Re: [PATCH 4/6] mm/page_alloc: Explicitly define what alloc flags deplete min reserves

[Mel Gorman <mgorman@techsingularity.net>]

On Thu, Dec 08, 2022 at 06:55:00PM +0100, Vlastimil Babka wrote:
> On 11/29/22 16:16, Mel Gorman wrote:
> > As there are more ALLOC_ flags that affect reserves, define what flags
> > affect reserves and clarify the effect of each flag.
> 
> Seems to me this does more than a clarification, but also some functional
> tweaks, so it could be helpful if those were spelled out in the changelog.
> 

I will to take out the problematic parts that need clarification. There
are two, one I'll drop and the other will be split. More details below.

> > @@ -3976,25 +3975,36 @@ bool __zone_watermark_ok(struct zone *z, unsigned int order, unsigned long mark,
> >  {
> >  	long min = mark;
> >  	int o;
> > -	const bool alloc_harder = (alloc_flags & (ALLOC_HARDER|ALLOC_OOM));
> >  
> >  	/* free_pages may go negative - that's OK */
> >  	free_pages -= __zone_watermark_unusable_free(z, order, alloc_flags);
> >  
> > -	if (alloc_flags & ALLOC_MIN_RESERVE)
> > -		min -= min / 2;
> > +	if (alloc_flags & ALLOC_RESERVES) {
> 
> Do we want to keep this unlikely() as alloc_harder did before?
> 

Added back in.

> > +		/*
> > +		 * __GFP_HIGH allows access to 50% of the min reserve as well
> > +		 * as OOM.
> > +		 */
> > +		if (alloc_flags & ALLOC_MIN_RESERVE)
> > +			min -= min / 2;
> >  
> > -	if (unlikely(alloc_harder)) {
> >  		/*
> > -		 * OOM victims can try even harder than normal ALLOC_HARDER
> > +		 * Non-blocking allocations can access some of the reserve
> > +		 * with more access if also __GFP_HIGH. The reasoning is that
> > +		 * a non-blocking caller may incur a more severe penalty
> > +		 * if it cannot get memory quickly, particularly if it's
> > +		 * also __GFP_HIGH.
> > +		 */
> > +		if (alloc_flags & (ALLOC_HARDER|ALLOC_HIGHATOMIC))
> > +			min -= min / 4;
> 
> For example this seems to change the allowed dip to reserves for
> ALLOC_HIGHATOMIC.
> 

You're right and this could cause problems. If high-order atomic allocation
failures start appearing again, this change would help but it should be
a standalone patch in response to a bug. I'll drop it for now.

> > +
> > +		/*
> > +		 * OOM victims can try even harder than the normal reserve
> >  		 * users on the grounds that it's definitely going to be in
> >  		 * the exit path shortly and free memory. Any allocation it
> >  		 * makes during the free path will be small and short-lived.
> >  		 */
> >  		if (alloc_flags & ALLOC_OOM)
> >  			min -= min / 2;
> > -		else
> > -			min -= min / 4;
> >  	}
> 
> (noted that this patch doesn't seem to change the concern I raised in
> previous patch)
> 

This might be addressed now with the chjanges to the patch that caused
you concerns about OOM handling.

> >  	/*
> > @@ -5293,7 +5303,7 @@ __alloc_pages_slowpath(gfp_t gfp_mask, unsigned int order,
> >  		 * could deplete whole memory reserves which would just make
> >  		 * the situation worse
> >  		 */
> > -		page = __alloc_pages_cpuset_fallback(gfp_mask, order, ALLOC_HARDER, ac);
> > +		page = __alloc_pages_cpuset_fallback(gfp_mask, order, ALLOC_MIN_RESERVE|ALLOC_HARDER, ac);
> 
> And this AFAICS seems to give __GFP_NOFAIL 3/4 of min reserves instead of
> 1/4, which seems like a significant change (but hopefully ok) so worth
> noting at least.
> 

It deserves a standalone patch. Below is the diff I intend to apply to
this patch and the standalone patch.

diff --git a/mm/page_alloc.c b/mm/page_alloc.c
index 58e01a31492e..6f41b84a97ac 100644
--- a/mm/page_alloc.c
+++ b/mm/page_alloc.c
@@ -3984,7 +3984,7 @@ bool __zone_watermark_ok(struct zone *z, unsigned int order, unsigned long mark,
 	/* free_pages may go negative - that's OK */
 	free_pages -= __zone_watermark_unusable_free(z, order, alloc_flags);
 
-	if (alloc_flags & ALLOC_RESERVES) {
+	if (unlikely(alloc_flags & ALLOC_RESERVES)) {
 		/*
 		 * __GFP_HIGH allows access to 50% of the min reserve as well
 		 * as OOM.
@@ -3999,7 +3999,7 @@ bool __zone_watermark_ok(struct zone *z, unsigned int order, unsigned long mark,
 		 * if it cannot get memory quickly, particularly if it's
 		 * also __GFP_HIGH.
 		 */
-		if (alloc_flags & (ALLOC_HARDER|ALLOC_HIGHATOMIC))
+		if (alloc_flags & ALLOC_HARDER)
 			min -= min / 4;
 
 		/*
@@ -5308,7 +5308,7 @@ __alloc_pages_slowpath(gfp_t gfp_mask, unsigned int order,
 		 * could deplete whole memory reserves which would just make
 		 * the situation worse
 		 */
-		page = __alloc_pages_cpuset_fallback(gfp_mask, order, ALLOC_MIN_RESERVE|ALLOC_HARDER, ac);
+		page = __alloc_pages_cpuset_fallback(gfp_mask, order, ALLOC_HARDER, ac);
 		if (page)
 			goto got_pg;
 
The patch to allow __GFP_NOFAIL deeper access is this

--8<--
mm/page_alloc.c: Allow __GFP_NOFAIL requests deeper access to reserves

Currently __GFP_NOFAIL allocations without any other flags can access 25%
of the reserves but these requests imply that the system cannot make forward
progress until the allocation succeeds. Allow __GFP_NOFAIL access to 75%
of the min reserve.

Signed-off-by: Mel Gorman <mgorman@techsingularity.net>
---
 mm/page_alloc.c | 2 +-
 1 file changed, 1 insertion(+), 1 deletion(-)

diff --git a/mm/page_alloc.c b/mm/page_alloc.c
index 6f41b84a97ac..d2df78f5baa2 100644
--- a/mm/page_alloc.c
+++ b/mm/page_alloc.c
@@ -5308,7 +5308,7 @@ __alloc_pages_slowpath(gfp_t gfp_mask, unsigned int order,
 		 * could deplete whole memory reserves which would just make
 		 * the situation worse
 		 */
-		page = __alloc_pages_cpuset_fallback(gfp_mask, order, ALLOC_HARDER, ac);
+		page = __alloc_pages_cpuset_fallback(gfp_mask, order, ALLOC_MIN_RESERVE|ALLOC_HARDER, ac);
 		if (page)
 			goto got_pg;