linux-kernel - Re: [PATCH] mm: for shm_open()/mmap() with OVERCOMMIT

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Date:	Wed, 31 Jul 2013 13:28:10 +0400
From:	Azat Khuzhin <a3at.mail@...il.com>
To:	Hugh Dickins <hughd@...gle.com>
Cc:	open list <linux-kernel@...r.kernel.org>, linux-mm@...ck.org
Subject: Re: [PATCH] mm: for shm_open()/mmap() with OVERCOMMIT_NEVER, return
 -1 if no memory avail

On Wed, Jul 31, 2013 at 10:32 AM, Hugh Dickins <hughd@...gle.com> wrote:
> On Tue, 30 Jul 2013, Azat Khuzhin wrote:
>
>> Otherwize if there is no left space on shmem device, there will be
>> "Bus error" when application will try to write to address space that was
>> returned by mmap(2)
>>
>> This patch also preserve old behaviour if MAP_NORESERVE/VM_NORESERVE
>> isset.
>>
>> So, with this patch, you will get next:
>>
>> a)
>> $ echo 2 >| /proc/sys/vm/overcommit_memory
>>   ....
>>   mmap() = MAP_FAILED;
>>   ....
>>
>> b)
>>   ....
>>   mmap(0, length, PROT_READ | PROT_WRITE, MAP_SHARED | MAP_NORESERVE) = !MAP_FAILED;
>>   write()
>>   killed by SIGBUS
>>   ....
>>
>> c)
>> $ echo 0 >| /proc/sys/vm/overcommit_memory
>>   ....
>>   mmap() = !MAP_FAILED;
>>   write()
>>   killed by SIGBUS
>>   ....
>>
>> Signed-off-by: Azat Khuzhin <a3at.mail@...il.com>
>
> Thanks for making the patch, but I'm afraid there are a number of
> things wrong with it; and even if it were perfect, I would still be
> reluctant to change the semantics of shmem_mmap() after all this time.

I was also think about this, but hence it only change behavior with
OVERCOMMIT_NEVER, I post this patch.

>
> Some comments on your implementation below; but if getting SIGBUS from
> a write to an mmapping, once the underlying filesystem (shmem/tmpfs or
> any other) fills up, if that SIGBUS is troublesome for you, then please
> try using fallocate() to allocate the space before accessing the mmapping.

Oh.. forgot about fallocate().
Thanks for you comments, I will keep in mind!

>
>> ---
>>  mm/shmem.c |   16 ++++++++++++++++
>>  1 file changed, 16 insertions(+)
>>
>> diff --git a/mm/shmem.c b/mm/shmem.c
>> index a87990c..965f4ba 100644
>> --- a/mm/shmem.c
>> +++ b/mm/shmem.c
>> @@ -32,6 +32,8 @@
>>  #include <linux/export.h>
>>  #include <linux/swap.h>
>>  #include <linux/aio.h>
>> +#include <linux/statfs.h>
>> +#include <linux/path.h>
>
> I'm surprised you need either of those: vfs.h should have already
> included statfs.h, and I don't see what path.h would be for.
>
>>
>>  static struct vfsmount *shm_mnt;
>>
>> @@ -1356,6 +1358,20 @@ out_nomem:
>>
>>  static int shmem_mmap(struct file *file, struct vm_area_struct *vma)
>>  {
>> +     if (!(vma->vm_flags & VM_NORESERVE) &&
>> +         sysctl_overcommit_memory == OVERCOMMIT_NEVER) {
>
> So, this would be a new and different usage of sysctl_overcommit_memory:
> usually it applies to vm_committed_as accounting, but you're extending
> it to affect tmpfs filesystem size accounting.  Hmm.
>
>> +             struct inode *inode = file_inode(file);
>> +             struct kstatfs sbuf;
>> +             u64 size;
>> +
>> +             inode->i_sb->s_op->statfs(file->f_dentry, &sbuf);
>
> You don't really need to go through ->statfs(), since that will arrive
> at shmem_statfs().  Where you can see there will be a problem in the
> case of an unlimited (max_blocks=0) mount - you will fail mmap() of
> every file of non-0 size - and mmaps of 0-size files aren't much use!
> But moving on from that case...

Nice catch, thanks!

>
>> +             size = sbuf.f_bfree * sbuf.f_bsize;
>> +
>> +             if (size < inode->i_size) {
>> +                     return -ENOMEM;
>
> So, if your filesystem is full, mmap() of any (i_size>0) file in it will
> fail?  I don't think that's what you want at all.  You seem to be assuming
> that no pages of the file you're mmap()ing have been allocated yet: that
> may be the case, but it's very often not so.
>
>> +             }
>
> And if we pass that test, there's stll no assurance that you won't get
> SIGBUS from accessing the mmapping: nothing has actually been reserved
> here, and other activity on the system can gobble up all the remaining
> space in the filesystem, or take vm_committed_as to its maximum.

Completely slipped my mind.

>
>> +     }
>> +
>>       file_accessed(file);
>>       vma->vm_ops = &shmem_vm_ops;
>>       return 0;
>> --
>> 1.7.10.4
>
> Please "man 2 fallocate" and use that instead.
>
> Hugh



-- 
Respectfully
Azat Khuzhin
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