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t=1789557878; bh=9yMRewUVdtk5FTKiyrOjdk+WbBNSVZ/plHkzP6P2SuQ=; h=Date:Subject:To:Cc:References:From:In-Reply-To; b=lSY66ISb3m9U6walw2ED+kaCgM2gABW2AgoPJjdeVg7dafwfXiiS2VY9yNMLoovS0 /HCNS6C7dbRdwLV1mezft/EYCTQWtw6Xi2W0ZCbP26gE+1GrfcGaLMTA4/DJvg/n38 X/L2iOHwfiwTNOWhnDMCMyh1rNq2RpYqDKDcErBTvZki/edYI8yFEwlyKCn31B4x/k j6xN5+ZIjaHBC0ob6CJz6pWn0+FlGblBUjDWi3ulUKl7e5lIjuwWR2baLBBAH/99MV CbshddnS3zF9W6QG8Nps7bkVMotFH0RLER0n6+Xxw/54ngx0wKejp3dXy08deTuSYr gN0Oz+amFoBEQ== Message-ID: <8d6a8a63-4adc-458a-b548-a47bf5ff8eb7@kernel.org> Date: Wed, 16 Sep 2026 13:24:31 +0200 MIME-Version: 1.0 User-Agent: Mozilla Thunderbird Subject: Re: [PATCH v4] mm/page_alloc: avoid direct compaction for costly __GFP_NORETRY allocations Content-Language: en-US To: Johannes Weiner , Salvatore Dipietro Cc: 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 References: <20260905174239.99e31515fabe220aa7d8e6fa@linux-foundation.org> <20260910114602.926944-1-dipiets@amazon.it> From: "Vlastimil Babka (SUSE)" Autocrypt: addr=vbabka@kernel.org; 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charset=UTF-8 Content-Transfer-Encoding: 7bit X-Rspamd-Server: rspam12 X-Rspamd-Queue-Id: EC31D40008 X-Rspam-User: X-Stat-Signature: 5xpbs3w3s68wn1mwjqoc5amdkhgd3exz X-HE-Tag: 1789557879-121011 X-HE-Meta: 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 BUhOgS19 AVE3sApzO8NCV6BJVYlVGfl11oROoFOcSWaqn3yfE5rdfAiE+aDnF2EWuPz/DL4ZzzHIqs/lzPKIcAZL5Rx0W6LpOpf6iKVcTGutcYDdx/PKxoeIgXf4IjAznxjcvOzzgw/iY8OYkzYWbg2B9551TNmVCH824P4H/DWcJo5iyE5EGvvqJm3v67mYt7iB4gFLQlSTTImHmRY+XapxfCTV2K7KF1YvVhbFnfwBp/UXcbNBUjd8GDi8fW0GIWmPKeTcrpTnOg3KdIISR0a7CcdNt7Mf7f4dp0kWjU9FMWUEWfeNn3Dz4N9ffm117BZuMVa+0JPGcz2B8wTQc2KPcouCHD4bqsAFsut8QusjE10I37yorqhn1vq5Xs8WtIEjlRuCUK/B3mbUazWlnIMFvmHFUfp6jSR30ulYWre26M7bx+7For2kJxkp3Tz/tLwr1m9bP5u4m6m+RTnmB8ltJpMefKDe3P5tz3En7AVO50ylT5yeatUjFv3b8Fp//Ww== Sender: owner-linux-mm@kvack.org Precedence: bulk X-Loop: owner-majordomo@kvack.org List-ID: List-Subscribe: List-Unsubscribe: On 9/11/26 17:59, Johannes Weiner wrote: > On Thu, Sep 10, 2026 at 11:46:02AM +0000, Salvatore Dipietro wrote: >> >> On Sat, 05 Sep 2026 17:42:39 -0700 Andrew Morton wrote: >> >> > Is there anything particularly unusual about this test case? >> >> It is a stock pgbench simple-update PostgreSQL workload on a large >> instance (96 vCPUs), using standard PostgreSQL settings and with no huge >> pages assigned to the database. We deliberately overprovision the >> pgbench clients: 1024 clients over 96 threads. That keeps enough writers >> in the buffered write path concurrently to hit the costly-order >> allocation failure path continuously. The memory fragmentation comes from >> page tables: PostgreSQL spawns a new process per client, and those page >> tables consume ~40% of memory, which significantly limits the page cache >> and the free memory available. >> >> >> > > Results (average of 3 runs, TPS): >> > > >> > > Config Avg TPS % vs Baseline >> > > baseline (no patch) 59,408 - >> > > With this patch 155,409 +161.6% >> > >> > Is this back to pre-5d8edfb900d5 performance? >> >> Yes - fully recovered. Here is the summary, same host and workload >> throughout, average of 3 runs: >> >> Config Avg TPS % vs baseline >> AL2023 stock 6.1 kernel 136,942 n/a >> v7.3-rc1 baseline (no patch) 59,408 - >> v7.3-rc1, 5d8edfb900d5 behaviour reverted 151,184 +154.5% >> v7.3-rc1 + v4 155,409 +161.6% >> >> The 6.1 row predates 5d8edfb900d5 entirely. A different kernel version, >> so not directly comparable, but it shows the performance this workload >> used to get on this host. >> >> A literal "git revert 5d8edfb900d5" does not apply to v7.3-rc1 - >> iomap_write_iter() has been rewritten since - so the reverted row is a >> one-line behavioural revert, forcing the write loop back to copying at >> most PAGE_SIZE per iteration: >> >> - size_t chunk = mapping_max_folio_size(mapping); >> + size_t chunk = PAGE_SIZE; >> >> which is what the pre-5d8edfb900d5 loop computed. That makes >> iomap_get_folio() pass no order hint, so the path issues only order-0 >> allocations. >> >> >> > AI review asked a few serious-looking questions: >> > https://sashiko.dev/#/patchset/20260904115629.3993331-1-dipiets@amazon.it >> >> Thanks for pointing that out. To address them, we can have something >> like the patch below. Performance results are still similar to v4. Happy >> to submit a formal v5 patch with it if you would like. > > The feedback looks misleading to me. Agreed. > ALLOC_NON_BLOCKING itself is fine, it usually only means something in > conjunction with other flags, like __GFP_HIGH. +1 > 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? > The rt issue seems made up too. We're talking about costly_order && > __GFP_NORETRY allocations. Why would they need access to reserves? Yep. And the concern about warning is also moot as __GFP_NORETRY is still needed and its users virtually always add __GFP_NOWARN as well. >> Config Avg TPS % vs baseline >> v7.3-rc1 baseline (no patch) 59,408 - >> v7.3-rc1 + v4 155,409 +161.6% >> v7.3-rc1 + proposed patch 161,994 +172.7% >> >> diff --git a/mm/page_alloc.c b/mm/page_alloc.c >> index 12fac9084c48..be8b0d72a2db 100644 >> --- a/mm/page_alloc.c >> +++ b/mm/page_alloc.c >> @@ -4784,10 +4784,22 @@ static inline struct page * >> __alloc_pages_slowpath(gfp_t gfp_mask, unsigned int order, >> struct alloc_context *ac) >> { >> - bool can_direct_reclaim = gfp_mask & __GFP_DIRECT_RECLAIM; >> + const bool costly_order = order > PAGE_ALLOC_COSTLY_ORDER; >> + /* >> + * Costly __GFP_NORETRY callers have a cheap fallback to a lower order, >> + * so don't stall them in direct reclaim or direct compaction. Exempt >> + * __GFP_THISNODE (the THP attempt from alloc_pages_mpol() needs direct >> + * compaction) and __GFP_NOFAIL (must not be made to fail). Don't >> + * clear __GFP_DIRECT_RECLAIM from gfp_mask instead: that would also >> + * change the alloc_flags derived by alloc_flags_slowpath(). >> + */ >> + const bool costly_noretry = costly_order && >> + (gfp_mask & __GFP_NORETRY) && >> + !(gfp_mask & (__GFP_THISNODE | __GFP_NOFAIL)); >> + bool can_direct_reclaim = !costly_noretry && >> + (gfp_mask & __GFP_DIRECT_RECLAIM); > > IMO this is worse because now there is a disconnect between these > flags and the gfp mask again. This sets us up for subtle bugs. > > I much preferred v4. 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.