From: Jaegeuk Hanse <jaegeuk.hanse@gmail.com>
To: Hugh Dickins <hughd@google.com>
Cc: Dave Jones <davej@redhat.com>,
Andrew Morton <akpm@linux-foundation.org>,
Johannes Weiner <hannes@cmpxchg.org>,
linux-mm@kvack.org, linux-kernel@vger.kernel.org
Subject: Re: [PATCH] tmpfs: fix shmem_getpage_gfp VM_BUG_ON
Date: Sun, 18 Nov 2012 08:57:45 +0800 [thread overview]
Message-ID: <50A83289.6020108@gmail.com> (raw)
In-Reply-To: <alpine.LNX.2.00.1211162018010.1164@eggly.anvils>
On 11/17/2012 12:48 PM, Hugh Dickins wrote:
> Further offtopic..
Thanks for your explanation, Hugh. :-)
>
> On Fri, 16 Nov 2012, Jaegeuk Hanse wrote:
>> Some questions about your shmem/tmpfs: misc and fallocate patchset.
>>
>> - Since shmem_setattr can truncate tmpfs files, why need add another similar
>> codes in function shmem_fallocate? What's the trick?
> I don't know if I understand you. In general, hole-punching is different
> from truncation. Supporting the hole-punch mode of the fallocate system
> call is different from supporting truncation. They're closely related,
> and share code, but meet different specifications.
What's the different between shmem/tmpfs hole-punching and
truncate_setsize/truncate_pagecache?
Do you mean one is punch hole in the file and the other one is shrink or
extent the size of a file?
>> - in tmpfs: support fallocate preallocation patch changelog:
>> "Christoph Hellwig: What for exactly? Please explain why preallocating on
>> tmpfs would make any sense.
>> Kay Sievers: To be able to safely use mmap(), regarding SIGBUS, on files on
>> the /dev/shm filesystem. The glibc fallback loop for -ENOSYS [or
>> -EOPNOTSUPP] on fallocate is just ugly."
>> Could shmem/tmpfs fallocate prevent one process truncate the file which the
>> second process mmap() and get SIGBUS when the second process access mmap but
>> out of current size of file?
> Again, I don't know if I understand you. fallocate does not prevent
> truncation or races or SIGBUS. I believe that Kay meant that without
> using fallocate to allocate the memory in advance, systemd found it hard
> to protect itself from the possibility of getting a SIGBUS, if access to
> a shmem mapping happened to run out of memory/space in the middle.
IIUC, it will return VM_xxx_OOM instead of SIGBUS if run out of memory.
Then how can get SIGBUS in this scene?
Regards,
Jaegeuk
> I never grasped why writing the file in advance was not good enough:
> fallocate happened to be what they hoped to use, and it was hard to
> deny it, given that tmpfs already supported hole-punching, and was
> about to convert to the fallocate interface for that.
> Hugh
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next prev parent reply other threads:[~2012-11-18 0:58 UTC|newest]
Thread overview: 35+ messages / expand[flat|nested] mbox.gz Atom feed top
2012-10-25 2:37 shmem_getpage_gfp VM_BUG_ON triggered. [3.7rc2] Dave Jones
2012-10-25 4:36 ` Hugh Dickins
2012-10-25 4:50 ` Ni zhan Chen
2012-10-25 6:59 ` Hugh Dickins
2012-10-25 9:53 ` Ni zhan Chen
2012-10-25 10:21 ` Ni zhan Chen
2012-10-25 21:27 ` Hugh Dickins
2012-10-26 1:48 ` Ni zhan Chen
2012-10-25 11:14 ` Dave Jones
2012-10-25 21:28 ` Hugh Dickins
2012-10-25 20:52 ` Johannes Weiner
2012-10-25 21:48 ` Hugh Dickins
2012-10-26 2:15 ` Ni zhan Chen
2012-11-01 19:10 ` Dave Jones
2012-11-01 23:03 ` Hugh Dickins
2012-11-01 23:20 ` Dave Jones
2012-11-01 23:48 ` Hugh Dickins
2012-11-02 1:43 ` Dave Jones
2012-11-02 23:26 ` Hugh Dickins
2012-11-06 1:32 ` [PATCH] tmpfs: fix shmem_getpage_gfp VM_BUG_ON Hugh Dickins
2012-11-06 13:54 ` Dave Jones
2012-11-06 23:48 ` Hugh Dickins
2012-11-07 22:38 ` Dave Jones
2012-11-14 1:36 ` [PATCH] tmpfs: fix shmem_getpage_gfp VM_BUG_ON fix Hugh Dickins
2012-11-14 3:07 ` [PATCH] tmpfs: fix shmem_getpage_gfp VM_BUG_ON Jaegeuk Hanse
2012-11-14 3:50 ` Hugh Dickins
2012-11-14 6:14 ` Dave Jones
2012-11-14 10:06 ` Hugh Dickins
2012-11-15 7:39 ` Jaegeuk Hanse
2012-11-15 19:56 ` Hugh Dickins
2012-11-16 0:40 ` Jaegeuk Hanse
2012-11-16 9:34 ` Jaegeuk Hanse
2012-11-17 4:48 ` Hugh Dickins
2012-11-18 0:57 ` Jaegeuk Hanse [this message]
2012-11-18 1:48 ` Jaegeuk Hanse
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