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=1794484377; b=hC2HqRFaNhEfyUJjhjH5wPEw7Zojp67w3+Oo70tVRGoBt8G8dnoWFarTvTUwUErFzqLz dRyelriGhiKUzN0MgmK3/4wZwCvwJIiCcH+dFGTPca8WUALJtuIKmLJ8z3Bsy0hbv3v+J TWvBwoMLZ1uaqTz01vQcGQGQoYJzoZMPcAKkK0meloNRyoCifcILww/OBDi4dP/7sAIur e+hj7OdGNZjtU6lOV50rst6/cSmNC46qai4sus0kJKqgj2XMAu3RrN8t7ZbNWZ0AaAfMp d/ZUZXcIxMwRJI64l6j56t1IwNfs8H7PexXuowvUj1oRLNnvYPtR0TdlNUzs6uqJpqQ== ARC-Message-Signature: i=1; d=subspace.kernel.org; s=arc20261015; a=rsa-sha256; c=relaxed/relaxed; t=1794484377; h=Received:DKIM-Signature:Date:From:To:Cc:Subject:Message-ID: References:MIME-Version:Content-Type:Content-Disposition:In-Reply-To; bh=mwn/H1ULONfD3r8MGUfyFZepAjOONqSHtVOfwqC7R44=; b=Oxq346R2fRCx+nru7mpRbeyhEfaSr0DobM06AY0j7kaIV9En/zmyEcyOcf0zEqoBNyXl ts36a1kqSLe5vaua1BPf4IUy/1XcyES8vxa7M6IlkG+dYZHgU8WEdz8Ds9HITK1IO+Pjb NmGozWIK2sAbVQobrag0S10/GwuO4jfAu1pUBcHiSRQsp25gwV0TR7rGho0TfZkabPPki xUANJEYIVtxMgyimPySWgnCCPFR5X+6IXo4glzsuhX1O+qWfjrRjETJ+vSj/GF2VT4Oup ex+cfTmLmf51pFZjqhohynTV5MUapnj/TmV6KjQdISHznLuDgZR4I8GC/kYu1N5V3Bw== 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=n73BoZfW; 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 E1A55606A33B; Thu, 12 Nov 2026 11:52:56 +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=n73BoZfW Received: by smtp.kernel.org (Postfix) with ESMTPSA id 69AFC1F00A06; Thu, 12 Nov 2026 11:52:49 +0000 (UTC) DKIM-Signature: v=1; a=rsa-sha256; c=relaxed/relaxed; d=kernel.org; s=k20260515; t=1794484376; bh=mwn/H1ULONfD3r8MGUfyFZepAjOONqSHtVOfwqC7R44=; h=Date:From:To:Cc:Subject:References:In-Reply-To; b=n73BoZfWq97QkN/EzmbuARqlOgkw571PAQbg9exHKtPkxPfLWofEF3xx49ExQI0+5 NP7AVrlK+9cNoWfkzTQe8yBsYysMxwPj7QUvfullML+BTgnwt+niwBFJ4djUkPZJhj u6DmKuJ63HSV8sLPOkQLGdgtwRQ6RQpEjkwL8WSFEcC5i1lk/REmtDn0ONN8B0fZFp KWkLDbCPyrj8njpmsNXUwgIiAnGNXbD/W3krNa+IIu6u5Jasv+5pY51IpfMF9/2E1L EJxh+zfJfG5qrKbAwulb0FfxElREfauArGku1EH6kDQbGoA0jT0iwqZaeqUalrXJE5 nEFAmwZULpIuQ== Date: Thu, 12 Nov 2026 11:52:46 +0000 From: "Lorenzo Stoakes (ARM)" To: Usama Arif Cc: Andrew Morton , apopple@nvidia.com, byungchul@sk.com, david@kernel.org, gourry@gourry.net, jannh@google.com, joshua.hahnjy@gmail.com, liam@infradead.org, linux-fsdevel@vger.kernel.org, linux-kernel@vger.kernel.org, linux-mm@kvack.org, matthew.brost@intel.com, mhocko@suse.com, pfalcato@suse.de, rppt@kernel.org, surenb@google.com, vbabka@kernel.org, ying.huang@linux.alibaba.com, ziy@nvidia.com, kas@kernel.org, hannes@cmpxchg.org, riel@surriel.com, shakeel.butt@linux.dev, kernel-team@meta.com, baolin.wang@linux.alibaba.com, Nico Pache , ryan.roberts@arm.com, dev.jain@arm.com, baohua@kernel.org, lance.yang@linux.dev, stable@vger.kernel.org Subject: Re: [PATCH 1/3] mm: don't PTE-batch a swap-in over a hardware-poisoned subpage Message-ID: References: <20261111183917.1093166-1-usama.arif@linux.dev> X-Mailing-List: linux-fsdevel@vger.kernel.org List-Id: List-Post: List-Help: List-Subscribe: List-Unsubscribe: List-Owner: Precedence: list MIME-Version: 1.0 Content-Type: text/plain; charset=us-ascii Content-Disposition: inline In-Reply-To: <20261111183917.1093166-1-usama.arif@linux.dev> On Wed, Nov 11, 2026 at 10:39:17AM -0800, Usama Arif wrote: > On Wed, 11 Nov 2026 13:18:36 +0000 "Lorenzo Stoakes (ARM)" wrote: > > > On Wed, Nov 11, 2026 at 03:38:29AM -0800, Usama Arif wrote: > > > do_swap_page() checks PageHWPoison() on the faulting page, but the > > > large-folio batching path then maps the rest of the folio without looking > > > at the other subpages. A poisoned subpage that is not the one being > > > faulted gets mapped anyway, and the fault that would have reported it > > > never happens. > > > > > > Skip batching for such a folio and let each page take its own fault, > > > where the existing check catches it. > > > > > > Scan the subpages rather than testing the folio-level PG_has_hwpoisoned: > > > memory_failure() sets PageHWPoison() on the subpage first and only sets > > > the folio flag much later, and the bail-outs in between - for example > > > get_hwpoison_page() losing a page lifecycle race - return with the > > > subpage marked and the folio flag never set. > > > > > > Fixes: 508758960b8d ("mm: swap: entirely map large folios found in swapcache") > > > Cc: stable@vger.kernel.org > > > Signed-off-by: Usama Arif > > > --- > > > mm/internal.h | 15 +++++++++++++++ > > > mm/memory.c | 4 ++++ > > > 2 files changed, 19 insertions(+) > > > > > > diff --git a/mm/internal.h b/mm/internal.h > > > index cd3675c044e0a..e61c02f4019b0 100644 > > > --- a/mm/internal.h > > > +++ b/mm/internal.h > > > @@ -28,6 +28,21 @@ struct folio_batch; > > > int __copy_remote_mm_str(struct mm_struct *mm, unsigned long addr, > > > void *buf, int len, unsigned int gup_flags); > > > > > > +/* > > > + * Check every page of the folio for HWPoison, rather than relying on the > > > + * folio-level PG_has_hwpoisoned, which memory_failure() can leave clear. > > > + */ > > > +static inline bool folio_has_any_hwpoisoned_page(const struct folio *folio) > > > +{ > > > + long nr = folio_nr_pages(folio); > > > + long i; > > > + > > > + for (i = 0; i < nr; i++) > > > + if (PageHWPoison(folio_page(folio, i))) > > > + return true; > > > + return false; > > > +} > > > + > > > /* mm/workingset.c */ > > > bool workingset_test_recent(void *shadow, bool file, bool *workingset, > > > bool flush); > > > diff --git a/mm/memory.c b/mm/memory.c > > > index d477a4f986bc8..4b7d456332200 100644 > > > --- a/mm/memory.c > > > +++ b/mm/memory.c > > > @@ -5158,6 +5158,10 @@ vm_fault_t do_swap_page(struct vm_fault *vmf) > > > swap_pte_batch(folio_ptep, nr, folio_pte) != nr) > > > goto check_folio; > > > > > > + /* Let a poisoned subpage take its own fault. */ > > > + if (folio_has_any_hwpoisoned_page(folio)) > > > + goto check_folio; > > > + > > Hello Lorenzo, > > Thanks for the reviews! > > > > > Hm, isn't scanning every single page in the folio potentially quite costly > > here? I thought the point of batching was to improve performance overall? > > Swapping in requires checking every subpage, so there would be a cost. > I guess trading it off for correctness :( (we don't say subpage apparently :) Yeah but the cost is now paid by literally everybody... And... > > > > > It seems broken to have stuff potentially interact with hwpoison but then > > to only set the folio-level flag much later? > > > > PG_has_hwpoisoned lives in the second page, so memory_failure() only > sets it once it holds a folio reference, otherwise it could race with a > THP split. If get_hwpoison_page() can't get a reference, > memory_failure() gives up with only the subpage marked. The comment > above folio_set_has_hwpoisoned() in memory_failure() says that is fine: > > Don't need care about the above error handling paths for > get_hwpoison_page() since they handle either free page > or unhandlable page. > > But a large folio that is briefly off the LRU, e.g. isolated by > reclaim, takes those paths too and can still be mapped afterwards. ...now you're narrowing this cost to literally a vague edge case. Can we not just do the scan _only in this case_? > > > Also why are we swapping out folios containing hwpoison pages? Shouldn't we > > migrate away from the hwpoison? :) (I'm not really familiar with the hw > > poison stuff) > > > > > > The error hits a folio that is already in the swap > cache, e.g. while reclaim has it off the LRU to write it out, and > get_hwpoison_page() fails for any folio that is off the LRU. Normally > memory_failure() would split the folio and isolate the bad page (for > an uncorrected error there is nothing to migrate; that is soft > offline). Here it does neither, and the folio stays in the swap cache > with only the subpage marked. Yeah this just seems like the swapcache code being broken in general. > > While looking at this I also noticed that the fallback drops a freshly > read (not yet anon) folio from the swap cache and frees it with the > poisoned page in it. We should actually actually also check folio_test_anon() > before checking folio_has_any_hwpoisoned_page(). Ugh lord, how much crap is handled improperly re: hw poisoning. Such a mess... -- Cheers, Lorenzo