Re: [PATCH 1/6] mlock: only hold mmap_sem in shared mode when faulting in pages

[Michel Lespinasse <walken@google.com>]

On Thu, Dec 9, 2010 at 10:11 PM, Linus Torvalds
<torvalds@linux-foundation.org> wrote:
> On Wednesday, December 8, 2010, Michel Lespinasse <walken@google.com> wrote:
>> Yes, patch 1/6 changes the long hold time to be in read mode instead
>> of write mode, which is only a band-aid. But, this prepares for patch
>> 5/6, which releases mmap_sem whenever there is contention on it or
>> when blocking on disk reads.
>
> I have to say that I'm not a huge fan of that horribly kludgy
> contention check case.
>
> The "move page-in to read-locked sequence" and the changes to
> get_user_pages look fine, but the contention thing is just disgusting.
> I'd really like to see some other approach if at all possible.

Are you OK with the part of patch 5/6 that drops mmap_sem when
blocking on disk ?

This by itself brings mmap_sem hold time down to a few seconds. Plus,
I could add something to limit the interval passed to
__mlock_vma_pages_range to a thousand pages or so, so that the hold
time would then be bounded by that constant.

I think rwsem_is_contended() actually sounds better than fiddling with
constants, but OTOH maybe the mlock use case is not significant enough
to justify introducing that new API.

-- 
Michel "Walken" Lespinasse
A program is never fully debugged until the last user dies.

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