From mboxrd@z Thu Jan 1 00:00:00 1970 Received: from smtp.kernel.org (aws-us-west-2-korg-mail-alma10-1.taild15c8.ts.net [100.103.45.18]) (using TLSv1.2 with cipher ECDHE-RSA-AES256-GCM-SHA384 (256/256 bits)) (No client certificate requested) by smtp.subspace.kernel.org (Postfix) with ESMTPS id 936484314AA; Tue, 28 Jul 2026 12:06:16 +0000 (UTC) Authentication-Results: smtp.subspace.kernel.org; arc=none smtp.client-ip=100.103.45.18 ARC-Seal:i=1; a=rsa-sha256; d=subspace.kernel.org; s=arc-20240116; t=1785240377; cv=none; b=m39bVFdMXRumsHbXvtq+zIB7yDFKQWNyDlIpAzp1YnYzAkZ/TVOmHfCeqyfbhjTDh8EjMjPvsmU06w1BQwW2rKTEgoYsT+SB0uyZy4e9i42+oYOGAoL07+a56b+dMam+Jv0jnJ305CytM9+adCPpBN5FUETqZdwBySGeRWpJ5vA= ARC-Message-Signature:i=1; a=rsa-sha256; d=subspace.kernel.org; s=arc-20240116; t=1785240377; c=relaxed/simple; bh=a2sgHY75/sCkkIEJ+2hyk3OnRW+f6UoOzAj9DJaHb5g=; h=From:Date:Subject:MIME-Version:Content-Type:Message-Id:References: In-Reply-To:To:Cc; b=DYQT1vwTg1MjMZoPv6sm0yRQPno7FGnNzDDL21EavPwb1H8+Vk7Ch0/CPxRiGUpIXx4OAKJ1fzcNkVQ2+AF8awm7x98HEIDhPplPoAWGje/4JXUBxxi4M8IswI56eCYV8t3wAHtkpeCkCjgMgKn+dPVQZs/sEfhAqRt2itP3/R4= ARC-Authentication-Results:i=1; smtp.subspace.kernel.org; dkim=pass (2048-bit key) header.d=kernel.org header.i=@kernel.org header.b=RTCPnacC; arc=none smtp.client-ip=100.103.45.18 Authentication-Results: smtp.subspace.kernel.org; dkim=pass (2048-bit key) header.d=kernel.org header.i=@kernel.org header.b="RTCPnacC" Received: by smtp.kernel.org (Postfix) with ESMTPSA id C17191F00A3A; Tue, 28 Jul 2026 12:06:11 +0000 (UTC) DKIM-Signature: v=1; a=rsa-sha256; c=relaxed/relaxed; d=kernel.org; s=k20260515; t=1785240376; bh=RjgKH/kkJdGm2VMudiDfggXGTukvUfp1G7/FRSrbX4M=; h=From:Date:Subject:References:In-Reply-To:To:Cc; b=RTCPnacCxEmlvezLDHXwR/oIW5fGnB70RoTj2f/08M6cydz+roZXGqI2TnwI3hV94 1uJYBvj34CrUoQ958hn1gfHGnRIZwlYg/wfVeP3pn1V9BxIpHzz8KTwp/305RZhHFq KyjqOSAJFt37wN66bl3ZlCMYrKpYVVucPEapaaknBtA2FJdE7V7iGwqbQtQPb1dbt+ VPZMgIn1DmajkR1je8GuMl2sNOh2ipHRJ+RDm2E41n6LO2E7zFJcqI0xZ6Sia/OLDV RSbTYbVs2wuwhbayZI+tbWO8nxipeLS/+JYjfJcRf5ryFUtRwvbagAZCEm3//tiGPC Iy38jRzYGicQg== From: "Lorenzo Stoakes (ARM)" Date: Tue, 28 Jul 2026 13:05:44 +0100 Subject: [PATCH mm-hotfixes 1/2] mm/huge_memory: separate out CONFIG_PERSISTENT_HUGE_ZERO_FOLIO logic Precedence: bulk X-Mailing-List: linux-kernel@vger.kernel.org List-Id: List-Subscribe: List-Unsubscribe: MIME-Version: 1.0 Content-Type: text/plain; charset="utf-8" Content-Transfer-Encoding: 7bit Message-Id: <20260728-fix-refcounted-huge-zero-v1-1-3f261f5447b4@kernel.org> References: <20260728-fix-refcounted-huge-zero-v1-0-3f261f5447b4@kernel.org> In-Reply-To: <20260728-fix-refcounted-huge-zero-v1-0-3f261f5447b4@kernel.org> To: Andrew Morton , David Hildenbrand , Zi Yan , Baolin Wang , "Liam R. Howlett" , Nico Pache , Ryan Roberts , Dev Jain , Barry Song , Lance Yang , Usama Arif , Pankaj Raghav , Hannes Reinecke , Hugh Dickins , Yang Shi , Kiryl Shutsemau Cc: linux-mm@kvack.org, linux-kernel@vger.kernel.org, Hengbin Zhang , "Lorenzo Stoakes (ARM)" , stable@vger.kernel.org X-Mailer: b4 0.15.2 X-Developer-Signature: v=1; a=openpgp-sha256; l=7572; i=ljs@kernel.org; h=from:subject:message-id; bh=a2sgHY75/sCkkIEJ+2hyk3OnRW+f6UoOzAj9DJaHb5g=; b=owGbwMvMwCV2fu7ZrsZH9SKMp9WSGLIyZist4VjtpSfbaZKku6qHgbnERVEjpfRg+//da/6vT rjesWtSRykLgxgXg6yYIsvzL+L7g0TC5nVe8HeDmcPKBDKEgYtTACby6C7DH47Xx/i/KHYvzF5R Ozd4bhy70u/7T2ZOn3C0TaIwZG3Mh80Mf+Vn2dnlnL6/WZfnX5P+clNVR4Unqu+7/skv37k9vfb KKjYA X-Developer-Key: i=ljs@kernel.org; a=openpgp; fpr=E7F417BF5214569E89D04F46CF9DCD8A81E27F14 Rather than mixing the refcounted and non-refcounted CONFIG_PERSISTENT_HUGE_ZERO_FOLIO logic, separate the two out cleanly so it is clear what happens when this configuration option is set and what happens when it is not. Introduce HUGE_ZERO_UNSET_PFN to abstract the ~0UL assignment, only introduce the refcount and shrinker if !CONFIG_PERSISTENT_HUGE_ZERO_FOLIO, abstract initialisation and teardown, abstract the huge zero folio allocation from refcounting. Also change a BUG_ON() to WARN_ON_ONCE() while we're at it. Without this change, the subsequent fix for a subtle race is harder to understand thus this is a dependency of it. Cc: stable@vger.kernel.org # 6.18.x: dependency of subsequent fix Signed-off-by: Lorenzo Stoakes (ARM) --- mm/huge_memory.c | 159 ++++++++++++++++++++++++++++++++----------------------- 1 file changed, 94 insertions(+), 65 deletions(-) diff --git a/mm/huge_memory.c b/mm/huge_memory.c index 032702a4637b..0f60bc82e87a 100644 --- a/mm/huge_memory.c +++ b/mm/huge_memory.c @@ -77,9 +77,14 @@ static unsigned long deferred_split_scan(struct shrinker *shrink, struct shrink_control *sc); static bool split_underused_thp = true; -static atomic_t huge_zero_refcount; +#define HUGE_ZERO_UNSET_PFN (~0UL) struct folio *huge_zero_folio __read_mostly; -unsigned long huge_zero_pfn __read_mostly = ~0UL; +unsigned long huge_zero_pfn __read_mostly = HUGE_ZERO_UNSET_PFN; +#ifndef CONFIG_PERSISTENT_HUGE_ZERO_FOLIO +static atomic_t huge_zero_refcount; +static struct shrinker *huge_zero_folio_shrinker; +#endif + unsigned long huge_anon_orders_always __read_mostly; unsigned long huge_anon_orders_madvise __read_mostly; unsigned long huge_anon_orders_inherit __read_mostly; @@ -221,22 +226,58 @@ unsigned long __thp_vma_allowable_orders(struct vm_area_struct *vma, return orders; } -static bool get_huge_zero_folio(void) +static struct folio *alloc_huge_zero_folio(void) { struct folio *zero_folio; -retry: - if (likely(atomic_inc_not_zero(&huge_zero_refcount))) - return true; zero_folio = folio_alloc((GFP_TRANSHUGE | __GFP_ZERO | __GFP_ZEROTAGS) & ~__GFP_MOVABLE, HPAGE_PMD_ORDER); if (!zero_folio) { count_vm_event(THP_ZERO_PAGE_ALLOC_FAILED); - return false; + return NULL; + } + folio_clear_large_rmappable(zero_folio); /* Explicitly not rmappable. */ + return zero_folio; +} + +#ifdef CONFIG_PERSISTENT_HUGE_ZERO_FOLIO +static int __init huge_zero_init(void) +{ + huge_zero_folio = alloc_huge_zero_folio(); + if (!huge_zero_folio) { + pr_warn("Allocating persistent huge zero folio failed\n"); + } else { + huge_zero_pfn = folio_pfn(huge_zero_folio); + count_vm_event(THP_ZERO_PAGE_ALLOC); } - /* Ensure zero folio won't have large_rmappable flag set. */ - folio_clear_large_rmappable(zero_folio); + return 0; +} + +static void __init huge_zero_shrinker_exit(void) +{ +} + +struct folio *mm_get_huge_zero_folio(struct mm_struct *mm) +{ + return huge_zero_folio; +} + +void mm_put_huge_zero_folio(struct mm_struct *mm) +{ +} +#else +static bool get_huge_zero_folio(void) +{ + struct folio *zero_folio; +retry: + if (likely(atomic_inc_not_zero(&huge_zero_refcount))) + return true; + + zero_folio = alloc_huge_zero_folio(); + if (unlikely(!zero_folio)) + return false; + preempt_disable(); if (cmpxchg(&huge_zero_folio, NULL, zero_folio)) { preempt_enable(); @@ -258,33 +299,7 @@ static void put_huge_zero_folio(void) * Counter should never go to zero here. Only shrinker can put * last reference. */ - BUG_ON(atomic_dec_and_test(&huge_zero_refcount)); -} - -struct folio *mm_get_huge_zero_folio(struct mm_struct *mm) -{ - if (IS_ENABLED(CONFIG_PERSISTENT_HUGE_ZERO_FOLIO)) - return huge_zero_folio; - - if (mm_flags_test(MMF_HUGE_ZERO_FOLIO, mm)) - return READ_ONCE(huge_zero_folio); - - if (!get_huge_zero_folio()) - return NULL; - - if (mm_flags_test_and_set(MMF_HUGE_ZERO_FOLIO, mm)) - put_huge_zero_folio(); - - return READ_ONCE(huge_zero_folio); -} - -void mm_put_huge_zero_folio(struct mm_struct *mm) -{ - if (IS_ENABLED(CONFIG_PERSISTENT_HUGE_ZERO_FOLIO)) - return; - - if (mm_flags_test(MMF_HUGE_ZERO_FOLIO, mm)) - put_huge_zero_folio(); + WARN_ON_ONCE(atomic_dec_and_test(&huge_zero_refcount)); } static unsigned long shrink_huge_zero_folio_count(struct shrinker *shrink, @@ -300,7 +315,7 @@ static unsigned long shrink_huge_zero_folio_scan(struct shrinker *shrink, if (atomic_cmpxchg(&huge_zero_refcount, 1, 0) == 1) { struct folio *zero_folio = xchg(&huge_zero_folio, NULL); BUG_ON(zero_folio == NULL); - WRITE_ONCE(huge_zero_pfn, ~0UL); + WRITE_ONCE(huge_zero_pfn, HUGE_ZERO_UNSET_PFN); folio_put(zero_folio); return HPAGE_PMD_NR; } @@ -308,7 +323,46 @@ static unsigned long shrink_huge_zero_folio_scan(struct shrinker *shrink, return 0; } -static struct shrinker *huge_zero_folio_shrinker; +static int __init huge_zero_init(void) +{ + huge_zero_folio_shrinker = shrinker_alloc(0, "thp-zero"); + if (!huge_zero_folio_shrinker) { + shrinker_free(deferred_split_shrinker); + list_lru_destroy(&deferred_split_lru); + return -ENOMEM; + } + + huge_zero_folio_shrinker->count_objects = shrink_huge_zero_folio_count; + huge_zero_folio_shrinker->scan_objects = shrink_huge_zero_folio_scan; + shrinker_register(huge_zero_folio_shrinker); + return 0; +} + +static void __init huge_zero_shrinker_exit(void) +{ + shrinker_free(huge_zero_folio_shrinker); +} + +struct folio *mm_get_huge_zero_folio(struct mm_struct *mm) +{ + if (mm_flags_test(MMF_HUGE_ZERO_FOLIO, mm)) + return READ_ONCE(huge_zero_folio); + + if (!get_huge_zero_folio()) + return NULL; + + if (mm_flags_test_and_set(MMF_HUGE_ZERO_FOLIO, mm)) + put_huge_zero_folio(); + + return READ_ONCE(huge_zero_folio); +} + +void mm_put_huge_zero_folio(struct mm_struct *mm) +{ + if (mm_flags_test(MMF_HUGE_ZERO_FOLIO, mm)) + put_huge_zero_folio(); +} +#endif /* CONFIG_PERSISTENT_HUGE_ZERO_FOLIO */ #ifdef CONFIG_SYSFS static ssize_t enabled_show(struct kobject *kobj, @@ -972,39 +1026,14 @@ static int __init thp_shrinker_init(void) deferred_split_shrinker->scan_objects = deferred_split_scan; shrinker_register(deferred_split_shrinker); - if (IS_ENABLED(CONFIG_PERSISTENT_HUGE_ZERO_FOLIO)) { - /* - * Bump the reference of the huge_zero_folio and do not - * initialize the shrinker. - * - * huge_zero_folio will always be NULL on failure. We assume - * that get_huge_zero_folio() will most likely not fail as - * thp_shrinker_init() is invoked early on during boot. - */ - if (!get_huge_zero_folio()) - pr_warn("Allocating persistent huge zero folio failed\n"); - return 0; - } - - huge_zero_folio_shrinker = shrinker_alloc(0, "thp-zero"); - if (!huge_zero_folio_shrinker) { - shrinker_free(deferred_split_shrinker); - list_lru_destroy(&deferred_split_lru); - return -ENOMEM; - } - - huge_zero_folio_shrinker->count_objects = shrink_huge_zero_folio_count; - huge_zero_folio_shrinker->scan_objects = shrink_huge_zero_folio_scan; - shrinker_register(huge_zero_folio_shrinker); - - return 0; + return huge_zero_init(); } static void __init thp_shrinker_exit(void) { - shrinker_free(huge_zero_folio_shrinker); shrinker_free(deferred_split_shrinker); list_lru_destroy(&deferred_split_lru); + huge_zero_shrinker_exit(); } static int __init hugepage_init(void) -- 2.55.0