From mboxrd@z Thu Jan 1 00:00:00 1970 Authentication-Results: smtp.subspace.kernel.org; arc=none smtp.remote-ip=100.103.45.18 ARC-Seal: i=1; d=subspace.kernel.org; s=arc20261015; a=rsa-sha256; cv=none; t=1794389518; b=v9EHGoX1wNWwV+n1i8UrVfl0BpzjFOudcQOF+mPMcLNemtFVLHPiwxC+GdJw6BIH9zXJ RIRH7S8/PByJQNn4sMJnUe8rKYkU68HtAGOdmuelZlmFNDBskRbFpWfdtsKazwpJs34nF 2yB0F6btM5fb3oeatFoavMIX8GOJwAA6QR77Yr2imlIs/MoetMnXwjdKuUDJ+Y+YTdktm cxH3KsA2MS6xQP52p4QB9xyfhQ8RtRSBd0DiQ8AWD5++OdnU0ZzoKQpJw8EAF35dr8dmK p4FXBmt1uiFiVW7Rijx9eSK298v2ChRlrmjJ6HbEMj81JhkNFw7A8Mb/3OrPgO1eAWQ== ARC-Message-Signature: i=1; d=subspace.kernel.org; s=arc20261015; a=rsa-sha256; c=relaxed/relaxed; t=1794389518; h=Received:DKIM-Signature:Message-ID:Date:MIME-Version:User-Agent: Subject:Content-Language:To:Cc:References:From:Autocrypt:In-Reply-To: Content-Type:Content-Transfer-Encoding; bh=u758KHAU32PODMlS6tG046G0kAihUmhpRxPra1+3skM=; b=pgS30CG9jsNa+lA2UKPaUr1rEAylo1DHXPQG9TzFU3TXv2mde+0efOLoorb8tvfiWjmY b/LEsTajMvyCeI62GSlE4/wIi5fvOK02N+EjEvnxx8715LP6SNORk1ogNmn+KivZ/URGC Jr9qFPIOkZV95dlGXorqw41yz7riL5285ZbTHOhgFfVGYrgMqQ2zHKp6PK0OjaANt42JH lQ3uJ09h8YLiMGBxi3krAao5tUOaTQDw0ovLG+8VrnTzWpYjswt16O+gfxQKq596pYM/B mGZhI1wyf5k3jeC8Dqe9RwjF0Vo4lYATBSlhYRgs0lQqLy0tCSo0h42NQEx+CZCY+4g== ARC-Authentication-Results: i=1; smtp.subspace.kernel.org; dkim=pass header.d=kernel.org header.i=@kernel.org header.a=rsa-sha256 header.s=k20260515 header.b="EknY/VKN"; 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 27410606A31C for ; Wed, 11 Nov 2026 09:31:58 +0000 (UTC) Authentication-Results: smtp.subspace.kernel.org; dkim=pass (2048-bit key) header.d=kernel.org header.i=@kernel.org header.a=rsa-sha256 header.s=k20260515 header.b=EknY/VKN Received: by smtp.kernel.org (Postfix) with ESMTPSA id 442531F00A05; Wed, 11 Nov 2026 09:31:54 +0000 (UTC) DKIM-Signature: v=1; a=rsa-sha256; c=relaxed/relaxed; d=kernel.org; s=k20260515; t=1794389517; bh=u758KHAU32PODMlS6tG046G0kAihUmhpRxPra1+3skM=; h=Date:Subject:To:Cc:References:From:In-Reply-To; b=EknY/VKNU6eFA8uwFtk8+psA0CHt0OgX5+D8T7S2I0GL9PblfvtRCOWrm+9NhOpnU I3Q8J59PNi8RkBtLHxX9juiU3U8jabKTIb17+ybbjlYDcap11rJxEqygA/p1LF5ryO UOhTDkT1SaODVRS45ScdPaDF6iik6/dk4Bl+UGJx6zQh2SdnJ3pjKQZMePcmBgpxvq R0Pu6TUF0NhgMX9xWeeSduRtiNvHMOxchBTdJsy5/J6yeVmbrp8k4kfQ9MS3J7Lbt0 GVa3OtUULfvUAH2Odx8oHt+AEWw2VeyZry0zy8yhv7HP5CpdXZoqQYuDWdMwlfaPGA TxWZzuyTh4xXg== Message-ID: Date: Wed, 11 Nov 2026 10:31:53 +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] mm/vma: predicate vma_assert_stabilised() on CONFIG_DEBUG_VM Content-Language: en-US To: "Lorenzo Stoakes (ARM)" , Andrew Morton , Suren Baghdasaryan , "Liam R. Howlett" , Shakeel Butt , Jann Horn , Pedro Falcato , Lance Yang , "David Hildenbrand (Arm)" , "Kiryl Shutsemau (Meta)" Cc: linux-mm@kvack.org, linux-kernel@vger.kernel.org, Seunghyun An References: <20261110-vma-fixup-debug-asserts-v1-1-0b2e0680cd7e@kernel.org> 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: <20261110-vma-fixup-debug-asserts-v1-1-0b2e0680cd7e@kernel.org> Content-Type: text/plain; charset=UTF-8 Content-Transfer-Encoding: 7bit On 11/10/26 16:10, Lorenzo Stoakes (ARM) wrote: > I introduced commit 17fd82c3abe0 ("mm/vma: add and use > vma_assert_stabilised()") with the intent of picking up programmatic bugs > when debugging mm code, but kept the general practice of asserting even > when CONFIG_DEBUG_VM is not set in line with existing code. > > However, it turns out doing so can cause performance regressions, > especially when asserted in vma_has_anon_rmap() which is referenced in a > number of hotpaths. > > Since the intent was always a debug-only assertion, explicitly check for > CONFIG_DEBUG_VM and do not assert if it isn't set. > > Really all of the assert paths need to be predicated on this, but start > with this one only, out of caution. > > Additionally, since a detached VMA is trivially stabilised (nothing to > stabilise against), check for this case at the outset, and update > vma_has_anon_rmap() to not explicitly need to check for this. > > This also, importantly, avoids reading any VMA fields if CONFIG_DEBUG_VM is > not set. > > Target the commit which introduced vma_has_anon_rmap() for the fix, as this > is where the issue started manifesting in practice. > > Fixes: 38564710f443 ("mm: implement and use vma_has_anon_rmap(), silence KCSAN") > Reported-by: Seunghyun An > Closes: https://lore.kernel.org/linux-mm/20261110000002.1834454-1-san55@wisc.edu/ > Signed-off-by: Lorenzo Stoakes (ARM) Reviewed-by: Vlastimil Babka (SUSE) > --- > Note that vma_is_anon_rmap() was introduced in 7.4, so no need for a cc: stable, > just needs to be sent to Linus as a hotfix. Thanks! > --- > include/linux/mmap_lock.h | 19 ++++++++++++++----- > mm/vma.h | 3 +-- > 2 files changed, 15 insertions(+), 7 deletions(-) > > diff --git a/include/linux/mmap_lock.h b/include/linux/mmap_lock.h > index 9ccbbcbd7aca..9c3a9c6525c1 100644 > --- a/include/linux/mmap_lock.h > +++ b/include/linux/mmap_lock.h > @@ -408,6 +408,11 @@ static inline void vma_assert_locked(const struct vm_area_struct *vma) > vma_assert_write_locked(vma); > } > > +static inline bool vma_is_attached(const struct vm_area_struct *vma) > +{ > + return refcount_read(&vma->vm_refcnt); > +} > + > /** > * vma_assert_stabilised() - assert that this VMA cannot be changed from > * underneath us either by having a VMA or mmap lock held. > @@ -418,9 +423,18 @@ static inline void vma_assert_locked(const struct vm_area_struct *vma) > * > * With lockdep disabled we may sometimes race with other threads acquiring the > * mmap read lock simultaneous with our VMA read lock. > + * > + * Assert only if CONFIG_DEBUG_VM is set. > */ > static inline void vma_assert_stabilised(const struct vm_area_struct *vma) > { > + if (!IS_ENABLED(CONFIG_DEBUG_VM)) > + return; > + > + /* Detached VMAs are trivially stabilised. */ > + if (!vma_is_attached(vma)) > + return; > + > /* > * If another thread owns an mmap lock, it may go away at any time, and > * thus is no guarantee of stability. > @@ -454,11 +468,6 @@ static inline void vma_assert_stabilised(const struct vm_area_struct *vma) > vma_assert_locked(vma); > } > > -static inline bool vma_is_attached(const struct vm_area_struct *vma) > -{ > - return refcount_read(&vma->vm_refcnt); > -} > - > /* > * WARNING: to avoid racing with vma_mark_attached()/vma_mark_detached(), these > * assertions should be made either under mmap_write_lock or when the object > diff --git a/mm/vma.h b/mm/vma.h > index 3fc3b47a954b..05300badd70b 100644 > --- a/mm/vma.h > +++ b/mm/vma.h > @@ -278,8 +278,7 @@ static inline pgoff_t vmg_end_pgoff(const struct vma_merge_struct *vmg) > */ > static inline bool vma_has_anon_rmap(const struct vm_area_struct *vma) > { > - if (vma_is_attached(vma)) > - vma_assert_stabilised(vma); > + vma_assert_stabilised(vma); > /* KCSAN gets confused about the optimistic check. Silence it. */ > return data_race(vma->anon_vma); > } > > --- > base-commit: 93d7a816a2df3e907e2719bd12b0073f1285fc5c > change-id: 20261110-vma-fixup-debug-asserts-2d327fe27f8c > > Cheers,