On Tue, 17 May 2011, Mel Gorman wrote:
> > > diff --git a/mm/page_alloc.c b/mm/page_alloc.c
> > > index 9f8a97b..057f1e2 100644
> > > --- a/mm/page_alloc.c
> > > +++ b/mm/page_alloc.c
> > > @@ -1972,6 +1972,7 @@ gfp_to_alloc_flags(gfp_t gfp_mask)
> > > {
> > > int alloc_flags = ALLOC_WMARK_MIN | ALLOC_CPUSET;
> > > const gfp_t wait = gfp_mask & __GFP_WAIT;
> > > + const gfp_t can_wake_kswapd = !(gfp_mask & __GFP_NO_KSWAPD);
> > >
> > > /* __GFP_HIGH is assumed to be the same as ALLOC_HIGH to save a branch. */
> > > BUILD_BUG_ON(__GFP_HIGH != (__force gfp_t) ALLOC_HIGH);
> > > @@ -1984,7 +1985,7 @@ gfp_to_alloc_flags(gfp_t gfp_mask)
> > > */
> > > alloc_flags |= (__force int) (gfp_mask & __GFP_HIGH);
> > >
> > > - if (!wait) {
> > > + if (!wait && can_wake_kswapd) {
> > > /*
> > > * Not worth trying to allocate harder for
> > > * __GFP_NOMEMALLOC even if it can't schedule.
> > > diff --git a/mm/slub.c b/mm/slub.c
> > > index 98c358d..c5797ab 100644
> > > --- a/mm/slub.c
> > > +++ b/mm/slub.c
> > > @@ -1170,7 +1170,8 @@ static struct page *allocate_slab(struct kmem_cache *s, gfp_t flags, int node)
> > > * Let the initial higher-order allocation fail under memory pressure
> > > * so we fall-back to the minimum order allocation.
> > > */
> > > - alloc_gfp = (flags | __GFP_NOWARN | __GFP_NORETRY | __GFP_NO_KSWAPD) & ~__GFP_NOFAIL;
> > > + alloc_gfp = (flags | __GFP_NOWARN | __GFP_NO_KSWAPD) &
> > > + ~(__GFP_NOFAIL | __GFP_WAIT | __GFP_REPEAT);
> > >
> > > page = alloc_slab_page(alloc_gfp, node, oo);
> > > if (unlikely(!page)) {
> >
> > It's unnecessary to clear __GFP_REPEAT, these !__GFP_NOFAIL allocations
> > will immediately fail.
> >
>
> We can enter enter direct compaction or direct reclaim
> at least once. If compaction is enabled and we enter
> reclaim/compaction, the presense of __GFP_REPEAT makes a difference
> in should_continue_reclaim(). With compaction disabled, the presense
> of the flag is relevant in should_alloc_retry() with it being possible
> to loop in the allocator instead of failing the SLUB allocation and
> dropping back.
>
You've cleared __GFP_WAIT, so it cannot enter direct compaction or direct
reclaim, so clearing __GFP_REPEAT here doesn't actually do anything.
That's why I suggested adding a comment about why you're clearing
__GFP_WAIT: to make it obvious that these allocations will immediately
fail if the alloc is unsuccessful and we don't need to add __GFP_NORETRY
or remove __GFP_REPEAT.
> Maybe you meant !__GFP_WAIT instead of !__GFP_NOFAIL which makes
> more sense.
No, I meant !__GFP_NOFAIL since the high priority allocations (if
PF_MEMALLOC or TIF_MEMDIE) will not loop forever looking for a page
without that bit. That allows this !__GFP_WAIT allocation to immediately
fail. __GFP_NORETRY and __GFP_REPEAT are no-ops unless you have
__GFP_WAIT.
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