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b=I0mNp4fipHMFLsjeiHarIAc/AwZDY9TKHt5YzeQ0/nt867h0ePIjiGzboazBeD7E/ qf3KhcpySwgWtRMQNm5/JpW03pqS6I+yw+4ZiNSlB2tv1R4Ah6SIhNWfWudHg5Fpwo mLJpvrfaDRa+5Obu3VMjc2xdccC3NcKIRkV2x4Yz3S3ZzOakmyhrqVeobC2Cx1C6l+ 6e1zs74Jdov87CNBJlD5NVUBetgD9peYD7SvmH7NEZC7FjaDhNxjCRE76Ru85t7FZd FTcNqVIh2orMCl4iF75xI6ioixzwDNuOfKsDa6RIhPOA56Cf7MeW0O2gxEBuHWjbCq +hwjmm5JiVU+A== Message-ID: <7d199cfd-cf45-4545-b9fd-19b9dbee4b6e@kernel.org> Date: Tue, 2 Jun 2026 21:05:19 +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: Kaitao Cheng , Andrew Morton , Vlastimil Babka , Suren Baghdasaryan , Michal Hocko , Brendan Jackman , Johannes Weiner , Zi Yan Cc: 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> 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: rspam06 X-Rspamd-Queue-Id: 3AEF61C000A X-Stat-Signature: t1zpaze9feozymuehma8ftgxrjeuefek X-Rspam-User: X-HE-Tag: 1780427125-63687 X-HE-Meta: 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 yXhRvwt0 9/EZvHX+3/VidpgJUwbz+u164CoIDKXUow4SzlOeVDohCj8k22FeEEG7MB38jJgrk6RElC3GrMwf8giMK2+F2Y8UsQm+ObgDNECnOwIeeKezoyF+cwlolyg3ZQp8EI5nfNX/OPaHY/gvowooXIARPZe/fpQD+c3kvb2k/MlVmsBpv6/KJP+2qKxD7Ysj8zE16sgpgIUsjIPtlhG3+DTr0tVEnH/4rSLZPqn9u+zbsgINeGU2QHotEh39IR32FApLY3qGpjRPUDaBLN9x6C97qDwO37vTyB1MEKczyLpnOx3fYyj33YxjlbS9dFUDmC2XhgIhSO/JLdKrhcr4= Sender: owner-linux-mm@kvack.org Precedence: bulk X-Loop: owner-majordomo@kvack.org List-ID: List-Subscribe: List-Unsubscribe: On 6/2/26 15:07, 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)) { Nit: you're not spelling that change out in the patch description, but it's alright. > 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)) This change might have been better in a separate patch, but it should also be alright (and never really trigger, ever ...). Acked-by: David Hildenbrand (Arm) -- Cheers, David