Re: [PATCH 02/30] mm: gfp_to_alloc_flags()

Peter Zijlstra
2008-08-12T07:33:07Z

On Tue, 2008-08-12 at 15:01 +1000, Neil Brown wrote:
> On Thursday July 24, a.p.zijlstra@chello.nl wrote:
> > Factor out the gfp to alloc_flags mapping so it can be used in other places.
> >
> > Signed-off-by: Peter Zijlstra <a.p.zijlstra@chello.nl>
> > ---
> > mm/internal.h | 10 +++++
> > mm/page_alloc.c | 95 +++++++++++++++++++++++++++++++-------------------------
> > 2 files changed, 64 insertions(+), 41 deletions(-)
>
> This patch all looks "obviously correct" and a nice factorisation of
> code, except the last little bit:
>
> > @@ -1618,6 +1627,10 @@ nofail_alloc:
> > if (!wait)
> > goto nopage;
> >
> > + /* Avoid recursion of direct reclaim */
> > + if (p->flags & PF_MEMALLOC)
> > + goto nopage;
> > +
> > cond_resched();
> >
> > /* We now go into synchronous reclaim */
> >
> > --
>
> I don't remember seeing it before (though my memory is imperfect) and
> it doesn't seem to fit with the rest of the patch (except spatially).
>
> There is a test above for PF_MEMALLOC which will result in a "goto"
> somewhere else unless "in_interrupt()".
> There is immediately above a test for "!wait".
> So the only way this test can fire is when in_interrupt and wait.
> But if that happens, then the
> might_sleep_if(wait)
> at the top should have thrown a warning... It really shouldn't happen.
>
> So it looks like it is useless code: there is already protection
> against recursion in this case.
>
> Did I miss something?
> If I did, maybe more text in the changelog entry (or the comment)
> would help.

Ok, so the old code did:

if (((p->flags & PF_MEMALLOC) || ...) && !in_interrupt) {
....
goto nopage;
}

which avoid anything that has PF_MEMALLOC set from entering into direct
reclaim, right?

Now, the new code reads:

if (alloc_flags & ALLOC_NO_WATERMARK) {
}

Which might be false, even though we have PF_MEMALLOC set -
__GFP_NOMEMALLOC comes to mind.

So we have to stop that recursion from happening.

so we add:

if (p->flags & PF_MEMALLOC)
goto nopage;

Now, if it were done before the !wait check, we'd have to consider
atomic contexts, but as those are - as you rightly pointed out - handled
by the !wait case, we can plainly do this check.