Re: XFS memory allocation deadlock in 2.6.38

[Dave Chinner]

On Mon, Mar 28, 2011 at 05:34:09PM -0400, Sean Noonan wrote:
> > Could you test if you see the deadlock before
> > 5ecfda041e4b4bd858d25bbf5a16c2a6c06d7272 without MAP_POPULATE ?
> 
> Built and tested 72ddc8f72270758951ccefb7d190f364d20215ab.
> Confirmed that the original bug does not present in this version.
> Confirmed that removing MAP_POPULATE does cause the deadlock to occur.
> 
> Here is the stack of the test:
> # cat /proc/3846/stack
> [<ffffffff812e8a64>] call_rwsem_down_read_failed+0x14/0x30
> [<ffffffff81271c1d>] xfs_ilock+0x9d/0x110
> [<ffffffff81271cae>] xfs_ilock_map_shared+0x1e/0x50
> [<ffffffff81294985>] __xfs_get_blocks+0xc5/0x4e0
> [<ffffffff81294dcc>] xfs_get_blocks+0xc/0x10
> [<ffffffff811322c2>] do_mpage_readpage+0x462/0x660
> [<ffffffff8113250a>] mpage_readpage+0x4a/0x60
> [<ffffffff81295433>] xfs_vm_readpage+0x13/0x20
> [<ffffffff810bb850>] filemap_fault+0x2d0/0x4e0
> [<ffffffff810d8680>] __do_fault+0x50/0x510
> [<ffffffff810da542>] handle_mm_fault+0x1a2/0xe60
> [<ffffffff8102a466>] do_page_fault+0x146/0x440
> [<ffffffff8164e6cf>] page_fault+0x1f/0x30
> [<ffffffffffffffff>] 0xffffffffffffffff

Something else is holding the inode locked here.

> xfssyncd is stuck in D state.
> # cat /proc/2484/stack
> [<ffffffff8106ee1c>] down+0x3c/0x50
> [<ffffffff81297802>] xfs_buf_lock+0x72/0x170
> [<ffffffff8128762d>] xfs_getsb+0x1d/0x50
> [<ffffffff8128e6af>] xfs_trans_getsb+0x5f/0x150
> [<ffffffff8128821e>] xfs_mod_sb+0x4e/0xe0
> [<ffffffff8126e4ea>] xfs_fs_log_dummy+0x5a/0xb0
> [<ffffffff812a2a13>] xfs_sync_worker+0x83/0x90
> [<ffffffff812a28e2>] xfssyncd+0x172/0x220
> [<ffffffff81069576>] kthread+0x96/0xa0
> [<ffffffff81003354>] kernel_thread_helper+0x4/0x10
> [<ffffffffffffffff>] 0xffffffffffffffff

And this is indicating that something else is holding the superblock
locked here. IOWs, whatever thread is having trouble with memory
allocation is causing these threads to block and so they can be
ignored. What's the stack trace of the thread that is throwing the
"I can't allocating a page" errors?

As it is, the question I'd really like answered is how a machine with
48GB RAM can possibly be short of memory when running mmap() on a
16GB file.  The error that XFS is throwing indicates that the
machine cannot allocate a single page of memory, so where has all
your memory gone, and why hasn't the OOM killer been let off the
leash?  What is consuming the other 32GB of RAM or preventing it
from being allocated? 

Also, I was unable to reproduce this at all on a machine with only
2GB of RAM, regardless of the kernel version and/or MAP_POPULATE, so
I'm left to wonder what is special about your test system...

Perhaps the output of xfs_bmap -vvp <file> after a successful vs
deadlocked run would be instructive....

Cheers,

Dave.
-- 
Dave Chinner
david@xxxxxxxxxxxxx
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