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 6D4B63815D9; Mon, 9 Feb 2026 17:36:41 +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=1770658601; cv=none; b=uOSLf8EHISDkVL/SRXDf15++aSXhJkX8sAD3yPXtbmmOLuJ9VYXSXagTLR5u7AufE9ire197vsAHPyjt6WXy+XmWESBhiHEYz5w+lZcg5ClCL6mX30SinhGEL8yIpGpg89qcplqgN82S0QSU/PedYCyDXm/f4Qd2TmA0dIY+Fis= ARC-Message-Signature:i=1; a=rsa-sha256; d=subspace.kernel.org; s=arc-20240116; t=1770658601; c=relaxed/simple; bh=gfMaqqBwGsOY8oyPuNLoKaW9sW4dgip3p9mqVldeH+4=; h=Message-ID:Date:MIME-Version:Subject:To:Cc:References:From: In-Reply-To:Content-Type; b=czFxbTA+Q+3bRvSDHKQjp4f43rGcmYDR93xuneAFXi/GIBP3ui8iAXgi1nXAFcPCHDea3KR4/6yjc//flDVdoDZyPUUAMl0icSO2zKr0xGOFpQRwUQMg2WN/s1G7lNp6i1Z/7BIrLauwJG7fH7iqr/9ENYszpjZlcaynvKDG0G0= ARC-Authentication-Results:i=1; smtp.subspace.kernel.org; dkim=pass (2048-bit key) header.d=kernel.org header.i=@kernel.org header.b=ducPG5td; 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="ducPG5td" Received: by smtp.kernel.org (Postfix) with ESMTPSA id 24815C116C6; Mon, 9 Feb 2026 17:36:35 +0000 (UTC) DKIM-Signature: v=1; a=rsa-sha256; c=relaxed/simple; d=kernel.org; s=k20201202; t=1770658601; bh=gfMaqqBwGsOY8oyPuNLoKaW9sW4dgip3p9mqVldeH+4=; h=Date:Subject:To:Cc:References:From:In-Reply-To:From; b=ducPG5tdCmRCCzFAHTXlQMHKfNtB9ChlU5y7orZSMHaVqb2HKo/wvtM90GTI+Gjgp CR75ETZQGnloF7cqrrofJ+tEY+elLZvu+OibSLcwksQyBPgQZJHwzjgR+PDJoUdjUu yWwN07NZPKukGr/ih6vf/OTgSwCNACwRHcUjY3Mkw3UMHqvM71VuLxJ07rgVMlW4fz PopIINCcDNH4grywm4UQmL2EpEcPwPJ7Ntt8SKcwqModWEOe/yy2r46aNEXtwvwt7o j0rBb1h9vQR/Ypks7r9Ohx4QsAOJuPj2hZC7L3GOno09CJTxXNXXf4p4N4vBHy7IJ9 /VTFZkYo7PUAA== Message-ID: <546b200d-5b70-4db4-99f1-f50f6a343c10@kernel.org> Date: Mon, 9 Feb 2026 18:36:32 +0100 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 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> <22431471-b569-4ade-9881-387debada00b@kernel.org> <91F2E741-5473-4D34-ADA1-C9E6EDCBF5E0@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: 8bit On 2/9/26 17:33, Zi Yan wrote: > On 9 Feb 2026, at 11:20, David Hildenbrand (Arm) wrote: > >> On 2/9/26 17:16, David Hildenbrand (Arm) wrote: >>> >>> Right. Or someone could use page->private on tail pages and free non- zero ->private that way. >>> >>> [...] >>> >>> >>> Thanks. >>> >>> >>> Right. >>> >>> >>> Right. And whether it is okay to have any tail->private be non-zero. >>> >>> >>> 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); >>>                 } >>>         } >> >> Thinking again, maybe it is indeed better to rework the code to not allow freeing pages with ->private on any page. Then, we only have to zero it out where we actually used it and could check here that all >> ->private is 0. >> >> I guess that's a bit more work, and any temporary fix would likely just do. > > I agree. Silently fixing non zero ->private just moves the work/responsibility > from users to core mm. They could do better. :) > > We can have a patch or multiple patches to fix users do not zero ->private > when freeing a page and add the patch below. Do we know roughly which ones don't zero it out? > The hassle would be that > catching all, especially non mm users might not be easy, but we could merge > the patch below (and obviously fixes) after next merge window is closed and > let rc tests tell us the remaining one. WDYT? LGTM, then we can look into stopping to zero for compound pages. > > > diff --git a/mm/page_alloc.c b/mm/page_alloc.c > index 24ac34199f95..0c5d117a251e 100644 > --- a/mm/page_alloc.c > +++ b/mm/page_alloc.c > @@ -1411,6 +1411,7 @@ __always_inline bool free_pages_prepare(struct page *page, > } > } > (page + i)->flags.f &= ~PAGE_FLAGS_CHECK_AT_PREP; > + VM_WARN_ON_ONCE((page + i)->private); > } > } > if (folio_test_anon(folio)) { > @@ -1430,6 +1431,7 @@ __always_inline bool free_pages_prepare(struct page *page, > > page_cpupid_reset_last(page); > page->flags.f &= ~PAGE_FLAGS_CHECK_AT_PREP; > + VM_WARN_ON_ONCE(page->private); > page->private = 0; > reset_page_owner(page, order); > page_table_check_free(page, order); > > > Best Regards, > Yan, Zi -- Cheers, David