On Wed, 3 Dec 2014 16:52:22 +0100 Michal Hocko <mhocko@xxxxxxx> wrote:
> On Mon 01-12-14 18:30:40, Johannes Weiner wrote:
> > On Thu, Nov 27, 2014 at 11:25:47AM +0100, Michal Hocko wrote:
> > > On Wed 26-11-14 14:17:32, David Rientjes wrote:
> > > > diff --git a/mm/page_alloc.c b/mm/page_alloc.c
> > > > --- a/mm/page_alloc.c
> > > > +++ b/mm/page_alloc.c
> > > > @@ -2706,7 +2706,7 @@ rebalance:
> > > > * running out of options and have to consider going OOM
> > > > */
> > > > if (!did_some_progress) {
> > > > - if (oom_gfp_allowed(gfp_mask)) {
> > > /*
> > > * Do not attempt to trigger OOM killer for !__GFP_FS
> > > * allocations because it would be premature to kill
> > > * anything just because the reclaim is stuck on
> > > * dirty/writeback pages.
> > > * __GFP_NORETRY allocations might fail and so the OOM
> > > * would be more harmful than useful.
> > > */
> >
> > I don't think we need to explain the individual flags, but it would
> > indeed be useful to remark here that we shouldn't OOM kill from
> > allocations contexts with (severely) limited reclaim abilities.
>
> Is __GFP_NORETRY really related to limited reclaim abilities? I thought
> it was merely a way to tell the allocator to fail rather than spend too
> much time reclaiming.
That's my understanding of __GFP_NORETRY. However it seems that I
really didn't have a plan:
: commit 75908778d91e92ca3c9ed587c4550866f4c903fc
: Author: Andrew Morton <akpm@xxxxxxxxx>
: Date: Sun Apr 20 00:28:12 2003 -0700
:
: [PATCH] implement __GFP_REPEAT, __GFP_NOFAIL, __GFP_NORETRY
:
: This is a cleanup patch.
:
: There are quite a lot of places in the kernel which will infinitely retry a
: memory allocation.
:
: Generally, they get it wrong. Some do yield(), the semantics of which have
: changed over time. Some do schedule(), which can lock up if the caller is
: SCHED_FIFO/RR. Some do schedule_timeout(), etc.
:
: And often it is unnecessary, because the page allocator will do the retry
: internally anyway. But we cannot rely on that - this behaviour may change
: (-aa and -rmap kernels do not do this, for instance).
:
: So it is good to formalise and to centralise this operation. If an
: allocation specifies __GFP_REPEAT then the page allocator must infinitely
: retry the allocation.
:
: The semantics of __GFP_REPEAT are "try harder". The allocation _may_ fail
: (the 2.4 -aa and -rmap VM's do not retry infinitely by default).
:
: The semantics of __GFP_NOFAIL are "cannot fail". It is a no-op in this VM,
: but needs to be honoured (or fix up the callers) if the VM ischanged to not
: retry infinitely by default.
:
: The semantics of __GFP_NOREPEAT are "try once, don't loop". This isn't used
: at present (although perhaps it should be, in swapoff). It is mainly for
: completeness.
(that's a braino in the changelog: it should be
s/__GFP_NOREPEAT/__GFP_NORETRY/)
> If you are referring to __GFP_FS part then I have
> no objections to be less specific, of course, but __GFP_IO would fall
> into the same category but we are not checking for it. I have no idea
> why we consider the first and not the later one, to be honest...
(__GFP_FS && !__GFP_IO) doesn't make much sense and probably doesn't
happen. "__GFP_FS implies __GFP_IO" is OK.
Anyway, yes, This particular piece of __alloc_pages_slowpath() sorely
needs documenting please. Once we manage to work out why we're doing
what we're doing!
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