(page);

no_page:
  if (!page && (fgp_flags & FGP_CREAT)) {
    int err;
    if ((fgp_flags & FGP_WRITE) && mapping_cap_account_dirty(mapping))
      gfp_mask |= __GFP_WRITE;
    if (fgp_flags & FGP_NOFS)
      gfp_mask &= ~__GFP_FS;

    page = __page_cache_alloc(gfp_mask);
    if (!page)
      return NULL;

    if (WARN_ON_ONCE(!(fgp_flags & FGP_LOCK)))
      fgp_flags |= FGP_LOCK;

    /* Init accessed so avoid atomic mark_page_accessed later */
    if (fgp_flags & FGP_ACCESSED)
      __SetPageReferenced(page);

    err = add_to_page_cache_lru(page, mapping, offset,
        gfp_mask & GFP_RECLAIM_MASK);
    if (unlikely(err)) {
      page_cache_release(page);
      page = NULL;
      if (err == -EEXIST)
        goto repeat;
    }
  }

  return page;
}

• find_get_entry 获取一页
• mark_page_accessed 标记页据说是给清除的时候用的
• no_page流程，如果没有获取到page就 __page_cache_alloc 一个 然后

add_to_page_cache_lru
struct page *find_get_entry(struct address_space *mapping, pgoff_t offset)
{
  void **pagep;
  struct page *page;

  rcu_read_lock();
repeat:
  page = NULL;
  pagep = radix_tree_lookup_slot(&mapping->page_tree, offset);
  if (pagep) {
    page = radix_tree_deref_slot(pagep);
    if (unlikely(!page))
      goto out;
    if (radix_tree_exception(page)) {
      if (radix_tree_deref_retry(page))
        goto repeat;
      goto out;
    }
    if (!page_cache_get_speculative(page))
      goto repeat;

    if (unlikely(page != *pagep)) {
      page_cache_release(page);
      goto repeat;
    }
  }
out:
  rcu_read_unlock();

  return page;
}

find_get_entry 也很简单，就是在radix_tree_lookup_slot 找一个。

最后

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