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    Subject[patch 3/3] ext2: use perform_write aop
    Date
    Convert ext2 to use ->perform_write. This uses the main loop out of
    generic_perform_write, but when encountering a short usercopy, it
    zeroes out new uninitialised blocks, and passes in a short-length commit
    to __block_commit_write, which does the right thing (in terms of not
    setting things uptodate).

    fs/buffer.c | 143 ++++++++++++++++++++++++++++++++++++++++++++
    fs/ext2/inode.c | 7 ++
    include/linux/buffer_head.h | 1
    include/linux/pagemap.h | 2
    4 files changed, 153 insertions(+)

    Index: linux-2.6/fs/buffer.c
    ===================================================================
    --- linux-2.6.orig/fs/buffer.c
    +++ linux-2.6/fs/buffer.c
    @@ -1866,6 +1866,50 @@ next_bh:
    return err;
    }

    +void page_zero_new_buffers(struct page *page, unsigned from, unsigned to)
    +{
    + unsigned int block_start, block_end;
    + struct buffer_head *head, *bh;
    +
    + BUG_ON(!PageLocked(page));
    + if (!page_has_buffers(page))
    + return;
    +
    + bh = head = page_buffers(page);
    + block_start = 0;
    + do {
    + block_end = block_start + bh->b_size;
    +
    + if (buffer_new(bh)) {
    + if (block_end > from && block_start < to) {
    + if (!PageUptodate(page)) {
    + unsigned start, end;
    + void *kaddr;
    +
    + start = max(from, block_start);
    + end = min(to, block_end);
    +
    + kaddr = kmap_atomic(page, KM_USER0);
    + memset(kaddr+start, 0, block_end-end);
    + flush_dcache_page(page);
    + kunmap_atomic(kaddr, KM_USER0);
    + set_buffer_uptodate(bh);
    + }
    +
    + /*
    + * XXX: make buffer_new behaviour more
    + * consistent.
    + * clear_buffer_new(bh);
    + */
    + mark_buffer_dirty(bh);
    + }
    + }
    +
    + block_start = block_end;
    + bh = bh->b_this_page;
    + } while (bh != head);
    +}
    +
    static int __block_commit_write(struct inode *inode, struct page *page,
    unsigned from, unsigned to)
    {
    @@ -1900,6 +1944,105 @@ static int __block_commit_write(struct i
    return 0;
    }

    +ssize_t block_perform_write(struct file *file, struct iovec_iterator *i,
    + loff_t pos, get_block_t *get_block)
    +{
    + struct address_space *mapping = file->f_mapping;
    + struct inode *inode = mapping->host;
    + long status = 0;
    + ssize_t written = 0;
    +
    + do {
    + struct page *page;
    + pgoff_t index; /* Pagecache index for current page */
    + unsigned long offset; /* Offset into pagecache page */
    + unsigned long bytes; /* Bytes to write to page */
    + size_t copied; /* Bytes copied from user */
    +
    + offset = (pos & (PAGE_CACHE_SIZE - 1));
    + index = pos >> PAGE_CACHE_SHIFT;
    + bytes = min_t(unsigned long, PAGE_CACHE_SIZE - offset,
    + iovec_iterator_count(i));
    +
    + /*
    + * Bring in the user page that we will copy from _first_.
    + * Otherwise there's a nasty deadlock on copying from the
    + * same page as we're writing to, without it being marked
    + * up-to-date.
    + *
    + * Not only is this an optimisation, but it is also required
    + * to check that the address is actually valid, when atomic
    + * usercopies are used, below.
    + */
    + if (unlikely(iovec_iterator_fault_in_readable(i))) {
    + status = -EFAULT;
    + break;
    + }
    +
    + page = __grab_cache_page(mapping, index);
    + if (!page) {
    + status = -ENOMEM;
    + break;
    + }
    +
    + status = __block_prepare_write(inode, page, offset,
    + offset+bytes, get_block);
    + if (unlikely(status)) {
    + ClearPageUptodate(page);
    +
    + page_cache_release(page);
    +
    + /*
    + * prepare_write() may have instantiated a few blocks
    + * outside i_size. Trim these off again. Don't need
    + * i_size_read because we hold i_mutex.
    + */
    + if (pos + bytes > inode->i_size)
    + vmtruncate(inode, inode->i_size);
    + break;
    + }
    +
    + /*
    + * Must not enter the pagefault handler here, because
    + * we hold the page lock. See mm/filemap.c for more
    + * details.
    + */
    + pagefault_disable();
    + copied = iovec_iterator_copy_from_user_atomic(page, i,
    + offset, bytes);
    + pagefault_enable();
    + if (unlikely(copied < bytes))
    + page_zero_new_buffers(page, offset+copied, offset+bytes);
    + flush_dcache_page(page);
    +
    + /* This could be a short (even 0-length) commit */
    + __block_commit_write(inode, page, offset, offset+copied);
    +
    + unlock_page(page);
    + mark_page_accessed(page);
    + page_cache_release(page);
    +
    + iovec_iterator_advance(i, copied);
    + pos += copied;
    + written += copied;
    +
    + balance_dirty_pages_ratelimited(mapping);
    + cond_resched();
    +
    + } while (iovec_iterator_count(i));
    +
    + /*
    + * No need to use i_size_read() here, the i_size
    + * cannot change under us because we hold i_mutex.
    + */
    + if (pos > inode->i_size) {
    + i_size_write(inode, pos);
    + mark_inode_dirty(inode);
    + }
    +
    + return written ? written : status;
    +}
    +
    /*
    * Generic "read page" function for block devices that have the normal
    * get_block functionality. This is most of the block device filesystems.
    Index: linux-2.6/fs/ext2/inode.c
    ===================================================================
    --- linux-2.6.orig/fs/ext2/inode.c
    +++ linux-2.6/fs/ext2/inode.c
    @@ -642,6 +642,12 @@ ext2_readpages(struct file *file, struct
    return mpage_readpages(mapping, pages, nr_pages, ext2_get_block);
    }

    +static ssize_t
    +ext2_perform_write(struct file *file, struct iovec_iterator *i, loff_t pos)
    +{
    + return block_perform_write(file, i, pos, ext2_get_block);
    +}
    +
    static int
    ext2_prepare_write(struct file *file, struct page *page,
    unsigned from, unsigned to)
    @@ -689,6 +695,7 @@ const struct address_space_operations ex
    .readpages = ext2_readpages,
    .writepage = ext2_writepage,
    .sync_page = block_sync_page,
    + .perform_write = ext2_perform_write,
    .prepare_write = ext2_prepare_write,
    .commit_write = generic_commit_write,
    .bmap = ext2_bmap,
    Index: linux-2.6/include/linux/buffer_head.h
    ===================================================================
    --- linux-2.6.orig/include/linux/buffer_head.h
    +++ linux-2.6/include/linux/buffer_head.h
    @@ -198,6 +198,7 @@ void block_invalidatepage(struct page *p
    int block_write_full_page(struct page *page, get_block_t *get_block,
    struct writeback_control *wbc);
    int block_read_full_page(struct page*, get_block_t*);
    +ssize_t block_perform_write(struct file *, struct iovec_iterator*, loff_t, get_block_t*);
    int block_prepare_write(struct page*, unsigned, unsigned, get_block_t*);
    int cont_prepare_write(struct page*, unsigned, unsigned, get_block_t*,
    loff_t *);
    Index: linux-2.6/include/linux/pagemap.h
    ===================================================================
    --- linux-2.6.orig/include/linux/pagemap.h
    +++ linux-2.6/include/linux/pagemap.h
    @@ -87,6 +87,8 @@ unsigned find_get_pages_contig(struct ad
    unsigned find_get_pages_tag(struct address_space *mapping, pgoff_t *index,
    int tag, unsigned int nr_pages, struct page **pages);

    +struct page *__grab_cache_page(struct address_space *mapping, pgoff_t index);
    +
    /*
    * Returns locked page at given index in given cache, creating it if needed.
    */
    -
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