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Re: REVIEW: Fix for incore extent corruption.

To: Russell Cattelan <cattelan@xxxxxxxxxxx>
Subject: Re: REVIEW: Fix for incore extent corruption.
From: Lachlan McIlroy <lachlan@xxxxxxx>
Date: Thu, 18 Sep 2008 17:02:07 +1000
Cc: xfs@xxxxxxxxxxx
In-reply-to: <48D1DCD5.7040502@xxxxxxxxxxx>
References: <48D19A83.4040608@xxxxxxxxxxx> <48D1CD46.4010104@xxxxxxx> <48D1DCD5.7040502@xxxxxxxxxxx>
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Russell Cattelan wrote:
Lachlan McIlroy wrote:
Russell Cattelan wrote:


It turns out that the code in question is still broken.

xfs_iext_irec_compact_full will still corrupt the incore extent list if it does
the the partial copy of extents from one page to the next.
I haven't quit figured out where things get out of sync but somehow
if_real_bytes which tracks the total number of bytes available in
the extent buffers and if_bytes (which tracks the total bytes used
for extents.

So at some point the inode thinks is has more extents than allocated pages allow.
So what happens is xfs_iext_idx_to_irec which uses idxp to pass IN the
absolute extent index is suppose to change idxp on the way OUT and erp_idxp to be a buffer index pair. What happens is that it doesn't find the extent so idxp
is left holding the same value as was passed in and erp_idx is 0.
This causes the extent code to then index way past the end of extent buffer 0
into garbage mem.

with 4k ext buffers max extent count per buffer is 256.
example being:
idxp = 400
should become:
idexp = 144
erp_idxp = 1

but we end up not finding the extent so
we have
idxp = 400
erp_idxp =0

so we now index 6400 bytes into a 4k buffer.

Which often times is a pages of mostly 0 so we end up with access to block 0 errors.

The more I looked at this code the more it didn't make sense to do partial moves. Since the list of extent buffers is only scanned once vs restarting the list whenever a partial move is done, it is very unlikely to actually free an extent buffer. (granted it's possible but unlikely)

xfs_iext_irec_compact_pages does the same thing as xfs_iext_irec_compact_full except that doesn't handle partial moves.

xfs_iext_irec_compact is written such that ratio of current extents has to be significantly smaller than the current allocated space
xfs_inode: 4513
nextents < ( nlists  * XFS_LINEAR_EXT) >> 3

As it turns out 99% of the time it calls xfs_iext_irec_compact_pages
(which is why it has been so hard to track this bug down).

If you change the 3 to a 1 so the code alway calls compact_full vs compact_pages the extent list will corrupt amost
Awesome work Russell, we'll give it a go.

Since the code is broken, almost never used, and provides only micro optimization of incore space I propose we just
remove it all together.
Are you sure the bug is in xfs_iext_irec_compact_full?

Is it possible that we are still indexing beyond the page when using
xfs_iext_irec_compact_pages but the pages just happen to be sequential
so the indexing gets the extent anyway?
I added a bunch of printk to track this down, the compact_pages path is hit a lot in fact as far as I can tell all running file systems that shrink extents and don't crash :-)

I should have done this originally my I'm including the modified makeextents that
I used to tickle this problem.
It reserves a bunch of space to create a contiguous extents then in unreserves space to poke a bunch of holes creating a big extent list, it then goes back and writes the whole
file hopefully collapsing extents as it goes.
I've got one just like it!  I made the change (3 -> 1) and ran it
and it asserted in xfs_bmap_add_extent_unwritten_real() with
ASSERT(PREV.br_state == oldext) failing.

I suspect the problem is that xfs_iext_irec_compact_full is not updating
the er_extoff field and that's why lookups are not finding extents.

makeextents -v -c 512 foo ; xfs_bmap -v foo
should give you 1024 extents
makeextents -v -f -c 512 foo ; xfs_bmap -v foo
will do the same thing but fill in the file with writes.
The number of resulting extents vary, but sometimes you
end up with one extent. sometimes more.

If you change the 3 to a 1 in the current code so compact_full is used vs compact_pages
and run the test it will hit some problem every time.
xexlist in kdb will show the corruption in the incore list.

This will run the code through all 3 formats so if you are lucky you end up hitting all
the cases indirect > 256, direct <= 256, and inline <= 2

note: xfs_iext_indirect_to_direct does call compact_full but in that case we are already down to under 256 extents (at least we should be ) and at that point compact_full will behave just like compact_pages.

I'm also including an xfsidbg patch that for xexlist that prints out buffer offset and index.

-Russell Cattelan

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