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t=1794763940; bh=jqEVXWdaDYy2iAwkzt+easl1YaYbtwylCIe+qCtSDRA=; h=Date:Subject:To:Cc:References:From:In-Reply-To; b=hkLB5ySA63uJy/oKlRFoLkV12uImhhP8T8+p3m8feeKxGPiq3DNfzOzYhilQMBfsh E3W1f4LmGd2CaxEnu5JRsvre2AXq/Fi10BhYw0/5mP/kycAxfvC9E5jCnHFZZgxVQ0 DCGeZaw9gEmWG3l00yW7cz7h7Kc0m/9+TbW+D+Ugi2uAggTxs4l+hkD1a67dSyMs93 QUVYsUz54GnIJstYxlTV4O62VxXb4DLrM9aSxkYb510V/v4TgmRbfEZyNES8GSeR+k kTfVCsN46ZWbA89WtiV17yXSH6SkQYdQfzOnygoObX+1hntbhhQQY6KxNCPbm4K8Sa k9194hCfkVzPw== Message-ID: <72dc7b45-ad20-450b-8ec7-3e86be8a41f8@kernel.org> Date: Sun, 15 Nov 2026 18:32:12 +0100 X-Archived-By: MIME-Version: 1.0 User-Agent: Mozilla Thunderbird Subject: Re: [PATCH] mm/util: fix off-by-one when checking folio index in snapshot_page() To: "Lorenzo Stoakes (ARM)" , shao.mingyin@zte.com.cn Cc: akpm@linux-foundation.org, linux-mm@kvack.org, liam@infradead.org, vbabka@kernel.org, rppt@kernel.org, surenb@google.com, mhocko@suse.com, linux-kernel@vger.kernel.org References: <20261115004657890Pz8ERDf5nCtE-B6M7KFzu@zte.com.cn> 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-Spam-Status: No, score=-17.3 required=3.0 tests=BAYES_00,DKIMWL_WL_HIGH, DKIM_SIGNED,DKIM_VALID,DKIM_VALID_AU,HEADER_FROM_DIFFERENT_DOMAINS, INCLUDES_CR_TRAILER,INCLUDES_PATCH,MAILING_LIST_MULTI,SPF_HELO_PASS, SPF_PASS,USER_AGENT_SANE_1 autolearn=ham autolearn_force=no version=4.0.1 X-Rspamd-Server: rspam07 X-Rspamd-Queue-Id: 00A091A0004 X-Stat-Signature: brjoroqum64kz7hzwrdfrztdhtmqkm5b X-Rspam-User: X-HE-Tag: 1794763941-120888 X-HE-Meta: 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 DbPmriOA FA+qhReg1I0xjc+A5YTdsgJMfys2O/6hAjyhTX2pMhHGg0gqOxFN79gz7/Bh52ixfD1blCx947ClVZGvZ9GvfD78bi70C0FHI/UWGhZmlFITWfxGWdTMWwMtRqCJfQqIP3CXwmdVrKQF0Bl2oGYeEuAwsmALvycaCqCBoBegTBhlv0Hk8jfd3Hz1o/lxYPplq4E0Lp7kg84QBYcxBzYtqXCaZaCWu/RHv3tbbmi7sRS0ok39ew4tcOYlnf9zv2Ddo141f+/YGYBQoAu9TbGFE6zstWlnqRdBjIyJoh4qGrIQvvuaNg0Bd2iO9WuOEULXDaiq7bVXly2lIUnyJ1mhx6N7b03uFoqXTUhtSCK3ZZkPXhOg7RT/lT8aEBd+my3ciVC/CUx6GWwjUgLQ= 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 11/14/26 18:33, Lorenzo Stoakes (ARM) wrote: > On Sun, Nov 15, 2026 at 12:46:57AM +0800, shao.mingyin@zte.com.cn wrote: >> From: Shao Mingyin >> >> mm/util: fix off-by-one when checking folio index in snapshot_page() >> >> snapshot_page() computes the page index before copying the folio state. >> If the folio changes between those reads, the index can equal the copied >> folio's page count. Valid indices are strictly less than that count, >> but the current check accepts equality and marks the snapshot faithful. >> >> Use >= so that this inconsistent snapshot is retried, or falls back to >> the unfaithful single-page representation. This also avoids reporting an >> out-of-range folio index without the corresponding dump_page() warning. >> >> Fixes: d863a12108f2 ("mm/util: introduce snapshot_page()") >> Signed-off-by: Shao Mingyin > > NAK. Let's do the following:. Thoughts? >From c7d1a79926e3b3448bb5e84f901c185234f43b52 Mon Sep 17 00:00:00 2001 From: "David Hildenbrand (Arm)" Date: Sun, 15 Nov 2026 18:11:08 +0100 Subject: [PATCH] mm/util: fix retry detection and handling in snapshot_page() snapshot_page() detects races with concurrent folio split+construction by checking whether the given page still falls into the folio. Unfortunately, we have an off-by-one error in there, and we should be checking for "ps->idx >= nr_pages", because idx 0 is valid. For example, we might currently report a tail page at idx 1 as faithful, although the snapshotted folio is now a small folio. However, there is one complication: As we don't hold a folio reference, folio_nr_pages() can return 0 on races (or when called from page freeing code). Handle that by just setting it to 1 in that case, so we don't wrongly report a head page as "page does not match folio". In addition, we should reset ps->flags when retrying, by just resetting it to PAGE_SNAPSHOT_FAITHFUL. Otherwise, PAGE_SNAPSHOT_PG_BUDDY and PAGE_SNAPSHOT_PG_IDLE from a failed attempt could stick around. These races are fairly rare, so this is not really critical, we simply report something slightly wrong. No need to CC stable. Fixes: d863a12108f2 ("mm/util: introduce snapshot_page()") Reported-by: Shao Mingyin Signed-off-by: David Hildenbrand (Arm) --- mm/util.c | 7 +++++-- 1 file changed, 5 insertions(+), 2 deletions(-) diff --git a/mm/util.c b/mm/util.c index 80f6c714844f2..3b10d4c2a4fd9 100644 --- a/mm/util.c +++ b/mm/util.c @@ -1406,9 +1406,9 @@ void snapshot_page(struct page_snapshot *ps, const struct page *page) int loops = 5; ps->pfn = page_to_pfn(page); - ps->flags = PAGE_SNAPSHOT_FAITHFUL; again: + ps->flags = PAGE_SNAPSHOT_FAITHFUL; memset(&ps->folio_snapshot, 0, sizeof(struct folio)); memcpy(&ps->page_snapshot, page, sizeof(*page)); info = ps->page_snapshot.compound_info; @@ -1438,13 +1438,16 @@ void snapshot_page(struct page_snapshot *ps, const struct page *page) if (ps->idx < MAX_FOLIO_NR_PAGES) { memcpy(&ps->folio_snapshot, foliop, 2 * sizeof(struct page)); nr_pages = folio_nr_pages(&ps->folio_snapshot); + /* On races, or when freeing pages, we can read 0. */ + if (nr_pages == 0) + nr_pages = 1; if (nr_pages > 2) memcpy(&ps->folio_snapshot.__page_2, &foliop->__page_2, sizeof(struct page)); set_ps_flags(ps, foliop, page); } - if (ps->idx > nr_pages) { + if (ps->idx >= nr_pages) { if (loops-- > 0) goto again; clear_compound_head(&ps->page_snapshot); -- 2.56.1 -- Cheers, David