Re: [PATCH] mm: make munlock fast when mlock is canceled by sigkill

From: KOSAKI Motohiro
Date: Tue Aug 25 2009 - 22:33:34 EST

> > > Yeah, GUP_FLAGS_NOFAULT is better.
> >
> > Me too.
> > I will change this flag name.
> >...
> > When I try to change __get_user_pages(), I got problem.
> > If remove NULLs from pages,
> > __mlock_vma_pages_range() cannot know how long __get_user_pages() readed.
> > So, I have to get the virtual address of the page from vma and page.
> > Because __mlock_vma_pages_range() have to call
> > __get_user_pages() many times with different `start' argument.
> >
> > I try to use page_address_in_vma(), but it failed.
> > (page_address_in_vma() returned -EFAULT)
> > I cannot find way to solve this problem.
> > Are there good ideas?
> > Please give me some ideas.
> I agree that this munlock issue needs to be addressed: it's not just a
> matter of speedup, I hit it when testing what happens when mlock takes
> you to OOM - which is currently a hanging disaster because munlock'ing
> in the exiting OOM-killed process gets stuck trying to fault in all
> those pages that couldn't be locked in the first place.

I agree too.

> I had intended to fix it by being more careful about splitting/merging
> vmas, noting how far the mlock had got, and munlocking just up to there.
> However, now that I've got in there, that looks wrong to me, given the
> traditional behaviour that mlock does its best, but pretends success
> to allow for later instantiation of the pages if necessary.
> You ask for ideas. My main idea is that so far we have added
> GUP_FLAGS_IGNORE_VMA_PERMISSIONS (Kosaki-san, what was that about?
> we already had the force flag),

MAY_WRITE and MAY_READ might be turned off at some special case.
but munlock should turn off PG_mlock bit. otherwise the page never be reclaimed.
This problem was explained by Lee about a year ago.

However, To use follow_page() solove this issue and we will be able to
remove this ugly flag.

> GUP_FLAGS_IGNORE_SIGKILL, and now you propose
> GUP_FLAGS_NOFAULT, all for the sole use of munlock.
> How about GUP_FLAGS_MUNLOCK, or more to the point, GUP_FLAGS_DONT_BE_GUP?
> By which I mean, don't all these added flags suggest that almost
> everything __get_user_pages() does is unsuited to the munlock case?
> My advice (but I sure hate giving advice before I've tried it myself)
> is to put __mlock_vma_pages_range() back to handling just the mlock
> case, and do your own follow_page() loop in munlock_vma_pages_range().

Agreed. follow_page() is better.

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