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Date: Wed, 4 Jan 2023 12:02:43 +0000
From: Mel Gorman <mgorman@...hsingularity.net>
To: Vlastimil Babka <vbabka@...e.cz>
Cc: Linux-MM <linux-mm@...ck.org>,
Andrew Morton <akpm@...ux-foundation.org>,
Michal Hocko <mhocko@...e.com>, NeilBrown <neilb@...e.de>,
Thierry Reding <thierry.reding@...il.com>,
Matthew Wilcox <willy@...radead.org>,
LKML <linux-kernel@...r.kernel.org>
Subject: Re: [PATCH 4/6] mm/page_alloc: Explicitly define what alloc flags
deplete min reserves
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@...hsingularity.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;
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