Message-ID: <4ECA8CA4.5040804@linux.vnet.ibm.com>
Date:	Mon, 21 Nov 2011 10:38:44 -0700
From:	Allison Henderson <achender@...ux.vnet.ibm.com>
To:	Yongqiang Yang <xiaoqiangnk@...il.com>
CC:	Hugh Dickins <hughd@...gle.com>,
	Curt Wohlgemuth <curtw@...gle.com>, "Ted Ts'o" <tytso@....edu>,
	linux-ext4@...r.kernel.org, linux-kernel@...r.kernel.org
Subject: Re: Bug with "fix partial page writes"

On 11/20/2011 06:59 PM, Yongqiang Yang wrote:
> Hi,
>
> I am curious about the reason we need this operation in write_begin
> functions.    I had a look at the commit log just now.   The commit
> log explains the intention is to handle writes on a hole and writes on
> EOF.   Two cases can be handled successfully by block_write_begin.
>
>
> Yongqiang.

Hi all,

Sorry I missed the first note that came through.  I have not been able 
to look at this in depth yet, but will do so when I get back from the 
holiday break next Thurs.  Basically this patch was addressing a bug I 
found when I was trying to get the punch hole patch through an overnight 
run of fsx.

With out this patch, fsx fails (after about 6 or so hours, with punch 
hole enabled).  The failure is triggered when a region of the test file 
that is supposed to contain zeros, ends up containing garbage.  In this 
case what I found was that a write operation that starts/ends in a hole 
or runs off the edge of the file, is supposed to zero out the part of 
the page that is still in the hole or beyond EOF.  But instead of 
zeroing to the end of the page, it would only zero to the edge of the 
block.  So it would only appear to work when blocksize = pagesize, but 
if blocksize < pagesize, we end up with extra garbage in the page.

ext4_discard_partial_page_buffers_no_lock() and 
ext4_discard_partial_page_buffers(), were modeled off of the original
ext4_block_zero_page_range routine, but modified to handle multiple 
blocks for a page.  ext4_discard_partial_page_buffers is simply a 
wrapper that locks the page before passing it to 
ext4_discard_partial_page_buffers_no_lock.  In most cases I found that 
the page needs to be locked, but for ext4_da_write_end and 
ext4_da_write_begin I ran into deadlocks, so I added the wrapper for 
optional locking.  I will look more into it when I get back, but perhaps 
all we need here is some more logic to figure out if the page is present 
and needs locking.

Allison Henderson

>
> On Mon, Nov 21, 2011 at 4:59 AM, Hugh Dickins<hughd@...gle.com>  wrote:
>> We've seen no response to this, so Cc'ing Ted and linux-kernel,
>> and I'll fill in some more detail below.
>>
>> On Tue, 8 Nov 2011, Curt Wohlgemuth wrote:
>>> It appears that there's a bug with this patch:
>>>
>>> -------------------------------------------
>>> commit 02fac1297eb3f471a27368271aadd285548297b0
>>> Author: Allison Henderson<achender@...ux.vnet.ibm.com>
>>> Date:   Tue Sep 6 21:53:01 2011 -0400
>>>
>>>      ext4: fix partial page writes
>>> ...
>>> -------------------------------------------
>>>
>>> Hugh Dickins found a bug with some nasty testing and lockdep that
>>
>> It's the tmpfs swapping test that I've been running, with variations,
>> for years.  System booted with mem=700M and 1.5G swap, two repetitious
>> make -j20 kernel builds (of a 2.6.24 kernel: I stuck with that because
>> the balance of built to unbuilt source grows smaller with later kernels),
>> one directly in a tmpfs (irrelevant in this case, except for the added
>> pressure it generates), the other in a 1k-block ext2 (that I drive with
>> ext4's CONFIG_EXT4_USE_FOR_EXT23) on /dev/loop0 on a 450MB tmpfs file.
>>
>> The first oops I got was indeed down in lockdep, but I've since seen
>> crashes from the same cause without lockdep configured in.  I've not
>> bothered to write down the stacks, beyond noting ext4_da_write_end()'s
>> call to ext4_discard_partial_page_buffers_no_lock() in them, since the
>> code there is clearly at fault as Curt describes.
>>
>>> crashed in ext4_da_write_end(), and after looking at the code with
>>> him, it appears that the call to
>>> ext4_discard_partial_page_buffers_no_lock() in this routine is
>>> manipulating an unlocked, and possibly non-existent page:
>>>
>>>
>>> -------------------------------------------
>>> ...
>>>        ret2 = generic_write_end(file, mapping, pos, len, copied,
>>>                                                        page, fsdata);
>>>
>>>        page_len = PAGE_CACHE_SIZE -
>>>                        ((pos + copied - 1)&  (PAGE_CACHE_SIZE - 1));
>>>
>>>        if (page_len>  0) {
>>>                ret = ext4_discard_partial_page_buffers_no_lock(handle,
>>>                        inode, page, pos + copied - 1, page_len,
>>>                        EXT4_DISCARD_PARTIAL_PG_ZERO_UNMAPPED);
>>>        }
>>> ...
>>> -------------------------------------------
>>>
>>> Note that generic_write_end() will unlock and release the page before
>>> it returns.
>>
>> Exactly.  And clearly the loop-on-tmpfs aspect of the test is
>> irrelevant, except in generating more pressure to trigger it.
>>
>>>
>>> I've no good answer for how to fix this properly, but I wanted to let
>>> Allison know about this, if she hadn't already.  I looked but didn't
>>> see any related email on the linux-ext4 list for this problem.
>>
>> There was a second problem I was seeing, more elusive and much harder
>> to attribute: occasionally the build on ext2 would fail with errors
>> from ld (almost always of the kind "In function `no symbol': multiple
>> definition of `no symbol'" and "Warning: size of symbol `' changed":
>> I don't know if there's anything to be deduced from that).  I took
>> these to indicate an error in filesystem or loop or tmpfs or swap.
>>
>> First suspect was loop changes from hch in 3.2-rc1, but backing those
>> out made no difference.  I thought I was facing a week's bisection
>> (since it would need at least a day to conclude any stage good), but
>> took a gamble on backing out *both* parts of 02fac1297eb3: page_len
>> additions to ext4_da_write_begin() as well as page_len additions to
>> ext4_da_write_end().
>>
>> That gamble paid off: the test then showed no problems in several
>> days running on two machines.  So, both parts of 02fac1297eb3 are
>> bad, but it's not so easy to see what's wrong with the write_begin.
>>
>> My *guess* is that the partial page fixes have interfered with the
>> subtle page-dirty buffer-dirty protocol in some way, which manifests
>> only under memory pressure.
>>
>> It's conceivable that loop and tmpfs and swap play a part in this
>> further error, but I don't think so: I have no evidence for that,
>> and no such problem was seen before 3.2-rc1.
>>
>> ---
>>
>> I wanted to find you an easier way to reproduce the problem, so I
>> tried fsx (I'm still using a pretty old fsx, no fallocate or punch
>> hole), run in ext2 on a kernel booted with mem=700M.  Sorry, I did
>> this a week ago, then didn't find time to write it up, and failed to
>> note when my ext2 was in /dev/loop0 and when it was directly on disk.
>>
>> fsx foo -q -c 100 -l 100000000&
>> while :
>> do      # memory hog mmaps and touches each page of 800MB private area
>>         swapout 800
>> done
>>
>> I did not reproduce either problem above with that.  Instead I found
>> that backing out 02fac1297eb3 made fsx on 3.2-rc1 fail in a few minutes.
>> But leaving 02fac1297eb3 in, fsx still failed in 20 minutes or an hour.
>> On 3.1, fsx failed in a few minutes.  On 3.0, fsx failed in half an hour.
>> On 2.6.39, fsx failed in a few minutes.  I had to go back to 2.6.38 for
>> fsx to run successfully under memory pressure for more than two hours.
>>
>> It looks as if ext4 testing has not been running fsx under memory
>> pressure recently.  And although I didn't reproduce my main problems
>> that way, it could well be that getting fsx to run reliably again
>> under memory pressure will be the way to fix those problems.
>>
>> Thanks,
>> Hugh
>> --
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>
>
>

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