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Date: Tue, 7 Feb 2023 14:32:32 +0100
From: Vlastimil Babka <vbabka@...e.cz>
To: Mel Gorman <mgorman@...hsingularity.net>,
Andrew Morton <akpm@...ux-foundation.org>
Cc: Michal Hocko <mhocko@...e.com>, NeilBrown <neilb@...e.de>,
Thierry Reding <thierry.reding@...il.com>,
Matthew Wilcox <willy@...radead.org>,
Linux-MM <linux-mm@...ck.org>,
LKML <linux-kernel@...r.kernel.org>
Subject: Re: [PATCH 5/6] mm/page_alloc: Explicitly define how __GFP_HIGH
non-blocking allocations accesses reserves
On 1/13/23 12:12, Mel Gorman wrote:
> GFP_ATOMIC allocations get flagged ALLOC_HARDER which is a vague
> description. In preparation for the removal of GFP_ATOMIC redefine
^ __GFP_ATOMC
> __GFP_ATOMIC to simply mean non-blocking and renaming ALLOC_HARDER to
> ALLOC_NON_BLOCK accordingly. __GFP_HIGH is required for access to reserves
> but non-blocking is granted more access. For example, GFP_NOWAIT is
> non-blocking but has no special access to reserves. A __GFP_NOFAIL
> blocking allocation is granted access similar to __GFP_HIGH if the
> only alternative is an OOM kill.
>
> Signed-off-by: Mel Gorman <mgorman@...hsingularity.net>
Well just for the lore record (too late for git)
Acked-by: Vlastimil Babka <vbabka@...e.cz>
Nit below:
> ---
> mm/internal.h | 7 +++++--
> mm/page_alloc.c | 44 ++++++++++++++++++++++++--------------------
> 2 files changed, 29 insertions(+), 22 deletions(-)
>
> diff --git a/mm/internal.h b/mm/internal.h
> index 8706d46863df..23a37588073a 100644
> --- a/mm/internal.h
> +++ b/mm/internal.h
> @@ -735,7 +735,10 @@ unsigned int reclaim_clean_pages_from_list(struct zone *zone,
> #define ALLOC_OOM ALLOC_NO_WATERMARKS
> #endif
>
> -#define ALLOC_HARDER 0x10 /* try to alloc harder */
> +#define ALLOC_NON_BLOCK 0x10 /* Caller cannot block. Allow access
> + * to 25% of the min watermark or
> + * 62.5% if __GFP_HIGH is set.
This is now (as of v3) inaccurate (the 25% part), right?
> + */
> #define ALLOC_MIN_RESERVE 0x20 /* __GFP_HIGH set. Allow access to 50%
> * of the min watermark.
> */
> @@ -750,7 +753,7 @@ unsigned int reclaim_clean_pages_from_list(struct zone *zone,
> #define ALLOC_KSWAPD 0x800 /* allow waking of kswapd, __GFP_KSWAPD_RECLAIM set */
>
> /* Flags that allow allocations below the min watermark. */
> -#define ALLOC_RESERVES (ALLOC_HARDER|ALLOC_MIN_RESERVE|ALLOC_HIGHATOMIC|ALLOC_OOM)
> +#define ALLOC_RESERVES (ALLOC_NON_BLOCK|ALLOC_MIN_RESERVE|ALLOC_HIGHATOMIC|ALLOC_OOM)
>
> enum ttu_flags;
> struct tlbflush_unmap_batch;
> diff --git a/mm/page_alloc.c b/mm/page_alloc.c
> index 6f41b84a97ac..b9ae0ba0a2ab 100644
> --- a/mm/page_alloc.c
> +++ b/mm/page_alloc.c
> @@ -3989,18 +3989,19 @@ bool __zone_watermark_ok(struct zone *z, unsigned int order, unsigned long mark,
> * __GFP_HIGH allows access to 50% of the min reserve as well
> * as OOM.
> */
> - if (alloc_flags & ALLOC_MIN_RESERVE)
> + if (alloc_flags & ALLOC_MIN_RESERVE) {
> min -= min / 2;
>
> - /*
> - * 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)
> - min -= min / 4;
> + /*
> + * Non-blocking allocations (e.g. GFP_ATOMIC) can
> + * access more reserves than just __GFP_HIGH. Other
> + * non-blocking allocations requests such as GFP_NOWAIT
> + * or (GFP_KERNEL & ~__GFP_DIRECT_RECLAIM) do not get
> + * access to the min reserve.
> + */
> + if (alloc_flags & ALLOC_NON_BLOCK)
> + min -= min / 4;
> + }
>
> /*
> * OOM victims can try even harder than the normal reserve
> @@ -4851,28 +4852,30 @@ gfp_to_alloc_flags(gfp_t gfp_mask, unsigned int order)
> * The caller may dip into page reserves a bit more if the caller
> * cannot run direct reclaim, or if the caller has realtime scheduling
> * policy or is asking for __GFP_HIGH memory. GFP_ATOMIC requests will
> - * set both ALLOC_HARDER (__GFP_ATOMIC) and ALLOC_MIN_RESERVE(__GFP_HIGH).
> + * set both ALLOC_NON_BLOCK and ALLOC_MIN_RESERVE(__GFP_HIGH).
> */
> alloc_flags |= (__force int)
> (gfp_mask & (__GFP_HIGH | __GFP_KSWAPD_RECLAIM));
>
> - if (gfp_mask & __GFP_ATOMIC) {
> + if (!(gfp_mask & __GFP_DIRECT_RECLAIM)) {
> /*
> * Not worth trying to allocate harder for __GFP_NOMEMALLOC even
> * if it can't schedule.
> */
> if (!(gfp_mask & __GFP_NOMEMALLOC)) {
> - alloc_flags |= ALLOC_HARDER;
> + alloc_flags |= ALLOC_NON_BLOCK;
>
> if (order > 0)
> alloc_flags |= ALLOC_HIGHATOMIC;
> }
>
> /*
> - * Ignore cpuset mems for GFP_ATOMIC rather than fail, see the
> - * comment for __cpuset_node_allowed().
> + * Ignore cpuset mems for non-blocking __GFP_HIGH (probably
> + * GFP_ATOMIC) rather than fail, see the comment for
> + * __cpuset_node_allowed().
> */
> - alloc_flags &= ~ALLOC_CPUSET;
> + if (alloc_flags & ALLOC_MIN_RESERVE)
> + alloc_flags &= ~ALLOC_CPUSET;
> } else if (unlikely(rt_task(current)) && in_task())
> alloc_flags |= ALLOC_MIN_RESERVE;
>
> @@ -5303,12 +5306,13 @@ __alloc_pages_slowpath(gfp_t gfp_mask, unsigned int order,
> WARN_ON_ONCE_GFP(costly_order, gfp_mask);
>
> /*
> - * Help non-failing allocations by giving them access to memory
> - * reserves but do not use ALLOC_NO_WATERMARKS because this
> + * Help non-failing allocations by giving some access to memory
> + * reserves normally used for high priority non-blocking
> + * allocations but do not use ALLOC_NO_WATERMARKS because this
> * could deplete whole memory reserves which would just make
> - * the situation worse
> + * 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, ac);
> if (page)
> goto got_pg;
>
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