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SubjectRe: [PATCH v12 00/31] Speculative page faults
On Mon 22-04-19 14:29:16, Michel Lespinasse wrote:
> I want to add a note about mmap_sem. In the past there has been
> discussions about replacing it with an interval lock, but these never
> went anywhere because, mostly, of the fact that such mechanisms were
> too expensive to use in the page fault path. I think adding the spf
> mechanism would invite us to revisit this issue - interval locks may
> be a great way to avoid blocking between unrelated mmap_sem writers
> (for example, do not delay stack creation for new threads while a
> large mmap or munmap may be going on), and probably also to handle
> mmap_sem readers that can't easily use the spf mechanism (for example,
> gup callers which make use of the returned vmas). But again that is a
> separate topic to explore which doesn't have to get resolved before
> spf goes in.

Well, I believe we should _really_ re-evaluate the range locking sooner
rather than later. Why? Because it looks like the most straightforward
approach to the mmap_sem contention for most usecases I have heard of
(mostly a mm{unm}ap, mremap standing in the way of page faults).
On a plus side it also makes us think about the current mmap (ab)users
which should lead to an overall code improvements and maintainability.

SPF sounds like a good idea but it is a really big and intrusive surgery
to the #PF path. And more importantly without any real world usecase
numbers which would justify this. That being said I am not opposed to
this change I just think it is a large hammer while we haven't seen
attempts to tackle problems in a simpler way.

Michal Hocko

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