Re: [PATCH] tmpfs: fix shmem_getpage_gfp VM_BUG_ON

[Jaegeuk Hanse <jaegeuk.hanse@gmail.com>]

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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