Re: [PATCH] Btrfs: clear the extent uptodate bits during parent transid failures
From: Chris Mason <hidden>
Date: 2012-02-23 14:24:55
On Thu, Feb 23, 2012 at 10:12:26AM +0800, Liu Bo wrote:
On 02/23/2012 01:43 AM, Chris Mason wrote:quoted
Normally I just toss patches into git, but this one is pretty subtle and I wanted to send it around for extra review. QA at Oracle did a test where they unplugged one drive of a btrfs raid1 mirror for a while and then plugged it back in. The end result is that we have a whole bunch of out-of-date blocks on the bad mirror. The btrfs parent transid pointers are supposed to detect these bad blocks and then we're supposed to read from the good copy instead. The good news is we did detect the bad blocks. The bad news is we didn't jump over to the good mirror instead. This patch explains why: Author: Chris Mason [off-list ref] Date: Wed Feb 22 12:36:24 2012 -0500 Btrfs: clear the extent uptodate bits during parent transid failures If btrfs reads a block and finds a parent transid mismatch, it clears the uptodate flags on the extent buffer, and the pages inside it. But we only clear the uptodate bits in the state tree if the block straddles more than one page. This is from an old optimization from to reduce contention on the extent state tree. But it is buggy because the code that retries a read from a different copy of the block is going to find the uptodate state bits set and skip the IO. The end result of the bug is that we'll never actually read the good copy (if there is one). The fix here is to always clear the uptodate state bits, which is safe because this code is only called when the parent transid fails.Reviewed-by: Liu Bo <redacted>
Thanks!
quoted hunk ↗ jump to hunk
or we can be safer:diff --git a/fs/btrfs/extent_io.c b/fs/btrfs/extent_io.c index fcf77e1..c1fe25d 100644 --- a/fs/btrfs/extent_io.c +++ b/fs/btrfs/extent_io.c@@ -3859,8 +3859,12 @@ int clear_extent_buffer_uptodate(struct extent_io_tree *tree, } for (i = 0; i < num_pages; i++) { page = extent_buffer_page(eb, i); - if (page) + if (page) { + u64 start = (u64)page->index << PAGE_CACHE_SHIFT; + u64 end = start + PAGE_CACHE_SIZE - 1; + ClearPageUptodate(page); + clear_extent_uptodate(tree, start, end, NULL, GFP_NOFS); } return 0; }
Hmmm, I'm not sure this is safer. Our readpage trusts the extent uptodate bits unconditionally, so we should really clear them unconditionally as well. -chris