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h=Date:From:To:Cc:Subject:References:In-Reply-To; b=afKyfzPwMZnX8RIkQ62Ya83mT2tiXh6A2Y4ngnA3DTORF80aPWcCsA9Ow9tWFaL8P Q3SeijIk5i44X5Pb5k1chgqbdmQBVYOP3/HhrbGvZIK87zpNHToS09FwTEQEOm0L4d nEs0ObzT+Pwcm7Rms4afNdAt62/YdphsvEugwPWGcRZ7hd9WuzURbZ446f88NHkACq KE0n9i1u+Hem+mJsgbcGvS0vJ1Q2wdwpgNUHCP4ztTBnHsukGxjgKCYYzx9RI7Dn/a n4HtV2VhwPI7pkIPFGuV+xJkLZblh3NOxl5FA5LRYtRH68rxb65adzMMRE1aiklf0P dwN5Hw0jqlhDg== Date: Wed, 3 Jun 2026 07:35:42 +0200 From: "Oscar Salvador (SUSE)" To: Kaitao Cheng Cc: Andrew Morton , Vlastimil Babka , Suren Baghdasaryan , Michal Hocko , Brendan Jackman , Johannes Weiner , Zi Yan , Liu Shixin , David Hildenbrand , Oscar Salvador , muchun.song@linux.dev, linux-mm@kvack.org, linux-kernel@vger.kernel.org, Kaitao Cheng Subject: Re: [PATCH v2] mm: page_isolation: avoid unsafe folio reads while scanning compound pages Message-ID: References: <20260602130755.38794-1-kaitao.cheng@linux.dev> MIME-Version: 1.0 Content-Type: text/plain; charset=us-ascii Content-Disposition: inline In-Reply-To: <20260602130755.38794-1-kaitao.cheng@linux.dev> X-Rspamd-Queue-Id: 7E91D1A0009 X-Stat-Signature: yxy8rxs7wx7qpw8afs7k96jweo1uzumi X-Rspamd-Server: rspam03 X-Rspam-User: X-HE-Tag: 1780464949-810312 X-HE-Meta: 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 Eoz1rdAY 6aVTY8TE1W9mHz2zU8knwZL5Y/KwI8p6XcWcyIoJ9fL1aYueWH0Jjo3VBl1Pnfw30eHDla4+oXigvgymn65pHKMx5OadSEXmJoItxGqW2IijRdw6eAV5hrycCQoD8F3LR7XhDreUBuOsr03M9viz6jqQhHUaM+SvzD8w85jj+tsVX3mymTd8salbApmbsw9SHDt3xcLpXu621ttIlt2JPml0pNqrSv0t+hIgDww9128HvOOuRM/C3hgxA+xDS8qGfow/P31ERVc8yjtHT7Le7xD65IYStG83bi+X9 Sender: owner-linux-mm@kvack.org Precedence: bulk X-Loop: owner-majordomo@kvack.org List-ID: List-Subscribe: List-Unsubscribe: On Tue, Jun 02, 2026 at 09:07:55PM +0800, 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 Acked-by: Oscar Salvador (SUSE) -- Oscar Salvador SUSE Labs