Re: + shmem-fix-faulting-into-a-hole-while-its-punched-take-2.patch added to -mm tree

From: Vlastimil Babka
Date: Wed Jul 09 2014 - 05:50:22 EST

On 07/09/2014 08:35 AM, Hugh Dickins wrote:
On Wed, 9 Jul 2014, Sasha Levin wrote:
On 07/02/2014 03:25 PM, akpm@xxxxxxxxxxxxxxxxxxxx wrote:
From: Hugh Dickins <hughd@xxxxxxxxxx>
Subject: shmem: fix faulting into a hole while it's punched, take 2

I suspect there's something off with this patch, as the shmem_fallocate
hangs are back... Pretty much same as before:

Thank you for reporting, but that is depressing news.

I don't see what's wrong with this (take 2) patch,
and I don't see that it's been garbled in any way in next-20140708.

[ 363.600969] INFO: task trinity-c327:9203 blocked for more than 120 seconds.
[ 363.605359] Not tainted 3.16.0-rc4-next-20140708-sasha-00022-g94c7290-dirty #772
[ 363.609730] "echo 0 > /proc/sys/kernel/hung_task_timeout_secs" disables this message.
[ 363.615861] trinity-c327 D 000000000000000b 13496 9203 8559 0x10000004
[ 363.620284] ffff8800b857bce8 0000000000000002 ffffffff9dc11b10 0000000000000001
[ 363.624468] ffff880104860000 ffff8800b857bfd8 00000000001d7740 00000000001d7740
[ 363.629118] ffff880104863000 ffff880104860000 ffff8800b857bcd8 ffff8801eaed8868
[ 363.633879] Call Trace:
[ 363.635442] [<ffffffff9a4dc535>] schedule+0x65/0x70
[ 363.638638] [<ffffffff9a4dc948>] schedule_preempt_disabled+0x18/0x30
[ 363.642833] [<ffffffff9a4df0a5>] mutex_lock_nested+0x2e5/0x550
[ 363.646599] [<ffffffff972a4d7c>] ? shmem_fallocate+0x6c/0x350
[ 363.651319] [<ffffffff9719b721>] ? get_parent_ip+0x11/0x50
[ 363.654683] [<ffffffff972a4d7c>] ? shmem_fallocate+0x6c/0x350
[ 363.658264] [<ffffffff972a4d7c>] shmem_fallocate+0x6c/0x350

So it's trying to acquire i_mutex at shmem_fallocate+0x6c...

[ 363.662010] [<ffffffff971bd96e>] ? put_lock_stats.isra.12+0xe/0x30
[ 363.665866] [<ffffffff9730c043>] do_fallocate+0x153/0x1d0
[ 363.669381] [<ffffffff972b472f>] SyS_madvise+0x33f/0x970
[ 363.672906] [<ffffffff9a4e3f13>] tracesys+0xe1/0xe6
[ 363.682900] 2 locks held by trinity-c327/9203:
[ 363.684928] #0: (sb_writers#12){.+.+.+}, at: [<ffffffff9730c02d>] do_fallocate+0x13d/0x1d0
[ 363.715102] #1: (&sb->s_type->i_mutex_key#16){+.+.+.}, at: [<ffffffff972a4d7c>] shmem_fallocate+0x6c/0x350

...but it already holds i_mutex, acquired at shmem_fallocate+0x6c.
Am I reading that correctly?

I wonder, why wouldn't lockdep fire here if it was a double lock? I assume lockdep is enabled. It seems to me that the lock #1 is being printed because it's being acquired, not because it already is acquired. __mutex_lock_common() calls mutex_acquire_nest() *before* it actually tries to acquire the mutex. So the output is just confusing.

So it would again help to see stacks of other tasks, to see who holds the i_mutex and where it's stuck...


In my source for next-20140708, the only return from shmem_fallocate()
which omits to mutex_unlock(&inode->i_mutex) is the "return -EOPNOTSUPP"
at the top, just before the mutex_lock(&inode->i_mutex). And inode
doesn't get reassigned in the middle.

Does 3.16.0-rc4-next-20140708-sasha-00022-g94c7290-dirty look different?


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