From mboxrd@z Thu Jan 1 00:00:00 1970 Received: from smtp.kernel.org (aws-us-west-2-korg-mail-alma10-1.taild15c8.ts.net [100.103.45.18]) (using TLSv1.2 with cipher ECDHE-RSA-AES256-GCM-SHA384 (256/256 bits)) (No client certificate requested) by smtp.subspace.kernel.org (Postfix) with ESMTPS id 2E5EC3AA187 for ; Wed, 20 May 2026 08:51:31 +0000 (UTC) Authentication-Results: smtp.subspace.kernel.org; arc=none smtp.client-ip=100.103.45.18 ARC-Seal:i=1; a=rsa-sha256; d=subspace.kernel.org; s=arc-20240116; t=1779267092; cv=none; b=WqeB+xXHPZeIq7wkXYBRSGN6TmsP+47vR6BICmEda00KiXSjVen+xhEx1EQsp8I7W3qztV4TuqemrLY6w1EP5VW31ZQ+/Hgs1IqJabl5O28zefVGm9S0MI5lfjSSeOQ+8Mzc4FKehVfkPCJ9k+2ceNcTvC0bp7QEvM2853nhqNA= ARC-Message-Signature:i=1; a=rsa-sha256; d=subspace.kernel.org; s=arc-20240116; t=1779267092; c=relaxed/simple; bh=48ggP2aw+ewO52ROHUaplXUvIM7l27QutY7RkM62isI=; h=Message-ID:Date:MIME-Version:Subject:To:Cc:References:From: In-Reply-To:Content-Type; b=reJ0MZ0OgrE5rZmkS2Mpc9eEeJxTf1lo+nWVoNQxpX6RYDBQoNOLjKFgEdU7iMjB7dapcYMjGQBTwdBwjUUM0X3DrWibIPceDPizVOgjXjMIWBFQwODEM0rgwc9+xdR1ynfKnK4WSEJtWpk2maYk1Kio80O43WdoGNJS9RpFNw4= ARC-Authentication-Results:i=1; smtp.subspace.kernel.org; dkim=pass (2048-bit key) header.d=kernel.org header.i=@kernel.org header.b=YITMVxI0; arc=none smtp.client-ip=100.103.45.18 Authentication-Results: smtp.subspace.kernel.org; dkim=pass (2048-bit key) header.d=kernel.org header.i=@kernel.org header.b="YITMVxI0" Received: by smtp.kernel.org (Postfix) with ESMTPSA id D9AD91F000E9; Wed, 20 May 2026 08:51:26 +0000 (UTC) DKIM-Signature: v=1; a=rsa-sha256; c=relaxed/relaxed; d=kernel.org; s=k20260515; t=1779267091; bh=xkHRzme42JJuxy3nJwsuuPvUrnL/cbY5gaMHg394CUg=; h=Date:Subject:To:Cc:References:From:In-Reply-To; b=YITMVxI0kmGqk5xyYUEAUISHRK6mrgw8RhS65mTnLFJ707/nzgBozxgQA31Wv1M2E RKD04ao4XcD239NOi0s7HTcxf9e6eziHO+ToRGbCJnyOQ/JGIf8HLPnQm2ZrYvRVju 11U4VTdHnusqlsWsrPPgT3MYIwcXOQmKsHP6IjXnCF2r+CAM3YdFYeeooSln7wDkJY T5AIsyCtoRfd1p6gLKlHz8R2w3WOhhUhkCxGfId9AzP8knIfb330KeSHe5ijdqxFS6 Fqnqeut2wSxWeUPWsYBBOHBmEhTTrKox4saJLc1cjpA9pXMmz3Zw9aTN/PHxwvQ5aI U4ptrT6sI65vw== Message-ID: Date: Wed, 20 May 2026 10:51:24 +0200 Precedence: bulk X-Mailing-List: linux-kernel@vger.kernel.org List-Id: List-Subscribe: List-Unsubscribe: MIME-Version: 1.0 User-Agent: Mozilla Thunderbird Subject: Re: [PATCH] mm: page_isolation: Avoid hugepage scan step underflow To: Kaitao Cheng , akpm@linux-foundation.org, vbabka@kernel.org, surenb@google.com, mhocko@suse.com, jackmanb@google.com, hannes@cmpxchg.org, ziy@nvidia.com Cc: liushixin2@huawei.com, osalvador@suse.de, linux-mm@kvack.org, linux-kernel@vger.kernel.org, Kaitao Cheng References: <20260519121646.40833-1-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 On 5/19/26 14:16, Kaitao Cheng wrote: > From: Kaitao Cheng > > page_is_unmovable() checks HugeTLB pages without holding hugetlb_lock and > without pinning the folio. This is intentional for the pageblock scanning > paths, but it means the HugeTLB folio can be freed concurrently after > PageHuge() or folio_test_hugetlb() succeeds. > > The existing code avoids folio_hstate() and uses size_to_hstate() because > the HugeTLB flag may already have been cleared. However, if > size_to_hstate() returns NULL, the code still falls through and computes > the scan step from folio_nr_pages(). If the folio has been freed and the > head/large state has been cleared, folio_nr_pages() can return 1. When the > current page is a tail page, subtracting folio_page_idx() from 1 can > underflow and make the scanner skip too far. > > Treat a NULL hstate as unmovable so the scanner does not try to skip over > an unstable HugeTLB folio. Once a valid hstate is found, derive the number > of pages from the hstate instead of reading the folio size again. Also > validate the page index before computing the step to avoid underflow if the > page/folio relationship changed concurrently. > > Fixes: a0a9f2180b90 ("mm: page_isolation: avoid calling folio_hstate() without hugetlb_lock") > Signed-off-by: Kaitao Cheng > --- > mm/page_isolation.c | 12 ++++++++++-- > 1 file changed, 10 insertions(+), 2 deletions(-) > > diff --git a/mm/page_isolation.c b/mm/page_isolation.c > index c48ff5c00244..99f0b06efaf6 100644 > --- a/mm/page_isolation.c > +++ b/mm/page_isolation.c > @@ -43,6 +43,7 @@ bool page_is_unmovable(struct zone *zone, struct page *page, > */ > if (PageHuge(page) || PageCompound(page)) { > struct folio *folio = page_folio(page); > + unsigned long idx, nr_pages; > > if (folio_test_hugetlb(folio)) { > struct hstate *h; > @@ -55,14 +56,21 @@ bool page_is_unmovable(struct zone *zone, struct page *page, > * use folio_hstate() directly. > */ > h = size_to_hstate(folio_size(folio)); > - if (h && !hugepage_migration_supported(h)) > + if (!h || !hugepage_migration_supported(h)) > return true; > > + nr_pages = pages_per_huge_page(h); > } else if (!folio_test_lru(folio)) { > return true; > + } else { > + nr_pages = folio_nr_pages(folio); > } > > - *step = folio_nr_pages(folio) - folio_page_idx(folio, page); > + idx = folio_page_idx(folio, page); > + if (idx >= nr_pages) > + return true; > + > + *step = nr_pages - idx; > return false; > } > We have very similar code in scan_movable_pages() that we previously fixed. Seems like we can avoid folio_page_idx() completely by just doing "pfn |= nr_pages - 1". If, in corner cases, we skip over some unmovable pages, too bad. The function is inherently racy. -- Cheers, David