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smtp.subspace.kernel.org; dkim=pass header.d=kernel.org header.i=@kernel.org header.a=rsa-sha256 header.s=k20260515 header.b=opV9ITIu; arc=none smtp.remote-ip=100.103.45.18 Received: from smtp.kernel.org (quasi.space.kernel.org [100.103.45.18]) (using TLSv1.3 with cipher TLS_AES_256_GCM_SHA384 (256/256 bits) key-exchange X25519 server-signature ECDSA (prime256v1)) (No client certificate requested) by smtp.subspace.kernel.org (Postfix) with ESMTPS id EEB30606A31C; Thu, 29 Oct 2026 17:47:31 +0000 (UTC) Authentication-Results: smtp.subspace.kernel.org; dkim=pass (2048-bit key, unprotected) header.d=kernel.org header.i=@kernel.org header.a=rsa-sha256 header.s=k20260515 header.b=opV9ITIu Received: by smtp.kernel.org (Postfix) with ESMTPSA id 580461F031E2; Thu, 29 Oct 2026 17:47:27 +0000 (UTC) DKIM-Signature: v=1; a=rsa-sha256; c=relaxed/relaxed; d=kernel.org; s=k20260515; t=1793296051; bh=Lu9NmCps7cwWhw9kB4kp6caSi9tSD3jrL00zfdQIbj4=; h=Date:Subject:To:Cc:References:From:In-Reply-To; b=opV9ITIuvYyN325Vo64Ehi4GBE5Kl15I5XlLgwfsb/0L0RXrVFvX7yax9DaCEFBLI tCCZeb5nYdYcbsSzz9BBeAsTi8a7mSSLdGIkLYZiIb3PxC/7BG2wMVPQzsbx3D+6X0 TkJX6LVYQhPAle9xIRIBYAa8mY9CjwjuKyiGafn1gJicnr6JSeNdmJWpsN4G3IIl73 Gnm83uHOkjSwpNDtqdgHbsVZ7NNCdlk4mTrO+aitlRqzFId//iFxYcEEFl21sDQfSP FYIEFnjenNPCVib+NByyKxd2pIsiczeQ3HHgy+icNG3hcxGQ3Z5+KMBqbR5LaYCclf f+MOnYd8wuLtQ== Message-ID: Date: Thu, 29 Oct 2026 18:47:25 +0100 X-Mailing-List: linux-kernel@vger.kernel.org List-Id: List-Post: List-Help: List-Subscribe: List-Unsubscribe: List-Owner: Precedence: list MIME-Version: 1.0 User-Agent: Mozilla Thunderbird Subject: Re: [PATCH v4 5/8] mm/page_alloc: retry allocations that raced with a zonelist rebuild Content-Language: en-US To: "Gregory Price (Meta)" , linux-mm@kvack.org Cc: linux-kernel@vger.kernel.org, kernel-team@meta.com, muchun.song@linux.dev, osalvador@suse.de, david@kernel.org, akpm@linux-foundation.org, ziy@nvidia.com, joshua.hahnjy@gmail.com, rakie.kim@sk.com, ying.huang@linux.alibaba.com, ljs@kernel.org, liam@infradead.org, rppt@kernel.org, surenb@google.com, mhocko@suse.com, brendan.jackman@linux.dev, hannes@cmpxchg.org, mgorman@techsingularity.net, anshuman.khandual@arm.com, stable@vger.kernel.org References: <20261028211930.3504634-1-gourry@gourry.net> <20261028211930.3504634-6-gourry@gourry.net> From: "Vlastimil Babka (SUSE)" Autocrypt: addr=vbabka@kernel.org; keydata= xsFNBFZdmxYBEADsw/SiUSjB0dM+vSh95UkgcHjzEVBlby/Fg+g42O7LAEkCYXi/vvq31JTB KxRWDHX0R2tgpFDXHnzZcQywawu8eSq0LxzxFNYMvtB7sV1pxYwej2qx9B75qW2plBs+7+YB 87tMFA+u+L4Z5xAzIimfLD5EKC56kJ1CsXlM8S/LHcmdD9Ctkn3trYDNnat0eoAcfPIP2OZ+ 9oe9IF/R28zmh0ifLXyJQQz5ofdj4bPf8ecEW0rhcqHfTD8k4yK0xxt3xW+6Exqp9n9bydiy tcSAw/TahjW6yrA+6JhSBv1v2tIm+itQc073zjSX8OFL51qQVzRFr7H2UQG33lw2QrvHRXqD Ot7ViKam7v0Ho9wEWiQOOZlHItOOXFphWb2yq3nzrKe45oWoSgkxKb97MVsQ+q2SYjJRBBH4 8qKhphADYxkIP6yut/eaj9ImvRUZZRi0DTc8xfnvHGTjKbJzC2xpFcY0DQbZzuwsIZ8OPJCc LM4S7mT25NE5kUTG/TKQCk922vRdGVMoLA7dIQrgXnRXtyT61sg8PG4wcfOnuWf8577aXP1x 6mzw3/jh3F+oSBHb/GcLC7mvWreJifUL2gEdssGfXhGWBo6zLS3qhgtwjay0Jl+kza1lo+Cv BB2T79D4WGdDuVa4eOrQ02TxqGN7G0Biz5ZLRSFzQSQwLn8fbwARAQABzSNWbGFzdGltaWwg QmFia2EgPHZiYWJrYUBrZXJuZWwub3JnPsLBsAQTAQoAWhYhBKlA1DSZLC6OmRA9UCJPp+fM gqZkBQJqFFy6GxSAAAAAAAQADm1hbnUyLDIuNSsxLjEyLDIsMgIbAwUJGtCBUAULCQgHAwUV CgkICwUWAgMBAAIeBQIXgAAKCRAiT6fnzIKmZJIUEADFx/tREzUImHrEwVHeSvDFmA7tJysI UVrlvrM09E7GIuzphzv7jYmo8n3ANpCczLEVr4G0syYQdTigaZgv3+FQDIIzhKih1IHhu1Ei XHlywNWKnQxxQEUNi5Mwx43wQz5XVw9F1A7gtKBKNtfogO511hAbrzagrYajyQacEJ/+sfhZ 9Da8ltHIXD8pcYaHUfQgEusCgmEd9+KrUwrTbckFKmYq5chuE6yJ4J0EmWknL096jIE6CnzF FRslQ3B1UKDjxVsm1ZHfir5NeWszLkTvGFsddFaWTgh8UycESG6VQzKXjjewXu2pG7YQYRpj QKm1W5X2TkwWkXRBZTmfmbhxIUMh3+zf5wQ463rSmDN/8v81tdqBtAW6rH/kzg1GvkaTHXn0 507yEHFzBksk2viAuIxxr7km8+/KARYLIdGtx30EG8cKzAUZOK6WqxtNCsXUJNrVE8CWrCaD icoNu7Fs1c5hmPHdSTnU48ce67449DdnO4neLSNhRiGlMHJgfJUmgrxu/hcYeOZ3haWmEQ2w uW1Mh01OHi8QZHCEyAbABrPs9GUgccc/4eYXX9hIgxfSkYzn8f+8NuIFPWl/0uTvjgqU29FQ SbzOLxHq9439Ox40G5mS5eZXRGxITYR+6TXvRGI6P/264jvflnr/pDGUttaikU+0W+1uxgKH cmYbEc7ATQRbGTU1AQgAn0H6UrFiWcovkh6EXVcl+SeqyO6JHOPm+e9Wu0Vw+VIUvXZVUVVQ La1PQDUi6j00ChlcR66g9/V0sPIcSutacPKfdKYOBvzd4rlhL8rfrdEsQw5ApZxrA8kYZVMh FmBRKAa6wos25moTlMKpCWzTH84+WO5+ziCTsTUZASAToz3RdunTD+vQcHj0GqNTPAHK63sf bAB2I0BslZkXkY1RLb/YhuA6E7JyEd2pilZOrIuBGl/5q2qSakgnAVFWFBR/DO27JuAksYnq +aH8vI0xGvwn75KqSk4UzAkDzWSmO4ZHuahKtQgZNsMYV+PGayRBX9b9zbldzopoLBdqHc4n jQARAQABwsF8BBgBCgAmAhsMFiEEqUDUNJksLo6ZED1QIk+n58yCpmQFAmfIHFQFCRYU6J8A CgkQIk+n58yCpmS2PA//bqN1LfcotmArgElsa+0EGZSQlYgK48pm8WAeTXTngudP9IJ4SuKY HR5RNjHcBeqN+Me0zxRqYzRb8nGanHEkDyf4Im8DQM8d6vbyU+FcPmG4skud4kgS1zMHnlVd SXfSIwKC/hKgdHG8aBV7545Lz9X6Iohea+94wneD0aw/hqF+QWewGZhWJriWAZtvEkzNjQOi 4U9F/trLten/x7bpphDSnDMKJtITbtzATT1Dq7o7VpIUK1nCTQALMuMjKCdi8OdU/+V+R3O4 0PXWvX8qrvqYapVbZ+9KqT74FsuB0Ya9uXwgBF2Q6cRuETZk5vqaqKxzqoQZCO8AOz/58j6O 2RHNy/mZEN+7tJ5Tsq42zVJ4jxsT8b9YplavCMsnBgDeRWhcbYhCyttoL7nYISyWg4kQYZ/P wIV3OuNv2f8iKYsxNsRuClOAF82+gvqOy1/1pprFjy8uo2pkoOrb63aOP3vO5VHnRKgra6dq NcaZ+c6J4H+nEJGi2SkHAUJz5oBzuThvPudLvPA/SK8sKoM01IRxSihev/S/5WLazXB1PGem OCbvzC1IjWJJraxiDJ5IygokapUa2RP7+WBR22skQ3SSl6G107QgWKSyTOGWEaRmV53vxQLV jXuCmzSSasTL60zq5yGrT4/DYQVSNEUiUbG4pYekxJujNeEDkUlky0Y= In-Reply-To: <20261028211930.3504634-6-gourry@gourry.net> Content-Type: text/plain; charset=UTF-8 Content-Transfer-Encoding: 7bit On 10/28/26 22:19, Gregory Price (Meta) wrote: > Onlining the first block of a zone rebuilds every zonelist in place, > shifting the slots after the new zone. The iterator approves a slot, > then the allocator loads the zone from it, so a GFP_KERNEL request can > land in ZONE_MOVABLE and a nodemask request on a node outside its mask: > > CPU 0 CPU 1 > prepare_alloc_pages() > first_zones_zonelist(nodemask {1}) > slot 0 (NORMAL1) approved > online_pages() > build_all_zonelists() > [NORMAL1, NORMAL2, ...] > -> [MOVABLE1, NORMAL1, ...] > get_page_from_freelist() > zone = zonelist_zone(slot 0) /* MOVABLE1 */ > allocates from MOVABLE1 > > zonelist_update_seq detects the rebuild, but only the slowpath's failure > exits check it. > > Check it whenever an allocation succeeds too, and if it changed, free > the page and retry, as the memcg charge failure path does. Eligible > pages are dropped as well: zonelists only change when memory comes or > goes, and a retry is simpler than rechecking the page. > > Freeing the page does not undo a block the allocation claimed. A > GFP_KERNEL request that lands in a fresh ZONE_MOVABLE converts a free > block to MIGRATE_UNMOVABLE, which stays until the zone's movable free > list runs out and a movable allocation takes it back. Yeah that's fine. > The fast and bulk paths read the seqlock's raw seqcount without waiting: > an ALLOC_NOLOCK caller can be an NMI that interrupted the writer, and on > PREEMPT_RT the seqlock accessor takes the spinlock. A sample taken > mid-write costs a retry, or NULL under ALLOC_NOLOCK. > check_retry_zonelist() returns 0 when compiled out, so a backport > without the hot-add change stays harmless. Hmm wasn't even aware there's waiting possible otherwise for seqcounts. > Fixes: 11cd8638c37f ("mm, page_alloc: remove stop_machine from build_all_zonelists") > Cc: stable@vger.kernel.org > Signed-off-by: Gregory Price (Meta) Reviewed-by: Vlastimil Babka (SUSE) > --- > mm/page_alloc.c | 34 +++++++++++++++++++++++++++++++++- > 1 file changed, 33 insertions(+), 1 deletion(-) > > diff --git a/mm/page_alloc.c b/mm/page_alloc.c > index 24106c881ec2..33aab9deaaa1 100644 > --- a/mm/page_alloc.c > +++ b/mm/page_alloc.c > @@ -4419,12 +4419,25 @@ static unsigned int zonelist_iter_begin(void) > return 0; > } > > +/* > + * Never wait for the writer, which an ALLOC_NOLOCK caller in NMI may have > + * interrupted. Read the raw seqcount: on PREEMPT_RT the seqlock accessor takes > + * the spinlock while a write is in progress. > + */ > +static unsigned int zonelist_iter_sample(void) > +{ > + if (IS_ENABLED(CONFIG_MEMORY_HOTPLUG)) > + return raw_seqcount_begin(&zonelist_update_seq.seqcount.seqcount); > + > + return 0; > +} > + > static unsigned int check_retry_zonelist(unsigned int seq) > { > if (IS_ENABLED(CONFIG_MEMORY_HOTPLUG)) > return read_seqretry(&zonelist_update_seq, seq); > > - return seq; > + return 0; > } > > /* Perform direct synchronous page reclaim */ > @@ -5115,6 +5128,12 @@ __alloc_pages_slowpath(gfp_t gfp_mask, unsigned int order, > warn_alloc(gfp_mask, ac->nodemask, > "page allocation failure: order:%u", order); > got_pg: > + /* A rebuild during the walk may have handed out an ineligible zone. */ > + if (page && check_retry_zonelist(zonelist_iter_cookie)) { > + __free_frozen_pages(page, order, FPI_NONE); > + page = NULL; > + goto restart; > + } > return page; > } > > @@ -5199,6 +5218,7 @@ unsigned long alloc_pages_bulk_noprof(gfp_t gfp, int preferred_nid, > struct list_head *pcp_list; > struct alloc_context ac; > unsigned int alloc_flags = ALLOC_WMARK_LOW; > + unsigned int zonelist_iter_cookie; > int nr_populated = 0, nr_account = 0; > > /* > @@ -5239,6 +5259,7 @@ unsigned long alloc_pages_bulk_noprof(gfp_t gfp, int preferred_nid, > /* May set ALLOC_NOFRAGMENT, fragmentation will return 1 page. */ > gfp &= gfp_allowed_mask; > gfp = current_gfp_context(gfp); > + zonelist_iter_cookie = zonelist_iter_sample(); > if (!prepare_alloc_pages(gfp, 0, preferred_nid, nodemask, &ac, &gfp, &alloc_flags)) > goto out; > > @@ -5288,6 +5309,10 @@ unsigned long alloc_pages_bulk_noprof(gfp_t gfp, int preferred_nid, > if (unlikely(!zone)) > goto failed; > > + /* The zone was chosen from a zonelist that a rebuild may have changed. */ > + if (unlikely(check_retry_zonelist(zonelist_iter_cookie))) > + goto failed; > + > /* spin_trylock may fail due to a parallel drain or IRQ reentrancy. */ > pcp = pcp_spin_trylock(zone->per_cpu_pageset); > if (!pcp) > @@ -5428,6 +5453,7 @@ struct page *__alloc_frozen_pages_noprof(gfp_t gfp, unsigned int order, > .alloc_flags = alloc_flags, > }; > unsigned int fastpath_alloc_flags = alloc_flags; > + unsigned int zonelist_iter_cookie; > > /* Other flags could be supported later if needed. */ > if (WARN_ON(alloc_flags & ~(ALLOC_NOLOCK | ALLOC_NO_CODETAG))) > @@ -5457,6 +5483,7 @@ struct page *__alloc_frozen_pages_noprof(gfp_t gfp, unsigned int order, > */ > gfp = current_gfp_context(gfp); > alloc_gfp = gfp; > + zonelist_iter_cookie = zonelist_iter_sample(); > if (!prepare_alloc_pages(gfp, order, preferred_nid, nodemask, &ac, > &alloc_gfp, &fastpath_alloc_flags)) > return NULL; > @@ -5473,6 +5500,11 @@ struct page *__alloc_frozen_pages_noprof(gfp_t gfp, unsigned int order, > > /* First allocation attempt (or, for nolock, only attempt) */ > page = get_page_from_freelist(alloc_gfp, order, fastpath_alloc_flags, &ac); > + if (likely(page) && unlikely(check_retry_zonelist(zonelist_iter_cookie))) { > + __free_frozen_pages(page, order, > + alloc_flags & ALLOC_NOLOCK ? FPI_NOLOCK : 0); > + page = NULL; > + } > if (likely(page) || (alloc_flags & ALLOC_NOLOCK)) > goto out; >