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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-Archived-By: 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 X-Rspamd-Server: rspam10 X-Rspamd-Queue-Id: 456BC20006 X-Stat-Signature: rcsumj7148q9a889dabi4wdztuzfyf8u X-Spam-Status: No, score=-12.3 required=3.0 tests=BAYES_00,DKIMWL_WL_HIGH, DKIM_SIGNED,DKIM_VALID,DKIM_VALID_AU,HEADER_FROM_DIFFERENT_DOMAINS, INCLUDES_PATCH,MAILING_LIST_MULTI,SPF_HELO_PASS,SPF_PASS, USER_AGENT_SANE_1 autolearn=ham autolearn_force=no version=4.0.1 X-Rspam-User: X-HE-Tag: 1793439297-249927 X-HE-Meta: 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 hqEmfyTi z5GaQ+mHfHrjrxYic9y+qPLpT/GVF/5lxPmxZF5eLtsWTDq3VSN2P9APfY5xTsaZrMw9Vi2L6gluJ4CVe4KMFT5qZzxw3EdjijppHNTmBYYHtQFHNnN4gZfUhdgwAVa4oc1jaBPibWGRMkk9qlJJIxmkTh1QMcaVQ9OSo7p06kdo2sFbLnOcOmcDLYw2nNSa52FSjpwaOTo707/6ozg8dS5QA96/+mRtk+JR3zhz6gDA43TJ1FICCubvXJJF4fXuAg2cJ5gkcDGnukAxuO401UexfDyMqKxl7QPotCgNy/dv2AoD7h+3J4cxi6Q== Sender: owner-linux-mm@kvack.org Precedence: bulk X-Loop: owner-majordomo@kvack.org List-ID: List-Subscribe: X-Spam-Checker-Version: SpamAssassin 4.0.1 (2024-03-26) on aws-us-west-2-korg-listserv-alma10-1 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