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b=Q4lP3MaDlvX0mnm+6mrSmEmXPxxkFpEi2fB2vW5lkMSVz8Zrz8VGVqJVy6GgIp8So QTnAE+C2nMV/0l1b+EWq+n1Edjt8lSCKs+BX20RARRcDfsFiA7rtVuQf/GX5UcCj0N LbeR04Kk04HGzVKm18l3h7lfWbZhGJX0HaVBN1EWZGjEICPefs7tqkToLgz3RX8KjG UIxMjrZQ8al+nyS8CNGDAgWHwWgPJTajRwOVp1BqzT8r2bUBaJ/lUJ3B9r2fzGmWCD dju8sklgpO+9GPZ8StjqxbHDED3XQT4zA21G+enU5n+Kv6Ek9rfSFWuQavH4e6n4MA N45+fzAEm+o2A== Message-ID: Date: Tue, 2 Jun 2026 20:56:36 +0200 MIME-Version: 1.0 User-Agent: Mozilla Thunderbird Subject: Re: [PATCH v2] mm: page_isolation: avoid unsafe folio reads while scanning compound pages To: Zi Yan , Kaitao Cheng Cc: Andrew Morton , Vlastimil Babka , Suren Baghdasaryan , Michal Hocko , Brendan Jackman , Johannes Weiner , Liu Shixin , Oscar Salvador , muchun.song@linux.dev, linux-mm@kvack.org, linux-kernel@vger.kernel.org, Kaitao Cheng References: <20260602130755.38794-1-kaitao.cheng@linux.dev> <34F0943E-4ACD-4655-B9B8-41F658FCDB7E@nvidia.com> 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-Queue-Id: 13FED100003 X-Stat-Signature: zt1bemad18jboxu7jzo5oacs6h8i8tpa X-Rspam-User: X-Rspamd-Server: rspam12 X-HE-Tag: 1780426601-799437 X-HE-Meta: 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 fNMN37h7 iUcECd9t7PcsEdZ1qR73/pCiASdfFzdO8gPz6sJBqmq4iw11DA/1vQskyO3Vc/ErBGCUDGQ8EAW/A1Oq4A5DrXqpb2j9/2xzLcFW86He2l9f/+J4MeoB55i1+4xQYjmMjMBVSrvd7ogrk2PDzalM8a+InAsYVSfXYavJK0EEG1l+5jrohYf7kIElBGKO/ODPK7zgAFwJ9v/IeqxvPT+n0f/lIVpxuU8VQH4ELDj9NgnMdxUQvvd+cKXNr2fxpXLdGJQd/dP3pMjB8glkzLtUBuq7eS41qbwjVLJHhRjOvCRW/wXYmPr2rsTebUZJ+PykAIYOltnrRhTxEMao= Sender: owner-linux-mm@kvack.org Precedence: bulk X-Loop: owner-majordomo@kvack.org List-ID: List-Subscribe: List-Unsubscribe: On 6/2/26 17:02, Zi Yan wrote: > On 2 Jun 2026, at 9:07, Kaitao Cheng wrote: > >> From: Kaitao Cheng >> >> page_is_unmovable() can inspect compound pages without holding a folio >> reference or any lock. The folio can therefore be freed, split or reused >> while the scanner is still looking at it. >> >> The existing HugeTLB handling already avoids folio_hstate() for this >> reason, but it still derives the hstate from folio_size() and later >> derives the scan step from folio_nr_pages() and folio_page_idx(). >> These helpers rely on the folio still being a valid folio head. If >> the folio changed concurrently, the scanner can read inconsistent folio >> metadata and compute a wrong step. In the worst case, folio_nr_pages() >> can return 1 for what used to be a tail page and the subtraction from >> folio_page_idx() can underflow. >> >> There is a similar issue for non-Hugetlb compound pages: folio_test_lru() >> expects a valid folio. If the previously observed head page has been >> reused as a tail page of another compound page, the folio flag checks >> can trigger VM_BUG_ON_PGFLAGS(). >> >> Read the compound order once with compound_order(), reject obviously >> bogus orders, and derive the hstate and scan step from that order >> instead of querying folio size information again. Also use PageLRU(page), >> which is safe for the page being scanned, instead of folio_test_lru() >> on a potentially stale folio pointer. >> >> Treat an unknown HugeTLB hstate as unmovable so the scanner does not try >> to skip over an unstable HugeTLB folio. >> >> Fixes: a0a9f2180b90 ("mm: page_isolation: avoid calling folio_hstate() without hugetlb_lock") >> Signed-off-by: Kaitao Cheng >> --- >> Changes in v2: >> - Avoid unsafe folio metadata reads in the unlocked scanner by deriving >> the hstate and scan step from compound_order(). (David Hildenbrand, >> Andrew Morton) >> - Treat invalid compound orders or unknown HugeTLB hstates as unmovable. >> - Use PageLRU(page) instead of folio_test_lru(folio) to avoid folio flag >> checks on a stale folio pointer. () >> - Update the commit log (David Hildenbrand) >> >> Link to v1: >> https://lore.kernel.org/all/20260519121646.40833-1-kaitao.cheng@linux.dev/ >> >> --- >> mm/page_isolation.c | 19 +++++++++++++------ >> 1 file changed, 13 insertions(+), 6 deletions(-) >> >> diff --git a/mm/page_isolation.c b/mm/page_isolation.c >> index 7a9d631945a3..32ce8a7d9df3 100644 >> --- a/mm/page_isolation.c >> +++ b/mm/page_isolation.c >> @@ -41,8 +41,14 @@ bool page_is_unmovable(struct zone *zone, struct page *page, >> * We need not scan over tail pages because we don't >> * handle each tail page individually in migration. >> */ >> - if (PageHuge(page) || PageCompound(page)) { >> + if (PageCompound(page)) { >> struct folio *folio = page_folio(page); >> + unsigned long nr_pages, pfn; >> + unsigned int order; >> + >> + order = compound_order(&folio->page); >> + if (order > MAX_FOLIO_ORDER) >> + return true; >> >> if (folio_test_hugetlb(folio)) { >> struct hstate *h; >> @@ -54,15 +60,16 @@ bool page_is_unmovable(struct zone *zone, struct page *page, >> * The huge page may be freed so can not >> * use folio_hstate() directly. >> */ >> - h = size_to_hstate(folio_size(folio)); >> - if (h && !hugepage_migration_supported(h)) >> + h = size_to_hstate(PAGE_SIZE << order); >> + if (!h || !hugepage_migration_supported(h)) >> return true; >> - >> - } else if (!folio_test_lru(folio)) { >> + } else if (!PageLRU(page)) { >> return true; >> } >> >> - *step = folio_nr_pages(folio) - folio_page_idx(folio, page); >> + nr_pages = 1UL << order; >> + pfn = page_to_pfn(page); >> + *step = (pfn | (nr_pages - 1)) + 1 - pfn; >> return false; >> } > > LGTM. Thanks. > > Just a comment, order can be dropped and use > nr_pages = compound_nr(&folio->page) instead: > 1. order > MAX_FOLIO_ORDER -> nr_pages > MAX_FOLIO_NR_PAGES > 2. PAGE_SIZE << order -> PAGE_SIZE * nr_pages. > But it is not worth a new version. If we use compound_nr, we have to check for power-of-2, like scan_movable_pages(), as folio_large_nr_pages() might use the per-computed value (which might read garbage). -- Cheers, David