Re: [PATCH 13/12] ksm: fix munlock during exit_mmap deadlock

From: Andrea Arcangeli
Date: Wed Aug 26 2009 - 15:45:02 EST

On Wed, Aug 26, 2009 at 08:17:50PM +0100, Hugh Dickins wrote:
> Has anyone looked at why Rawhide's mlockall is not faulting in the
> pages, I wonder if there's a separate bug there?)

I reproduced only with self compiled mmotm kernel with full self
compiled userland with just a mlockall and exit (not rawhide
kernel/userland) so there's certainly no bug in rawhide, or at least
nothing special about it.

> No, not while it's down inside page allocation.

There's a slight difference if it's ksm inside page allocation and not
the task itself for other reasons. See the TIF_MEMDIE check in
page_alloc.c, those won't trigger when it's ksm causing a page
fault. So that's the problem left to tackle to make oom killer fully
happy with KSK unshare.

> But you don't like that approach at all, hmm. It sounds like we'll
> have a fight if I try either that or to reintroduce the ksm_test_exits

;) Well I'd rather have a more unfixable issue if we have to
reintroduce the mm_users check the in page faults.

All is left to address is to teach page_alloc.c that the mm is going
away in a second patch. That might also help when it's aio triggering
gup page allocations or other kernel threads with use_mm just like ksm
and the oom killer selected those "mm" for release.

Having ksm using use_mm before triggering the handle_mm_fault (so
tsk->mm points to the mm of the task) and adding a MMF_MEMDIE to
mm->flags checked by page_alloc would work just fine and should solve
the double task killed... but then I'm unsure.. this is just the first
idea I had.

> in memory.c, once the munlock faulting is eliminated. Well, I'll give
> it more thought: your patch is a lot better than the status quo,
> and should go in for now - thanks.

Ok, agreed!
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