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b=fAWWQW+Y+CK3KEpZnau1N5gWdiMD5xMnDis+H57BJy3MqgvUHH0XsZQk4NWb8zNNk f3SDtdDFKtuMQ5MA0WwSbrS6KAn2N74/Z13FOIFHz5d3nF7ShGg9HI/QvHTWv70K7c KPT+cvyOZ9HYFuABx/SF3GmfHk/ciVcwh9LPb9joqlpBoW2UegkoDDKLO95vgbr3ll FN3hfCKgVv7ucxPdDlyuKuuKPMafYDGmB6Wmlwelp5VgqMqKAgaMF0kEQJHKsAVv2n oFXv3CqbdOGk9GJ52+JsBnl6NedYm4nQUAVFUzLCjLl8P8fTDQUvy+lNvRLn5S8HMY JM/hjbJV4adBA== Message-ID: <54f42979-fc19-44ff-bd11-cb43f13bcd32@kernel.org> Date: Thu, 24 Sep 2026 16:34:04 +0200 MIME-Version: 1.0 User-Agent: Mozilla Thunderbird Subject: Re: [PATCH 1/2] mm: page_alloc: do not give all non-blocking requests reserve access Content-Language: en-US To: Johannes Weiner Cc: Matthew Wilcox , Matt Fleming , 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, ziy@nvidia.com References: <20260910114602.926944-1-dipiets@amazon.it> <8d6a8a63-4adc-458a-b548-a47bf5ff8eb7@kernel.org> <9f415dc7-adad-4161-b20d-7c3173f50ff3@kernel.org> From: "Vlastimil Babka (SUSE)" Autocrypt: addr=vbabka@kernel.org; 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charset=UTF-8 Content-Transfer-Encoding: 7bit X-Rspamd-Server: rspam05 X-Rspamd-Queue-Id: 6F922A0007 X-Stat-Signature: q543gwiisr4q899pd5udxfhybensnhen X-Rspam-User: X-HE-Tag: 1790260452-826314 X-HE-Meta: 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 MWFkkOVN ejdviwMiQ2zxVwaXOjXdJbtEa1znspcGfxN8Z8yq5m+gjxVXNO8k6VxqwCzGSXh5spzWGyUoNfAaqgDSH3YaTSlTASSw9rnuEQkAbG+vfsU3IFcwQPRbllFzxnD29MTvmAjJMJXdpL9+ETdOKzmxhUd6GVQFQRvl61BV4yQYe2HAvRTf44ikC66zpW69upZqFbxPNZBG498CpS5yHgUEF3L2QgtOhpPkxJ2KVDmaCfDqyQJpeR8TCq0nMveWJL51PT3FpcrgH0s6Si56RJJviT9zRhQ4Bp4Pz1xqZNLi6MbrUVz+hyh8z4q2i5JRTeMo1xDh4lUMc5LfjFYKbompq4GFwXaUScrs+SjubcfnThlS1QiS6TX9zrkc4b7e4GP5K47vM Sender: owner-linux-mm@kvack.org Precedence: bulk X-Loop: owner-majordomo@kvack.org List-ID: List-Subscribe: List-Unsubscribe: On 9/22/26 15:56, Johannes Weiner wrote: > On Tue, Sep 22, 2026 at 01:52:41PM +0200, Vlastimil Babka (SUSE) wrote: >> >> >> On 9/21/26 5:58 PM, Johannes Weiner wrote: >> > On Mon, Sep 21, 2026 at 03:54:32PM +0100, Matthew Wilcox wrote: >> >> On Mon, Sep 21, 2026 at 10:38:18AM -0400, Johannes Weiner wrote: >> >>> +++ b/mm/page_alloc.c >> >>> @@ -3246,7 +3246,9 @@ 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); >> >> >> >> Would this be slightly neater? >> >> >> >> static inline bool may_access_reserves(unsigned int alloc_flags) >> >> { >> >> if (alloc_flags & ALLOC_OOM) >> >> return true; >> >> if (alloc_flags & (ALLOC_NON_BLOCK | ALLOC_MIN_RESERVE)) == >> >> (ALLOC_NON_BLOCK | ALLOC_MIN_RESERVE) >> >> return true; >> >> return false; >> >> } >> >> I think it should be named e.g. may_access_highatomic_reserves() as >> may_access_reserves() is rather generic and would seem to imply an >> ALLOC_RESERVES match (see 2/2). > > Note that it doesn't actually check ALLOC_HIGHATOMIC itself. It's just > the hail-mary AFTER trying the primary migratetype. So the name still > doesn't look right, and rmqueue_buddy() reads kind of awkardly: > > if (alloc_flags & ALLOC_HIGHATOMIC) > page = __rmqueue_smallest(..., MIGRATE_HIGHATOMIC); > if (!page) { > page = __rmqueue(..., migratetype, ...); > > /* Allow OOM and order-0 atomic */ > if (!page && may_access_highatomic_reserve()) > page = __rmqueue_smallest(..., MIGRATE_HIGHATOMIC); > } > > It would have to be may_access_highatomic_reserve_as_last_resort() or > something? > > Hm, this is a mess. Looking closer, I think there is more breakage, > name aside. > > You suggest in 2/2 to use the same helper in unusable_free. But I > think that's broken. My 2/2 does not look like the full fix either: > > The fundamental problem is including the highatomics reserves in the > watermark check for any allocations that first prefer a different > migratetype - "regular memory". Any time we do this, we allow those > allocations to draw down regular memory to 0 based on the presence of > the highatomic reserves. And when reclaim, swap etc. come along there > is nothing left for them. ALLOC_NO_WATERMARKS e.g. permits ignoring > the wmarks but doesn't grant access to highatomic *freelists*. Hmm yes, we'd have to be nuanced about which freelists we allow allocating from depending on their particular watermarks. I.e. if we passed the watermark checks only thanks to zone->nr_free_highatomic, we would have to try those first. I assume it would be complicated and perhaps with perf impact. But maybe if we managed to keep this complexity outside of fastpath attempts? The CMA handling had a similar issue and now we have that balancing heuristic since 16867664936e ("mm,page_alloc,cma: conditionally prefer cma pageblocks for movable allocations") > So I think there are two choices: > > (1) Let *everything* with some sort of reserve access fall back to > highatomic, or > > (2) Only allow ALLOC_HIGHATOMIC to include highatomic reserves in > their watermarks check. With limited opportunistic fallback to the > highatomics *freelists*, like the order-0 atomics above. > > My intuition is that (1) might weaken the highatomic reserves to the > point of uselessness, and we should probably go with (2): I think the goal of commit 281dd25c1a01 ("mm/page_alloc: let GFP_ATOMIC order-0 allocs access highatomic reserves") would be defeated with (2). The order-0 atomic fallback to MIGRATE_HIGHATOMIC freelists would be there, but the watermark check would never let such allocation through anymore, if there wasn't a free page on the regular freelists? So it would become (some races aside maybe) a dead code? I guess your current patches are a compromise that can work, as patch 2/2 improves the current worse-than-(1) state to exclude the plain non-block allocations. > watermarks: > if (alloc_flags & ALLOC_HIGHATOMIC) > unusable_free += zone->nr_free_highatomic > > freelists: > if (alloc_flags & ALLOC_HIGHATOMIC) > page = __rmqueue_smallest(..., MIGRATE_HIGHATOMIC); > if (!page) { > page = __rmqueue(..., migratetype, ...); > /* Opportunistic fallback for order-0 atomics */ > if (!page && opportunistic_highatomic_fallback()) > page = __rmqueue_smallest(..., MIGRATE_HIGHATOMIC); > } > >> > I tend to be hesitant with single-use abstractions, but no objection >> > if people think this is better. >> >> True but single-use ALLOC_MASK_ATOMIC is also not that great, and the >> usage makes the code hard to decipher. And see my reply to 2/2. > > There is one small upside, which is that it pairs with the > ALLOC_HIGHATOMIC check that precedes it. The comment says "order-0 > atomics" get a hail mary, but there is no order check. That order-0 > comes out of the sequence of events here: we first check highatomic, > which is order > 0 && atomic. If that, and the native type, fail, we > do the hail mary for atomic, which must be by definition order-0. > > If you abstract that privilege into a generic "can access highatomic > reserves" without an order check, it tempts refactors that cause bugs > like the above. Ack.