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Re: [PATCH] Prevent lookup from finding bad buffers

To: lachlan@xxxxxxx
Subject: Re: [PATCH] Prevent lookup from finding bad buffers
From: Eric Sandeen <sandeen@xxxxxxxxxxx>
Date: Wed, 25 Nov 2009 14:26:19 -0600
Cc: xfs-oss <xfs@xxxxxxxxxxx>
In-reply-to: <4990EAF9.9010607@xxxxxxx>
References: <4990EAF9.9010607@xxxxxxx>
User-agent: Thunderbird 2.0.0.21 (X11/20090320)
Lachlan McIlroy wrote:
> There's a bug in _xfs_buf_find() that will cause it to return buffers
> that failed to be initialised.
> 
> If a thread has a buffer locked and is waiting for I/O to initialise
> it and another thread wants the same buffer the second thread will
> wait on the buffer lock in _xfs_buf_find().  If the initial thread
> gets an I/O error it marks the buffer in error and releases the
> buffer lock.  The second thread gets the buffer lock, assumes the
> buffer has been successfully initialised, and then tries to use it.
> 
> Some callers of xfs_buf_get_flags() will check for B_DONE, and if
> it's not set then re-issue the I/O, bust most callers assume the
> buffer and it's contents are good and then use the uninitialised
> data.
> 
> The solution I've come up with is if we lookup a buffer and find
> it's got b_error set or has been marked stale then unhash it from
> the buffer hashtable and retry the lookup.  Also if we fail to setup
> the buffer correctly in xfs_buf_get_flags() then mark the buffer in
> error and unhash it.  If the buffer is marked stale then in
> xfs_buf_free() inform the page cache that the contents of the pages
> are no longer valid.

I managed to come up with a sorta-kinda testcase for this.

Fragmented freespace, many files in a dir, on raid5; simply doing
drop caches / ls in a loop triggered it.

I guess raid5 is bad in this respect; in it's make_request() we have:

                } else {
                        /* cannot get stripe for read-ahead, just give-up */
                        clear_bit(BIO_UPTODATE, &bi->bi_flags);
                        finish_wait(&conf->wait_for_overlap, &w);
                        break;
                }

and this happens fairly often.  This probably explains a large
percentage of our xfs_da_do_buf(2) errors we've seen on the list.

>From my testing, I think this suffices - and interestingly, Lachlan's
original patch doesn't seem to help...

Comments?

Maybe could clean up the logic a bit... should this only be
tested for XBF_READ buffers as well ... or maybe an assert that
if !uptodate, error should be set ...

diff --git a/fs/xfs/linux-2.6/xfs_buf.c b/fs/xfs/linux-2.6/xfs_buf.c
index 965df12..cbc0541 100644
--- a/fs/xfs/linux-2.6/xfs_buf.c
+++ b/fs/xfs/linux-2.6/xfs_buf.c
@@ -1142,6 +1165,8 @@ xfs_buf_bio_end_io(
                if (unlikely(bp->b_error)) {
                        if (bp->b_flags & XBF_READ)
                                ClearPageUptodate(page);
+               } else if (!test_bit(BIO_UPTODATE, &bio->bi_flags)) {
+                       ClearPageUptodate(bp);
                } else if (blocksize >= PAGE_CACHE_SIZE) {
                        SetPageUptodate(page);
                } else if (!PagePrivate(page) &&

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