Re: [PATCH v4 29/30] btrfs: fix a subpage relocation data corruption

[Qu Wenruo <wqu@suse.com>]

On 2021/5/31 下午4:51, Qu Wenruo wrote:
> [BUG]
> When using the following script, btrfs will report data corruption after
> one data balance with subpage support:
> 
>    mkfs.btrfs -f -s 4k $dev
>    mount $dev -o nospace_cache $mnt
>    $fsstress -w -n 8 -s 1620948986 -d $mnt/ -v > /tmp/fsstress
>    sync
>    btrfs balance start -d $mnt
>    btrfs scrub start -B $mnt
> 
> Similar problem can be easily observed in btrfs/028 test case, there
> will be tons of balance failure with -EIO.
> 
> [CAUSE]
> Above fsstress will result the following data extents layout in extent
> tree:
>          item 10 key (13631488 EXTENT_ITEM 98304) itemoff 15889 itemsize 82
>                  refs 2 gen 7 flags DATA
>                  extent data backref root FS_TREE objectid 259 offset 1339392 count 1
>                  extent data backref root FS_TREE objectid 259 offset 647168 count 1
>          item 11 key (13631488 BLOCK_GROUP_ITEM 8388608) itemoff 15865 itemsize 24
>                  block group used 102400 chunk_objectid 256 flags DATA
>          item 12 key (13733888 EXTENT_ITEM 4096) itemoff 15812 itemsize 53
>                  refs 1 gen 7 flags DATA
>                  extent data backref root FS_TREE objectid 259 offset 729088 count 1
> 
> Then when creating the data reloc inode, the data reloc inode will look
> like this:
> 
> 	0	32K	64K	96K 100K	104K
> 	|<------ Extent A ----->|   |<- Ext B ->|
> 
> Then when we first try to relocate extent A, we setup the data reloc
> inode with iszie 96K, then read both page [0, 64K) and page [64K, 128K).
> 
> For page 64K, since the isize is just 96K, we fill range [96K, 128K)
> with 0 and set it uptodate.
> 
> Then when we come to extent B, we update isize to 104K, then try to read
> page [64K, 128K).
> Then we find the page is already uptodate, so we skip the read.
> But range [96K, 128K) is filled with 0, not the real data.
> 
> Then we writeback the data reloc inode to disk, with 0 filling range
> [96K, 128K), corrupting the content of extent B.
> 
> The behavior is caused by the fact that we still do full page read for
> subpage case.
> 
> The bug won't really happen for regular sectorsize, as one page only
> contains one sector.
> 
> [FIX]
> This patch will fix the problem by invalidating range [isize, PAGE_END]
> in prealloc_file_extent_cluster().

The fix is enough to fix the data corruption, but it leaves a very rare 
deadlock.

Above invalidating is in fact not safe, since we're not doing a proper 
btrfs_invalidatepage().

The biggest problem here is, we can leave the page half dirty, and half 
out-of-date.

Then later btrfs_readpage() can trigger a deadlock like this:
btrfs_readpage()
|  We already have the page locked.
|
|- btrfs_lock_and_flush_ordered_range()
    |- btrfs_start_ordered_extent()
       |- extent_write_cache_pages()
          |- pagevec_lookup_range_tag()
          |- lock_page()
             We try to lock a page which is already locked by ourselves.

This can only happen for subpage case, and normally it should never 
happen for regular subpage opeartions.
As we either read the full the page, then update part of the page to 
dirty, dirty the full page without reading it.

This shortcut in relocation code breaks the assumption, and could lead 
to above deadlock.

Although I still don't like to call btrfs_invalidatepage(), here we can 
workaround the half-dirty-half-out-of-date problem by just writing the 
page back to disk.

This will clear the page dirty bits, and allow later clear_uptodate() 
call to be safe.

I'll update the patchset in github repo first, and hope to merge it with 
other feedback into next update.

Currently the test looks very promising, as the Power8 VM has survived 
over 100 loops without crashing.

Thanks,
Qu

> 
> So that if above example happens, when we preallocate the file extent
> for extent B, we will clear the uptodate bits for range [96K, 128K),
> allowing later relocate_one_page() to re-read the needed range.
> 
> Reported-by: Ritesh Harjani <riteshh@linux.ibm.com>
> Signed-off-by: Qu Wenruo <wqu@suse.com>
> ---
>   fs/btrfs/relocation.c | 38 ++++++++++++++++++++++++++++++++++++++
>   1 file changed, 38 insertions(+)
> 
> diff --git a/fs/btrfs/relocation.c b/fs/btrfs/relocation.c
> index cd50559c6d17..b50ee800993d 100644
> --- a/fs/btrfs/relocation.c
> +++ b/fs/btrfs/relocation.c
> @@ -2782,10 +2782,48 @@ static noinline_for_stack int prealloc_file_extent_cluster(
>   	u64 num_bytes;
>   	int nr;
>   	int ret = 0;
> +	u64 isize = i_size_read(&inode->vfs_inode);
>   	u64 prealloc_start = cluster->start - offset;
>   	u64 prealloc_end = cluster->end - offset;
>   	u64 cur_offset = prealloc_start;
>   
> +	/*
> +	 * For subpage case, previous isize may not be aligned to PAGE_SIZE.
> +	 * This means the range [isize, PAGE_END + 1) is filled with 0 by
> +	 * btrfs_do_readpage() call of previously relocated file cluster.
> +	 *
> +	 * If the current cluster starts in above range, btrfs_do_readpage()
> +	 * will skip the read, and relocate_one_page() will later writeback
> +	 * the padding 0 as new data, causing data corruption.
> +	 *
> +	 * Here we have to manually invalidate the range (isize, PAGE_END + 1).
> +	 */
> +	if (!IS_ALIGNED(isize, PAGE_SIZE)) {
> +		struct btrfs_fs_info *fs_info = inode->root->fs_info;
> +		const u32 sectorsize = fs_info->sectorsize;
> +		struct page *page;
> +
> +		ASSERT(sectorsize < PAGE_SIZE);
> +		ASSERT(IS_ALIGNED(isize, sectorsize));
> +
> +		page = find_lock_page(inode->vfs_inode.i_mapping,
> +				      isize >> PAGE_SHIFT);
> +		/*
> +		 * If page is freed we don't need to do anything then, as
> +		 * we will re-read the whole page anyway.
> +		 */
> +		if (page) {
> +			u64 page_end = page_offset(page) + PAGE_SIZE - 1;
> +
> +			clear_extent_bits(&inode->io_tree, isize, page_end,
> +					  EXTENT_UPTODATE);
> +			btrfs_subpage_clear_uptodate(fs_info, page, isize,
> +						     page_end + 1 - isize);
> +			unlock_page(page);
> +			put_page(page);
> +		}
> +	}
> +
>   	BUG_ON(cluster->start != cluster->boundary[0]);
>   	ret = btrfs_alloc_data_chunk_ondemand(inode,
>   					      prealloc_end + 1 - prealloc_start);
>