linux-ext4 - Re: [PATCH 4/4] ext4: fix suboptimal seek

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Date:	Sun, 28 Dec 2014 22:55:46 +0400
From:	Dmitry Monakhov <dmonakhov@...nvz.org>
To:	Theodore Ts'o <tytso@....edu>
Cc:	linux-ext4@...r.kernel.org
Subject: Re: [PATCH 4/4] ext4: fix suboptimal seek_{data,hole} extents traversial


Theodore Ts'o <tytso@....edu> writes:

> On Wed, Dec 17, 2014 at 06:06:27PM +0300, Dmitry Monakhov wrote:
>> I'm working on that. Sorry for delay. Delay is caused by confusion of
>> existing seek_xxx implementation (even w/o my changes)
>> For example: it is not obvious to believe that if bh is (bh_unwritten |
>> bh_uptodate) then it contains valid data. IMHO it is just a fallocated
>> area.  I already have simple fix for actual regression. But I try to
>> review,optimize and retest all related logic.
>
> Hi Dmitry,
>
> Any update on how your testing is going?  I was wondering if I could
> take a look at your fix just so I can understand what is going on.
Yes. I've already sent you updated patches
https://patchwork.ozlabs.org/patch/422594/
https://patchwork.ozlabs.org/patch/422595/
>
>> FYI: Currently test failed only on EXT4 w/o extents but w/ -odelalloc enabled.
>> This configuration is absent in xfstest-bld where conf/ext3 is EXT4 w/o
>> extents and w/ nodelalloc. That is why we oversee this from very beginning.
>
> One of the reasons why I'm having trouble understanding what is
> happening is that I can *only* reproduce the test if I use
>
> ./kvm-xfstests -c ext3 generic/285
>
> If I run it by hand, I get this instead:
>
> # xfstests/src/seek_sanity_test  /mnt/test
> File system magic#: 0xef53
> Allocation size: 4096
> Kernel does not support llseek(2) extensions SEEK_HOLE and/or SEEK_DATA. Aborting.
>
> ... which is what I would expect, since this test should be refusing
> to run in the first place if extents aren't available.  Also, if
> extents aren't available, then we shouldn't be seeing *any* fallocated
> space, so if the fault is in handling (bh_unwritten | bh_uptodate),
> then this case shouldn't be happening at all on a file system w/o
> extents, right?
>
> One of the reasons why I am worrying is now that it's post -rc1, more
> people will be testing out the kernel, and I'm concerned that I don't
> understand what the risk exposure might be, especially since there are
> programs like /bin/cp that assume SEEK_DATA and SEEK_HOLE work
> correctly, and early testers might suffer data loss if they aren't
> working correctly.
Agree. This bug us serious because  may result in bad data lose 
for backup/copy software which use this API.
So if you see any failures on my updated patches please let me know.
>
> So I'm trying to determine whether this is something where we
> understand what is going on so we can fix the bug, or whether I'm
> better off reverting the commit and waiting until we have fully
> characterized the bug.
>
> Thanks,
>
> 					- Ted
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