Messages in this thread | | | Date | Sun, 18 Nov 2012 08:57:45 +0800 | From | Jaegeuk Hanse <> | Subject | Re: [PATCH] tmpfs: fix shmem_getpage_gfp VM_BUG_ON |
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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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