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..

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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