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Wed, 16 Sep 2026 08:58:32 -0700 (PDT) Date: Wed, 16 Sep 2026 11:58:29 -0400 From: Johannes Weiner To: "Vlastimil Babka (SUSE)" Cc: Salvatore Dipietro , akpm@linux-foundation.org, abuehaze@amazon.com, alisaidi@amazon.com, blakgeof@amazon.com, brauner@kernel.org, brendan.jackman@linux.dev, david@redhat.com, dgc@kernel.org, dipietro.salvatore@gmail.com, djwong@kernel.org, hch@infradead.org, hch@lst.de, linux-fsdevel@vger.kernel.org, linux-kernel@vger.kernel.org, linux-mm@kvack.org, linux-xfs@vger.kernel.org, mhocko@suse.com, ritesh.list@gmail.com, rvvandan@amazon.com, stable@vger.kernel.org, surenb@google.com, willy@infradead.org, ziy@nvidia.com Subject: Re: [PATCH v4] mm/page_alloc: avoid direct compaction for costly __GFP_NORETRY allocations Message-ID: References: <20260905174239.99e31515fabe220aa7d8e6fa@linux-foundation.org> <20260910114602.926944-1-dipiets@amazon.it> <8d6a8a63-4adc-458a-b548-a47bf5ff8eb7@kernel.org> Precedence: bulk X-Mailing-List: linux-fsdevel@vger.kernel.org List-Id: List-Subscribe: List-Unsubscribe: MIME-Version: 1.0 Content-Type: text/plain; charset=us-ascii Content-Disposition: inline In-Reply-To: <8d6a8a63-4adc-458a-b548-a47bf5ff8eb7@kernel.org> On Wed, Sep 16, 2026 at 01:24:31PM +0200, Vlastimil Babka (SUSE) wrote: > On 9/11/26 17:59, Johannes Weiner wrote: > > The access to > > MIGRATE_HIGHATOMIC that it points out is in itself too generous. This > > seems like a real but separate bug. It will allow GFP_TRANSHUGE_LIGHT > > into the highatomic reserves as well, for example. > > I don't follow this part. For ALLOC_HIGHATOMIC you need __GFP_HIGH in > alloc_flags_nonblocking(). So GFP_TRANSHUGE_LIGHT won't get the access, no? rmqueue_buddy() has this: /* * If the allocation fails, allow OOM handling and * order-0 (atomic) allocs access to HIGHATOMIC * reserves as failing now is worse than failing a * high-order atomic allocation in the future. */ if (!page && (alloc_flags & (ALLOC_OOM|ALLOC_NON_BLOCK))) page = __rmqueue_smallest(zone, order, MIGRATE_HIGHATOMIC); It says "atomic", but it's checking only ALLOC_NON_BLOCK, which is broader than GFP_ATOMIC. alloc_flags_nonblocking(); if (gfp_mask & __GFP_DIRECT_RECLAIM) return 0; if (gfp_mask & __GFP_NOMEMALLOC) return 0; alloc_flags |= ALLOC_NON_BLOCK; if (order > 0 && (gfp_mask & __GFP_HIGH)) alloc_flags |= ALLOC_HIGHATOMIC; So this can apply to random !direct_reclaim requests, no? I have to correct myself on GFP_TRANSHUGE_LIGHT because it happens to include __GFP_NOMEMALLOC, and so won't actually get ALLOC_NON_BLOCK. But what about random GFP_NOWAIT and & ~__GFP_DIRECT_RECLAIM sites? Those explicitly don't get watermark exemptions already, and weren't the intent of the rmqueue_buddy() exemption above. Something like this? diff --git a/mm/page_alloc.c b/mm/page_alloc.c index 12fac9084c48..c79cc7aa4dff 100644 --- a/mm/page_alloc.c +++ b/mm/page_alloc.c @@ -3246,7 +3246,8 @@ struct page *rmqueue_buddy(struct zone *preferred_zone, struct zone *zone, * reserves as failing now is worse than failing a * high-order atomic allocation in the future. */ - if (!page && (alloc_flags & (ALLOC_OOM|ALLOC_NON_BLOCK))) + if (!page && ((alloc_flags & ALLOC_OOM) || + ((alloc_flags & ALLOC_MASK_ATOMIC) == ALLOC_MASK_ATOMIC))) page = __rmqueue_smallest(zone, order, MIGRATE_HIGHATOMIC); if (!page) { diff --git a/mm/page_alloc.h b/mm/page_alloc.h index b9259deddb59..11714ddca254 100644 --- a/mm/page_alloc.h +++ b/mm/page_alloc.h @@ -60,6 +60,9 @@ /* Flags that allow allocations below the min watermark. */ #define ALLOC_RESERVES (ALLOC_NON_BLOCK|ALLOC_MIN_RESERVE|ALLOC_HIGHATOMIC|ALLOC_OOM) +/* Flag combination from GFP_ATOMIC */ +#define ALLOC_MASK_ATOMIC (ALLOC_NON_BLOCK|ALLOC_MIN_RESERVE) + /* * Structure for holding the mostly immutable allocation parameters passed * between functions involved in allocations, including the alloc_pages* > Unless I'm mistaken about that MIGRATE_HIGHATOMIC part, it seems all sashiko > concerns can be dismissed and then indeed v4 is the better version. It looks like a real issue to me, but one that already exists independent of Salvatore's change.