Messages in this thread | | | Date | Sun, 18 Nov 2012 09:48:14 +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..
Hi Hugh,
- I see you add this in vfs.txt: + fallocate: called by the VFS to preallocate blocks or punch a hole. I want to know if it's necessary to add it to man page since users still don't know fallocate can punch a hole from man fallocate. - in function shmem_fallocate: + else if (shmem_falloc.nr_unswapped > shmem_falloc.nr_falloced) + error = -ENOMEM; If this changelog "shmem_fallocate() compare counts and give up once the reactivated pages have started to coming back to writepage (approximately: some zones would in fact recycle faster than others)." describe why need this change? If the answer is yes, I have two questions. 1) how can guarantee it really don't need preallocation if just one or a few pages always reactivated, in this scene, nr_unswapped maybe grow bigger enough than shmem_falloc.nr_falloced 2) why return -ENOMEM, it's not really OOM, is it a trick or ...?
Regards, Jaegeuk
> > 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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