From mboxrd@z Thu Jan 1 00:00:00 1970 Received: from smtp.kernel.org (aws-us-west-2-korg-mail-1.web.codeaurora.org [10.30.226.201]) (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 5E4D525A321; Mon, 9 Feb 2026 16:16:31 +0000 (UTC) Authentication-Results: smtp.subspace.kernel.org; arc=none smtp.client-ip=10.30.226.201 ARC-Seal:i=1; a=rsa-sha256; d=subspace.kernel.org; s=arc-20240116; t=1770653791; cv=none; b=UFPXt0Sddx6SoDQvvyvy7Aug2nt1iwZ4+82gx+7I7wbLVQKbJMCAgn79rvewGDokYa3jsl2m+Ry4rhAS15i/w6uWL1xGYmcZS92Staa/yxJKUbCkbvWeSDQMYJxaucQNoo/y6eSZnKimqjApO8zCJL52x7H3yUpiSHtrraPDItc= ARC-Message-Signature:i=1; a=rsa-sha256; d=subspace.kernel.org; s=arc-20240116; t=1770653791; c=relaxed/simple; bh=vt1m/YOd9IGoIYCdeHUIlqZ2Oqusir4jrcBZWBjYrL0=; h=Message-ID:Date:MIME-Version:Subject:To:Cc:References:From: In-Reply-To:Content-Type; b=JzYPiyEXpd3DaldsJYJYFKoFDJRUKwIYSX1FSbpIoJyqrYWE7i9FfSeo8g8a9olbkMrRtuehgUIiQZf4cPuF45Il0DGXXY0o+2Mj9H1aXy8DSr920t4kiLkARIu1EPTmwSgyB7dIqBwM7M608YrOqI03AMwGvxX8muTSS1o1DAI= ARC-Authentication-Results:i=1; smtp.subspace.kernel.org; dkim=pass (2048-bit key) header.d=kernel.org header.i=@kernel.org header.b=oQEG6vdy; arc=none smtp.client-ip=10.30.226.201 Authentication-Results: smtp.subspace.kernel.org; dkim=pass (2048-bit key) header.d=kernel.org header.i=@kernel.org header.b="oQEG6vdy" Received: by smtp.kernel.org (Postfix) with ESMTPSA id ECD5EC116C6; Mon, 9 Feb 2026 16:16:25 +0000 (UTC) DKIM-Signature: v=1; a=rsa-sha256; c=relaxed/simple; d=kernel.org; s=k20201202; t=1770653791; bh=vt1m/YOd9IGoIYCdeHUIlqZ2Oqusir4jrcBZWBjYrL0=; h=Date:Subject:To:Cc:References:From:In-Reply-To:From; b=oQEG6vdym0d7y+mDS4mckjspiFrBRMy57rK7QQCVbwtglGbBixQOnocFEnuomyYgU XJGp6JvCvqxa5RyOLw1GlIHlU+EA59hbAclZbUf9fHbtgZsTrElXNUs1dzXqAWX7Hd aNOisi0TmkJr1ZPpcbAMzUps35Jq8FV0/ON11B8L2Dew2WOB+esSYOJWstM446v9ZK VlI1NSA9GmnFlPrpD6Kn1HDOwryS32He90Al746jC5fdRT+xVetB06P6gvzvDag2xj faXEKPtayMSiQk5Eo9V/fOJzHGB5cdEub80Xex37WYJKe+KUGjHiVxCnFlLdL4dVsg +gUOy2abjKAbA== Message-ID: Date: Mon, 9 Feb 2026 17:16:22 +0100 Precedence: bulk X-Mailing-List: stable@vger.kernel.org List-Id: List-Subscribe: List-Unsubscribe: MIME-Version: 1.0 User-Agent: Mozilla Thunderbird Subject: Re: [PATCH v3] mm/page_alloc: clear page->private in free_pages_prepare() To: Zi Yan Cc: Mikhail Gavrilov , linux-mm@kvack.org, akpm@linux-foundation.org, vbabka@suse.cz, surenb@google.com, mhocko@suse.com, jackmanb@google.com, hannes@cmpxchg.org, npiggin@gmail.com, linux-kernel@vger.kernel.org, kasong@tencent.com, hughd@google.com, chrisl@kernel.org, ryncsn@gmail.com, stable@vger.kernel.org, willy@infradead.org References: <209207FE-D3A9-4BE2-8DA7-9BE38A19F387@nvidia.com> <20260207173615.146159-1-mikhail.v.gavrilov@gmail.com> <0BC1D792-80CA-4E60-AEA0-187F73BD4723@nvidia.com> From: "David Hildenbrand (Arm)" Content-Language: en-US Autocrypt: addr=david@kernel.org; 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charset=UTF-8; format=flowed Content-Transfer-Encoding: 7bit >> I recall that freeing pages with page->private set was allowed. Although >> I once wondered whether we should actually change that. > > But if that is allowed, we can end up with tail page's private non zero, > because that free page can merge with a lower PFN buddy and its ->private > is not reset. See __free_one_page(). Right. Or someone could use page->private on tail pages and free non-zero ->private that way. [...] > > For the issue reported by Mikhail[2], the page comes from vmalloc(), so it will not be split. > For other cases, a page/folio needs to be compound to be splittable and prep_compound_tail() > sets all tail page's private to 0. So that check is not that useful. Thanks. > > And the issue we are handling here is non compound high order page allocation. No one is > clearing ->private for all pages right now. Right. > > OK, I think we want to decide whether it is OK to have a page with set ->private at > page free time. Right. And whether it is okay to have any tail->private be non-zero. > If no, we can get this patch in and add a VM_WARN_ON_ONCE(page->private) > to catch all violators. If yes, we can use Mikhail's original patch, zeroing ->private > in split_page() and add a comment on ->private: > > 1. for compound page allocation, prep_compound_tail() is responsible for resetting ->private; > 2. for non compound high order page allocation, split_page() is responsible for resetting ->private. Ideally, I guess, we would minimize the clearing of the ->private fields. If we could guarantee that *any* pages in the buddy have ->private clear, maybe prep_compound_tail() could stop clearing it (and check instead). So similar to what Vlasta said, maybe we want to (not check but actually clear): diff --git a/mm/page_alloc.c b/mm/page_alloc.c index e4104973e22f..4960a36145fe 100644 --- a/mm/page_alloc.c +++ b/mm/page_alloc.c @@ -1410,6 +1410,7 @@ __always_inline bool free_pages_prepare(struct page *page, } } (page + i)->flags.f &= ~PAGE_FLAGS_CHECK_AT_PREP; + set_page_private(page + i, 0); } } if (folio_test_anon(folio)) { And the try removing any other unnecessary clearing (like in prep_compound_tail()). -- Cheers, David