Re: ftruncate-mmap: pages are lost after writing to mmaped file.

[Ying Han <yinghan@google.com>]

On Tue, Mar 24, 2009 at 3:32 AM, Andrew Morton
<akpm@linux-foundation.org> wrote:
> On Tue, 24 Mar 2009 18:44:21 +1100 Nick Piggin <nickpiggin@yahoo.com.au> wrote:
>
>> On Friday 20 March 2009 03:46:39 Jan Kara wrote:
>> > On Fri 20-03-09 02:48:21, Nick Piggin wrote:
>>
>> > > Holding mapping->private_lock over the __set_page_dirty should
>> > > fix it, although I guess you'd want to release it before calling
>> > > __mark_inode_dirty so as not to put inode_lock under there. I
>> > > have a patch for this if it sounds reasonable.
>> >
>> >   Yes, that seems to be a bug - the function actually looked suspitious to
>> > me yesterday but I somehow convinced myself that it's fine. Probably
>> > because fsx-linux is single-threaded.
>>
>>
>> After a whole lot of chasing my own tail in the VM and buffer layers,
>> I think it is a problem in ext2 (and I haven't been able to reproduce
>> with ext3 yet, which might lend weight to that, although as we have
>> seen, it is very timing dependent).
>>
>> That would be slightly unfortunate because we still have Jan's ext3
>> problem, and also another reported problem of corruption on ext3 (on
>> brd driver).
>>
>> Anyway, when I have reproduced the problem with the test case, the
>> "lost" writes are all reported to be holes. Unfortunately, that doesn't
>> point straight to the filesystem, because ext2 allocates blocks in this
>> case at writeout time, so if dirty bits are getting lost, then it would
>> be normal to see holes.
>>
>> I then put in a whole lot of extra infrastructure to track metadata about
>> each struct page (when it was last written out, when it last had the number
>> of writable ptes reach 0, when the dirty bits were last cleared etc). And
>> none of the normal asertions were triggering: eg. when any page is removed
>> from pagecache (except truncates), it has always had all its buffers
>> written out *after* all ptes were made readonly or unmapped. Lots of other
>> tests and crap like that.
>>
>> So I tried what I should have done to start with and did an e2fsck after
>> seeing corruption. Yes, it comes up with errors.
>
> Do you recall what the errors were?

I run e2fsck on the partition after the failure happened and here is
what i saw, not sure if that is the same message Jan looked at:

e2fsck 1.41.3 (12-Oct-2008)
Warning!  /dev/hda1 is mounted.
/dev/hda1 contains a file system with errors, check forced.
Pass 1: Checking inodes, blocks, and sizes
Pass 2: Checking directory structure
Pass 3: Checking directory connectivity
Pass 4: Checking reference counts
Pass 5: Checking group summary information
Block bitmap differences:  +74915 -195111 -224680
Fix? no

Free blocks count wrong for group #6 (170, counted=169).
Fix? no

Free blocks count wrong (10120, counted=523).
Fix? no

Free inodes count wrong (95678, counted=95672).
Fix? no


/dev/hda1: ********** WARNING: Filesystem still has errors **********

/dev/hda1: 35938/131616 files (1.5% non-contiguous), 252936/263056 blocks

--Ying


>
>> Now that is unusual
>> because that should be largely insulated from the vm: if a dirty bit gets
>> lost, then the filesystem image should be quite happy and error-free with
>> a hole or unwritten data there.
>>
>> I don't know ext? locking very well, except that it looks pretty overly
>> complex and crufty.
>>
>> Usually, blocks are instantiated by write(2), under i_mutex, serialising
>> the allocator somewhat. mmap-write blocks are instantiated at writeout
>> time, unserialised. I moved truncate_mutex to cover the entire get_blocks
>> function, and can no longer trigger the problem. Might be a timing issue
>> though -- Ying, can you try this and see if you can still reproduce?
>>
>> I close my eyes and pick something out of a hat. a686cd89. Search for XXX.
>> Nice. Whether or not this cased the problem, can someone please tell me
>> why it got merged in that state?
>>
>> I'm leaving ext3 running for now. It looks like a nasty task to bisect
>> ext2 down to that commit :( and I would be more interested in trying to
>> reproduce Jan's ext3 problem, however, because I'm not too interested in
>> diving into ext2 locking to work out exactly what is racing and how to
>> fix it properly. I suspect it would be most productive to wire up some
>> ioctls right into the block allocator/lookup and code up a userspace
>> tester for it that could probably stress it a lot harder than kernel
>> writeout can.
>

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