Messages in this thread | | | Subject | Re: [PATCH 02/30] mm: gfp_to_alloc_flags() | From | Peter Zijlstra <> | Date | Tue, 12 Aug 2008 09:33:05 +0200 |
| |
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.
| |