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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="HrSF/e4j"; 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 67796606A31D; Fri, 30 Oct 2026 22:15:24 +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=HrSF/e4j Received: by smtp.kernel.org (Postfix) with ESMTPSA id C26BF1F000C1; Fri, 30 Oct 2026 22:15:20 +0000 (UTC) DKIM-Signature: v=1; a=rsa-sha256; c=relaxed/relaxed; d=kernel.org; s=k20260515; t=1793398524; bh=ae/WNpwhDO2VnWY2svGWtlkmRzAah686LwChcMW5xuY=; h=Date:Subject:To:Cc:References:From:In-Reply-To; b=HrSF/e4jvxLJdiVvNyJjOVr3iEP3BLH4f4XAOlLGgXaIvlPhikoj4YcSfGH7ZIMVF wHUCHkhC6F1spfp2lEuuTiwV0MiUnz53NpxMPVlI+ImhsdAvAAF2hYYkJsCp4hc+hH byDuDl5TI3NUj/SelCEpqDact3Uutl+KgcA2wnfGPEAilnnOBODMx+JoxsiRjYDO6E FH9dNeKGww6ELtUIcxcOHYY63B4XaEZw7LZIRDH95QkrBz6eEd2csTie08F49UaUO4 z1AJuSoyoa1fX+EBB3lU0NvvuhtJETgT4+kg8Hk1HqsdhhKjmPkCKL5MSUVaOyNA5J OvrHw4iesX/tQ== Message-ID: <04ad7001-bdd4-47e1-9a80-5b2464da1044@kernel.org> Date: Fri, 30 Oct 2026 23:15:18 +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/swapfile: drop a large unmapped swap-in folio in unuse_pte() To: mt4022@columbia.edu, Andrew Morton , Chris Li , Kairui Song Cc: Kemeng Shi , Nhat Pham , Baoquan He , Barry Song , Youngjun Park , linux-mm@kvack.org, linux-kernel@vger.kernel.org, tz2294@columbia.edu, stable@vger.kernel.org References: <20261030-swapin-large-unmapped-v1-1-41e4ae378e6c@columbia.edu> From: "David Hildenbrand (Arm)" Content-Language: en-US Autocrypt: addr=david@kernel.org; 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charset=UTF-8 Content-Transfer-Encoding: 7bit On 10/30/26 20:11, Mani Tofigh via B4 Relay wrote: > From: Mani Tofigh > Thanks for the report! > Since commit f1879e8a0c60 ("mm, swap: never bypass the swap cache even > for SWP_SYNCHRONOUS_IO"), a fault can read a swapped-out mTHP range into > one large folio in the swap cache. The folio only becomes anon once > do_swap_page() maps all of it, so if the faulting PTE changes in the > meantime the folio stays in the swap cache while not being mapped or > anon: > > Task A (fault) Task B > do_swap_page() > /* swaps in large folio F */ > madvise(MADV_DONTNEED) > /* zaps the faulting PTE */ > /* PTE changed (F not mapped) */ > > swapoff then maps it, which marks all of its pages as mapped and > exclusive, and the next fault on one of the other PTEs crashes in > do_swap_page(): Ugh. Why is this special to large folios? I guess because a single PTE keeps the large folio alive, right? The problem really is that unuse_pte() open-codes whet do_swap_page() would usually do. And it doesn't handle large folios in this scenario correctly. What a mess. I guess we could just teach unuse_pte() to only map a single PTE. folio_add_new_anon_rmap() assumes the folios is fully mapped. We could add up to twp folio_remove_rmap_ptes() calls with everything except the page we want to map. But reading below ... > > [ 6.424485] kernel BUG at mm/memory.c:5195! > [ 6.424503] Oops: invalid opcode: 0000 [#1] SMP KASAN NOPTI > [ 6.427654] CPU: 0 UID: 0 PID: 67 Comm: swapmthp Tainted: G W 7.3.0-17087-gd865f2677c44 #1 PREEMPT > [ 6.432665] RIP: 0010:do_swap_page+0x24eb/0x2fa0 > [ 6.451035] Call Trace: > [ 6.451567] > [ 6.453841] __handle_mm_fault+0x13b5/0x3b40 > [ 6.458525] handle_mm_fault+0x16f/0x480 > [ 6.461143] do_user_addr_fault+0x312/0xa30 > [ 6.462044] exc_page_fault+0x66/0xc0 > [ 6.462846] asm_exc_page_fault+0x26/0x30 > [ 6.476708] > > do_swap_page() already drops such a folio from the swap cache when it > can't map all of it. Do the same in unuse_pte() so that swapoff swaps > the entries in again. With the fix, nothing is reported. We indeed have this if (!folio_test_anon(folio) && folio_test_large(folio) && nr_pages != folio_nr_pages(folio)) { ... } check. The nr_pages check makes sense as in your case below it's simply nr_pages == 1. > > This bug was identified and confirmed with the help of a deterministic > hypervisor currently under development. The C reproducer below however > hits it within a second. Rings a bell :) > > Fixes: f1879e8a0c60 ("mm, swap: never bypass the swap cache even for SWP_SYNCHRONOUS_IO") > Cc: stable@vger.kernel.org > Signed-off-by: Mani Tofigh > --- > This conflicts in context with patch 41/47 of David's v2 series [1], > which changes the line right below in unuse_pte() for KSM folios. I > can rebase on top of it once it's applied if needed. It will likely get applied on top of git git://git.kernel.org/pub/scm/linux/kernel/git/mm/core.git for-next Tomorrow and then quickly find its way into next. So if a resend is required, best to send against that. > > [1] https://lore.kernel.org/all/20261029-fixes_core-v2-41-a64ce7f83abb@kernel.org/ > > Reproducer: https://github.com/manitofigh/files/blob/main/linux-repros/swapmthp.c > --- > mm/swapfile.c | 12 ++++++++++++ > 1 file changed, 12 insertions(+) > > diff --git a/mm/swapfile.c b/mm/swapfile.c > index 189f04d8838f..b0b75ff16835 100644 > --- a/mm/swapfile.c > +++ b/mm/swapfile.c > @@ -2518,6 +2518,18 @@ static int unuse_pte(struct vm_area_struct *vma, pmd_t *pmd, > if (!folio_matches_swap_entry(folio, entry)) > return 0; > > + /* > + * A large folio that is not anon yet was swapped in by a page fault > + * that did not map it (yet). It can only be mapped as a whole, by > + * do_swap_page(). Drop it from the swap cache and swap in small > + * folios instead. > + */ > + if (unlikely(folio_test_large(folio) && !folio_test_anon(folio))) { > + if (!WARN_ON_ONCE(folio_test_dirty(folio))) > + swap_cache_del_folio(folio); > + return 0; > + } I don't enjoy the additional code duplication, including the comment. Can you pack the existing handling in do_swap_page() into a helper, pass nr_pages, and reuse it here? I guess finding a good name is the main challenge ... static bool swapcache_folio_mappable(struct folio *folio, unsigned int nr_pages) { if (!folio_test_anon(folio) || !folio_test_large(folio) || nr_pages == folio_nr_pages(folio)) return true; if (!WARN_ON_ONCE(folio_test_dirty(folio))) swap_cache_del_folio(folio); return false; } Just a thought. But it does look like the right fix. -- Cheers, David