Re: [PATCH] tmpfs: fix shmem_getpage_gfp VM_BUG_ON

[Hugh Dickins <hughd@google.com>]

Further offtopic..

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.

> - 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.

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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