Does shmem_getpage==>shmem_alloc_page==>alloc_page_vma hold mmap_sem?

Does shmem_getpage==>shmem_alloc_page==>alloc_page_vma hold mmap_sem?

[Paul Jackson]

Andi,

The comment in mm/mempolicy.c for alloc_page_vma() states:

  Should be called with the mm_sem of the vma hold.

However it seems that the call chain (#ifdef CONFIG_NUMA):

  shmem_getpage ==> shmem_alloc_page ==> alloc_page_vma

where shmem_getpage() is called from many of the mm/shmem.c file
operations, is called without holding mmap_sem.  There is no
mention of mmap_sem in the entire mm/shmem.c file.

This doesn't seem right.

-- 
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                  Programmer, Linux Scalability
                  Paul Jackson <pj@sgi.com> 1.925.600.0401

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Re: Does shmem_getpage==>shmem_alloc_page==>alloc_page_vma hold mmap_sem?

[Hugh Dickins]

On Sat, 5 Nov 2005, Paul Jackson wrote:
> 
> The comment in mm/mempolicy.c for alloc_page_vma() states:
> 
>   Should be called with the mm_sem of the vma hold.
> 
> However it seems that the call chain (#ifdef CONFIG_NUMA):
> 
>   shmem_getpage ==> shmem_alloc_page ==> alloc_page_vma
> 
> where shmem_getpage() is called from many of the mm/shmem.c file
> operations, is called without holding mmap_sem.  There is no
> mention of mmap_sem in the entire mm/shmem.c file.

It's safe but horrid.  Look closer and you'll find there isn't even
an mm to hold the mmap_sem of.  The struct vm_area_struct is on the
stack of shmem_alloc_page, and exists solely to apply mempolicy to
a shmem file via an interface designed for mempolicy on vmas.

So far as I know, it works fine; but that interface really ought
to be redesigned some time - it looks like a quick hack that stuck.

Hugh

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Re: Does shmem_getpage==>shmem_alloc_page==>alloc_page_vma hold mmap_sem?

[Paul Jackson]

Hugh wrote:
> It's safe but horrid.

Ok - thanks for the explanation.

Now I don't feel so bad about some of my cpuset locking hacks.

(Yes, Andrew, I'm still looking at my latest hack, 
the down_write_trylock() call in cpuset.c refresh_mems.)

-- 
                  I won't rest till it's the best ...
                  Programmer, Linux Scalability
                  Paul Jackson <pj@sgi.com> 1.925.600.0401

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