Re: [PATCH v2] mm: per-thread vma caching

[Linus Torvalds <torvalds@linux-foundation.org>]

On Tue, Feb 25, 2014 at 10:37 AM, Linus Torvalds
<torvalds@linux-foundation.org> wrote:
>
>  - clear all the cache entries (of the new 'struct task_struct'! - so
> not in dup_mmap, but make sure it's zeroed when allocating!)(
>
>  - set vmcache_seqnum to 0 in dup_mmap (since any sequence number is
> fine when it got invalidated, and 0 is best for "avoid overflow").

Btw, as far as I can tell, that also makes the per-thread vmacache
automatically do the right thing for the non-MMU case, so that you
could just remove the difference between CONFIG_MMU and NOMMU.

Basically, dup_mmap() should no longer have anything to do with the
vmacache, since it is now per-thread, not per-mm.

So :

 - allocating a new "struct mm_struct" should clear the
vmacache_seqnum for that new mm, to try to minimize unnecessary future
overflow.

 - thread allocation should just zero the cache entries, and set
"tsk->vmacache_seqnum = mm->vmacache_seqnum" (after dup_mm()) to avoid
future unnecessary flushes.

and as far as I can tell, the logic would be exactly the same on NOMMU
(the dup_mm just doesn't happen, since all forks are basically sharing
mm).

And maybe you'd want to make VMACACHE_SIZE be 1 on NOMMU (and make
sure to change the "& 3" to "& (VMACACHE_SIZE-1)". Just to keep the
size down on small systems that really don't need it.

          Linus

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