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Message-ID: <20110804005001.GI3150@thunk.org>
Date: Wed, 3 Aug 2011 20:50:01 -0400
From: Ted Ts'o <tytso@....edu>
To: Allison Henderson <achender@...ux.vnet.ibm.com>
Cc: Ext4 Developers List <linux-ext4@...r.kernel.org>
Subject: Re: [PATCH 1/1 v3] ext4: fix xfstests 75, 112, 127 punch hole
failure
On Wed, Aug 03, 2011 at 08:20:36AM -0700, Allison Henderson wrote:
> @@ -4227,6 +4227,28 @@ int ext4_ext_punch_hole(struct file *file, loff_t offset, loff_t length)
> }
> }
>
> + /*
> + * Now we need to unmap the un page aligned buffers.
> + * If the file is smaller than a page, just
> + * unmap the middle
> + */
This should be "non-page-aligned buffers". And it's not "if the file
is smaller than a page", but rather "if the file space being truncated
is smaller than a page". (The comment above this patch hunk is
similarly wrong).
> + if (first_page > last_page)
> + ext4_unmap_page_range(handle, mapping, offset, length);
> + else {
> + /* unmap page buffers before the first aligned page */
> + page_len = first_page_offset - offset;
> + if (page_len > 0)
> + ext4_unmap_page_range(handle, mapping,
> + offset, page_len);
> +
> + /* unmap the page buffers after the last aligned page */
> + page_len = offset + length - last_page_offset;
> + if (page_len > 0) {
> + ext4_unmap_page_range(handle, mapping,
> + last_page_offset, page_len);
> + }
> + }
> +
We unconditionally call ext4_unmap_page_range() at least once, and
possibly twice. The ext4_unmap_page_range() function is always going
to be calling find_or_create_page(), which requires locking pages,
taking RCU locks, etc.. None of this code should be needed if:
inode->i_sb->s_blocksize == PAGE_CACHE_SIZE
or
(offset % PAGE_CACHE_SIZE == 0) && (length % PAGE_CACHE_SIZE == 0)
> +/*
> + * ext4_unmap_page_range() unmaps a page range of length 'length'
> + * starting from file offset 'from'. The range to be unmaped must
> + * be contained with in one page. If the specified range exceeds
> + * the end of the page it will be shortened to end of the page
> + * that cooresponds to 'from'. Only block aligned buffers will
> + * be unmapped and unblock aligned buffers are skipped
> + */
> +int ext4_unmap_page_range(handle_t *handle,
> + struct address_space *mapping, loff_t from, loff_t length)
This function is only used by extents.c; maybe it should be placed in
extents.c and declared static, instead of being placed in inode.c?
> +{
> + ext4_fsblk_t index = from >> PAGE_CACHE_SHIFT;
> + unsigned int offset = from & (PAGE_CACHE_SIZE-1);
> + unsigned int blocksize, max, pos;
> + unsigned int end_of_block, range_to_unmap;
> + ext4_lblk_t iblock;
> + struct inode *inode = mapping->host;
> + struct buffer_head *bh;
> + struct page *page;
> + int err = 0;
> +
> + page = find_or_create_page(mapping, from >> PAGE_CACHE_SHIFT,
> + mapping_gfp_mask(mapping) & ~__GFP_FS);
> + if (!page)
> + return -EINVAL;
> +
> + blocksize = inode->i_sb->s_blocksize;
> + max = PAGE_CACHE_SIZE - offset;
> +
> + /*
> + * correct length if it does not fall between
> + * 'from' and the end of the page
> + */
> + if (length > max || length < 0)
> + length = max;
> +
> + iblock = index << (PAGE_CACHE_SHIFT - inode->i_sb->s_blocksize_bits);
> +
> + if (!page_has_buffers(page))
> + create_empty_buffers(page, blocksize, 0);
If the page doesn't have any buffers, we could just return 0; there's
no point calling create_empty_buffers() and then looping over them
all.
> +
> + /* Find the buffer that contains "offset" */
> + bh = page_buffers(page);
> + pos = blocksize;
> + while (offset >= pos) {
> + bh = bh->b_this_page;
> + iblock++;
> + pos += blocksize;
> + }
> +
> + pos = offset;
> + while (pos < offset + length) {
> + err = 0;
> +
> + /* The length of space left to zero */
> + range_to_unmap = offset + length - pos;
> +
> + /* The length of space until the end of the block */
> + end_of_block = blocksize - (pos & (blocksize-1));
> +
> + /* Do not unmap past end of block */
> + if (range_to_unmap > end_of_block)
> + range_to_unmap = end_of_block;
> +
> + if (buffer_freed(bh)) {
> + BUFFER_TRACE(bh, "freed: skip");
> + goto next;
> + }
> +
> + if (!buffer_mapped(bh)) {
> + BUFFER_TRACE(bh, "unmapped");
> + ext4_get_block(inode, iblock, bh, 0);
> + /* unmapped? It's a hole - nothing to do */
> + if (!buffer_mapped(bh)) {
> + BUFFER_TRACE(bh, "still unmapped");
> + goto next;
> + }
> + }
If the buffer is unmapped, why not just goto next right away? Why
bother calling ext4_get_block() and mapping it, when all we're going
to do is declare the buffer as unmapped anyway.
> +
> + /* If the range is not block aligned, skip */
> + if (range_to_unmap != blocksize)
> + goto next;
> +
> + if (ext4_should_journal_data(inode)) {
> + BUFFER_TRACE(bh, "get write access");
> + err = ext4_journal_get_write_access(handle, bh);
> + if (err)
> + goto unlock;
> + }
> +
> + clear_buffer_dirty(bh);
> + bh->b_bdev = NULL;
> + clear_buffer_mapped(bh);
> + clear_buffer_req(bh);
> + clear_buffer_new(bh);
> + clear_buffer_delay(bh);
> + clear_buffer_unwritten(bh);
> + clear_buffer_uptodate(bh);
> + ClearPageUptodate(page);
> +
> + BUFFER_TRACE(bh, "buffer unmapped");
> +
> + if (ext4_should_journal_data(inode)) {
> + err = ext4_handle_dirty_metadata(handle, inode, bh);
> + } else {
> + if (ext4_should_order_data(inode) &&
> + EXT4_I(inode)->jinode)
> + err = ext4_jbd2_file_inode(handle, inode);
> + }
Why are we calling ext4_handle_dirty_metadata() or
ext4_jbd2_file_inode() here? It's not necessary, since we're not
actually dirtying any buffers here. We're just marking buffer heads
as unmarked.
> +
> +next:
> + bh = bh->b_this_page;
> + iblock++;
> + pos += range_to_unmap;
> + }
> +unlock:
> + unlock_page(page);
> + page_cache_release(page);
> + return err;
> +}
> +
> +
> int ext4_can_truncate(struct inode *inode)
> {
> if (S_ISREG(inode->i_mode))
> --
> 1.7.1
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