Re: [PATCH 6/7] mm/page_alloc: Give GFP_ATOMIC and non-blocking allocations access to reserves

[Mel Gorman <mgorman@techsingularity.net>]

On Wed, Jan 11, 2023 at 04:58:02PM +0100, Michal Hocko wrote:
> On Mon 09-01-23 15:16:30, Mel Gorman wrote:
> > Explicit GFP_ATOMIC allocations get flagged ALLOC_HARDER which is a bit
> > vague. In preparation for removing __GFP_ATOMIC, give GFP_ATOMIC and
> > other non-blocking allocation requests equal access to reserve.  Rename
> > ALLOC_HARDER to ALLOC_NON_BLOCK to make it more clear what the flag
> > means.
> 
> GFP_NOWAIT can be also used for opportunistic allocations which can and
> should fail quickly if the memory is tight and more elaborate path
> should be taken (e.g. try higher order allocation first but fall back to
> smaller request if the memory is fragmented). Do we really want to give
> those access to memory reserves as well?

Good question. Without __GFP_ATOMIC, GFP_NOWAIT only differs from GFP_ATOMIC
by __GFP_HIGH but that is not enough to distinguish between a caller that
cannot sleep versus one that is speculatively attempting an allocation but
has other options. That changelog is misleading, it's not equal access
as GFP_NOWAIT ends up with 25% of the reserves which is less than what
GFP_ATOMIC gets.

Because it becomes impossible to distinguish between non-blocking and
atomic without __GFP_ATOMIC, there is some justification for allowing
access to reserves for GFP_NOWAIT. bio for example attempts an allocation
(clears __GFP_DIRECT_RECLAIM) before falling back to mempool but delays
in IO can also lead to further allocation pressure. mmu gather failing
GFP_WAIT slows the rate memory can be freed. NFS failing GFP_NOWAIT will
have to retry IOs multiple times. The examples were picked at random but
the point is that there are cases where failing GFP_NOWAIT can degrade
the system, particularly delay the cleaning of pages before reclaim.

A lot of the truly speculative users appear to use GFP_NOWAIT | __GFP_NOWARN
so one compromise would be to avoid using reserves if __GFP_NOWARN is
also specified.

Something like this as a separate patch?

diff --git a/mm/page_alloc.c b/mm/page_alloc.c
index 7244ab522028..0a7a0ac1b46d 100644
--- a/mm/page_alloc.c
+++ b/mm/page_alloc.c
@@ -4860,9 +4860,11 @@ gfp_to_alloc_flags(gfp_t gfp_mask, unsigned int order)
 	if (!(gfp_mask & __GFP_DIRECT_RECLAIM)) {
 		/*
 		 * Not worth trying to allocate harder for __GFP_NOMEMALLOC even
-		 * if it can't schedule.
+		 * if it can't schedule. Similarly, a caller specifying
+		 * __GFP_NOWARN is likely a speculative allocation with a
+		 * graceful recovery path.
 		 */
-		if (!(gfp_mask & __GFP_NOMEMALLOC)) {
+		if (!(gfp_mask & (__GFP_NOMEMALLOC|__GFP_NOWARN))) {
 			alloc_flags |= ALLOC_NON_BLOCK;
 
 			if (order > 0)