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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=PMeX0QQv; 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 EBAA2606A324; Sat, 31 Oct 2026 09:34:55 +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=PMeX0QQv Received: by smtp.kernel.org (Postfix) with ESMTPSA id BCAA11F000C1; Sat, 31 Oct 2026 09:34:51 +0000 (UTC) DKIM-Signature: v=1; a=rsa-sha256; c=relaxed/relaxed; d=kernel.org; s=k20260515; t=1793439295; bh=P9o1mQzkbmTdaiTKQdIgL8fzuR8hlpHa+FcuDOUlG7g=; h=Date:Subject:To:Cc:References:From:In-Reply-To; b=PMeX0QQvPBgkOrecOplu7IqNxYMnMEyiD5AjPxAjtugRusW9fJiuNj9IXZNPcnSH/ WMqSf4rUuty4Ax+BW2IdK8zetJkUQAMx3/TqvyUINiyjs6NfC0rfqsO3X79DBBi4Sc WhfQaM1PTWd6z3XfFqVDPxoYFuZCr8x+mzLIQ503fxBBUSUX6dp+bH4tgB3FpN4yeA JimfPT0DLgkb4O0DeQsG/FdpYWITpOIdsyvhtAmXLxivSzB4EiIbnVSluZg62j/4Ro txxIMOW7G42uEhpVi6RVyYto7K6f2/WDA2zTfShc/YtXJ3HZghe/m3wXmmC0cGiTT8 MG8uqaygp9/SQ== Message-ID: <04155e81-a88f-4bba-a77f-0aad1ffdcde0@kernel.org> Date: Sat, 31 Oct 2026 10:34:47 +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: Andrew Morton , Mani Tofigh Cc: Chris Li , Kairui Song , 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> <20261030155419.b10056c8fb37bc784b0d2778@linux-foundation.org> 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 23:54, Andrew Morton wrote: > On Fri, 30 Oct 2026 15:11:56 -0400 Mani Tofigh wrote: > >> 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(): >> >> [ 6.424485] kernel BUG at mm/memory.c:5195! >> >> ... >> >> 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. >> >> 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. >> >> --- 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; >> + } > > Let's not WARN here. If it hits and someone reports it, what do we > tell them? "uh, that's working as designed, no action needed". I thought harder about this, and it just shouldn't happen. Something that was swapped in and never mapped (!anon) that would be dirty would reveal a bug elsewhere. I think we can just turn that into a VM_WARN_ON_ONCE() and still drop the folio. (there is a reason we never saw this so far in do_swap_page()) -- Cheers, David