Re: [PATCH v3 2/4] mm: failfast mode with __GFP_NORETRY in alloc_contig_range

[Minchan Kim <minchan@kernel.org>]

On Wed, Jan 13, 2021 at 09:39:26AM +0100, David Hildenbrand wrote:
> On 13.01.21 02:21, Minchan Kim wrote:
> > Contiguous memory allocation can be stalled due to waiting
> > on page writeback and/or page lock which causes unpredictable
> > delay. It's a unavoidable cost for the requestor to get *big*
> > contiguous memory but it's expensive for *small* contiguous
> > memory(e.g., order-4) because caller could retry the request
> > in diffrent range where would have easy migratable pages
> > without stalling.
> 
> s/diffrent/different/
> 
> > 
> > This patch introduce __GFP_NORETRY as compaction gfp_mask in
> > alloc_contig_range so it will fail fast without blocking
> > when it encounters pages needed waitting.
> 
> s/waitting/waiting/

Fxed both. Thanks.
Let me resend once I get some review.

Michal, I appreciate if you could give an review before
next revision.

Thanks!

> 
> > 
> > Signed-off-by: Minchan Kim <minchan@kernel.org>
> > ---
> >  mm/page_alloc.c | 8 ++++++--
> >  1 file changed, 6 insertions(+), 2 deletions(-)
> > 
> > diff --git a/mm/page_alloc.c b/mm/page_alloc.c
> > index 5b3923db9158..ff41ceb4db51 100644
> > --- a/mm/page_alloc.c
> > +++ b/mm/page_alloc.c
> > @@ -8489,12 +8489,16 @@ static int __alloc_contig_migrate_range(struct compact_control *cc,
> >  	unsigned int nr_reclaimed;
> >  	unsigned long pfn = start;
> >  	unsigned int tries = 0;
> > +	unsigned int max_tries = 5;
> >  	int ret = 0;
> >  	struct migration_target_control mtc = {
> >  		.nid = zone_to_nid(cc->zone),
> >  		.gfp_mask = GFP_USER | __GFP_MOVABLE | __GFP_RETRY_MAYFAIL,
> >  	};
> >  
> > +	if (cc->alloc_contig && cc->mode == MIGRATE_ASYNC)
> > +		max_tries = 1;
> > +
> >  	migrate_prep();
> >  
> >  	while (pfn < end || !list_empty(&cc->migratepages)) {
> > @@ -8511,7 +8515,7 @@ static int __alloc_contig_migrate_range(struct compact_control *cc,
> >  				break;
> >  			}
> >  			tries = 0;
> > -		} else if (++tries == 5) {
> > +		} else if (++tries == max_tries) {
> >  			ret = ret < 0 ? ret : -EBUSY;
> >  			break;
> >  		}
> > @@ -8562,7 +8566,7 @@ int alloc_contig_range(unsigned long start, unsigned long end,
> >  		.nr_migratepages = 0,
> >  		.order = -1,
> >  		.zone = page_zone(pfn_to_page(start)),
> > -		.mode = MIGRATE_SYNC,
> > +		.mode = gfp_mask & __GFP_NORETRY ? MIGRATE_ASYNC : MIGRATE_SYNC,
> >  		.ignore_skip_hint = true,
> >  		.no_set_skip_hint = true,
> >  		.gfp_mask = current_gfp_context(gfp_mask),
> > 
> 
> I'm fine with using gfp flags (e.g., __GFP_NORETRY) as long as they
> don't enable other implicit behavior (e.g., move draining X to the
> caller) that's hard to get from the flag name.
> 
> IMHO, if we ever want to move draining to the caller, or change the
> behavior of alloc_contig_range() in different ways (e.g., disable PCP),
> we won't get around introducing a separate set of flags for
> alloc_contig_range().
> 
> Let's see what Michal thinks. Thanks!
> 
> -- 
> Thanks,
> 
> David / dhildenb
>