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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