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 6F1683939C2 for ; Wed, 1 Jul 2026 06:34:55 +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=1782887696; cv=none; b=GXJqwDt6xwcqeIBkVJDxcOvAac7eyU+LrWAi6hWOfD/UaDV+qVUpy4d4zu4S6FEqlIXlO7lfP3UULemVg2MzKudQ1I1ExBgy5q/PwFGpc4VSuRCHqrrFuH4qIHikr5wqHzrGYLeprECpcBcfrcyJm7mcp0WsDbR3Q9SvulDD5zg= ARC-Message-Signature:i=1; a=rsa-sha256; d=subspace.kernel.org; s=arc-20240116; t=1782887696; c=relaxed/simple; bh=CGPRzKWrGOGhebhKvc0cR9nhiO4WsZUz4JX9s/W1+Dg=; h=Message-ID:Date:MIME-Version:Subject:To:Cc:References:From: In-Reply-To:Content-Type; b=E61FT0tH1v+lxUWTXukdUxiBRYV5VCa4DVVHWl20eUWXNbFynhmpcwIW9LQJskJRZcvuVJLeL07KAm1l/k7u49LGk6GlCyWklqV/n5EvmnwB9oxy0Ol6hhElggTwRJulvq158msyYLEtBBdCTozXNkV2/vmYPJt0l0v+SHYGbf8= ARC-Authentication-Results:i=1; smtp.subspace.kernel.org; dkim=pass (2048-bit key) header.d=kernel.org header.i=@kernel.org header.b=hcdE4sGa; 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="hcdE4sGa" Received: by smtp.kernel.org (Postfix) with ESMTPSA id 535331F000E9; Wed, 1 Jul 2026 06:34:53 +0000 (UTC) DKIM-Signature: v=1; a=rsa-sha256; c=relaxed/relaxed; d=kernel.org; s=k20260515; t=1782887695; bh=ArEdv1dW3RnbP6iwECWGTwNieMpk9J9I1LK+vA9CFI4=; h=Date:Subject:To:Cc:References:From:In-Reply-To; b=hcdE4sGaKrMTw9Ez6cwQbV3LqQDi39agtcLpvIWBZ1Pr+ZnhZZoUrRNzUNqtTJJzz 12NlCUaeCZuLOVuLS4uPeI0KQDpvDiuFjdugeHQgUA5BQIMXwqNTyyI69A8zwfbPE5 OamGBpggYOoeFjTjcZm3OWG83fQ3kjGDpaEAoI6sVOpeXKbXyuPu8qqL+OrwoudhKv ZjhPvzn1czfw47+FMyTR3h/kqLo5BudB3HW2toqFZNOK3Fc83LviibvJNiRbnzKH3V rUWPg8Ruxvo99gr8C9UbwmeHha0GS1aZeuI4BmgBuqLcEt2i7dZeK74AeDYKIiFx5c bh+H+tMpUEksg== Message-ID: Date: Wed, 1 Jul 2026 08:34:51 +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 v5 8/9] mm/page_owner: clamp skip_buddy_pages() PFN advance at MAX_ORDER_NR_PAGES boundary To: Ye Liu , Andrew Morton Cc: Suren Baghdasaryan , Michal Hocko , Brendan Jackman , Johannes Weiner , Zi Yan , linux-mm@kvack.org, linux-kernel@vger.kernel.org References: <20260701061101.344679-1-ye.liu@linux.dev> <20260701061101.344679-9-ye.liu@linux.dev> From: "Vlastimil Babka (SUSE)" Content-Language: en-US Autocrypt: addr=vbabka@kernel.org; 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charset=UTF-8 Content-Transfer-Encoding: 7bit On 7/1/26 08:10, Ye Liu wrote: > The lockless buddy_order_unsafe() read can return a garbage order > value if the page is concurrently allocated between the PageBuddy > check and the private read. If this bogus order is <= MAX_PAGE_ORDER, > skip_buddy_pages() would arbitrarily advance the PFN, potentially > jumping past a MAX_ORDER_NR_PAGES boundary whose pfn_valid() check > would have caught an offline memory section. > > In read_page_owner(), which relies solely on boundary-aligned > pfn_valid() to guard pfn_to_page(), skipping the boundary could > cause pfn_to_page() to access an unmapped mem_section. > > Clamp the advance so it never crosses the next MAX_ORDER_NR_PAGES > boundary. This is safe for all three callers: the pageblock-iterating > ones already handle boundary transitions in their outer loops, and > for read_page_owner() the worst case is one extra PageBuddy check per > 1024 pages for a huge buddy block straddling the boundary. I don't see how a huge buddy block can straddle the boundary, as the largest buddy block order is MAX_ORDER? > Signed-off-by: Ye Liu Other than that, LGTM Reviewed-by: Vlastimil Babka (SUSE) > --- > mm/page_owner.c | 14 ++++++++++++-- > 1 file changed, 12 insertions(+), 2 deletions(-) > > diff --git a/mm/page_owner.c b/mm/page_owner.c > index 46a933f9c229..2e3880053a34 100644 > --- a/mm/page_owner.c > +++ b/mm/page_owner.c > @@ -428,6 +428,12 @@ void __folio_copy_owner(struct folio *newfolio, struct folio *old) > * to skip less than the full buddy block, but that is acceptable for page owner > * iteration purposes. > * > + * The lockless read of buddy_order_unsafe() can also return a garbage order if > + * the page is concurrently allocated and PageBuddy is cleared between the check > + * and the read. Clamp the advance at the next MAX_ORDER_NR_PAGES boundary so > + * that a bogus order cannot carry @pfn into an unvalidated memory section, > + * which would break callers that rely on boundary-aligned pfn_valid() checks. > + * > * Return: true if the page was skipped (caller should continue its loop), > * false if the page is not a buddy page and should be processed normally. > */ > @@ -439,8 +445,12 @@ static inline bool skip_buddy_pages(unsigned long *pfn, struct page *page) > return false; > > order = buddy_order_unsafe(page); > - if (order <= MAX_PAGE_ORDER) > - *pfn += (1UL << order) - 1; > + if (order <= MAX_PAGE_ORDER) { > + unsigned long new_pfn = *pfn + (1UL << order); > + unsigned long boundary = ALIGN(*pfn + 1, MAX_ORDER_NR_PAGES); > + > + *pfn = min(new_pfn, boundary) - 1; > + } > > return true; > }