Re: [PATCH] tmpfs: fix shmem_getpage_gfp VM_BUG_ON

[Hugh Dickins]

Offtopic...

On Thu, 15 Nov 2012, Jaegeuk Hanse wrote:
> 
> Another question. Why the function shmem_fallocate which you add to kernel
> need call shmem_getpage?

Because shmem_getpage(_gfp) is where shmem's
page lookup and allocation complexities are handled.

I assume the question behind your question is: why does shmem actually
allocate pages for its fallocate, instead of just reserving the space?

I did play with just reserving the space, with more special entries in
the radix_tree to note the reservations made.  It should be doable for
the vm_enough_memory and sbinfo->used_blocks reservations.

What absolutely deterred me from taking that path was the mem_cgroup
case: shmem and swap and memcg are not easy to get working right together,
and nobody would thank me for complicating memcg just for shmem_fallocate.

By allocating pages, the pre-existing memcg code just works; if we used
reservations instead, we would have to track their memcg charges in some
additional new way.  I see no justification for that complication.

Hugh
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