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 EA4EF1F3BA4; Mon, 9 Feb 2026 16:20:37 +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=1770654038; cv=none; b=QSCWR7/U06hkviUx1HxHb5Stxzvz+oyBBnV4YSdHrpmQC5A/cN8VKDOEuVBieMx9XUqdeRRTP1RpuHRuBDr9afc8QzFQNGe9n8kKuOSmPvMClj+RM8DZTm9jgKO/nhKEq0dsOOoHtcEge3sG2gz0S9DoFWOBtbmwNZ3BUWhhiW4= ARC-Message-Signature:i=1; a=rsa-sha256; d=subspace.kernel.org; s=arc-20240116; t=1770654038; c=relaxed/simple; bh=nrLNw+FxvZHojX8ryLVPAy3U5gCQbGmv9uX9Z6FB9EU=; h=Message-ID:Date:MIME-Version:Subject:From:To:Cc:References: In-Reply-To:Content-Type; b=YhOsVD++aeoWwVnLtxpDdBcxAZJDVSsR2gB0TL9olzVastZgGD6MTjG8Ztgjnd5gHK87ysNZJ41L/GVpTxTKagpQJd66uXiumoFSstOYIARGEnJ7S+xo2ipvaM3u2J4lSMEjNvmbRkzrInJ7cijHjCyCOTa0wt5y83zrHGLKTpI= ARC-Authentication-Results:i=1; smtp.subspace.kernel.org; dkim=pass (2048-bit key) header.d=kernel.org header.i=@kernel.org header.b=N+NxrIrM; 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="N+NxrIrM" Received: by smtp.kernel.org (Postfix) with ESMTPSA id 3A5E1C116C6; Mon, 9 Feb 2026 16:20:31 +0000 (UTC) DKIM-Signature: v=1; a=rsa-sha256; c=relaxed/simple; d=kernel.org; s=k20201202; t=1770654037; bh=nrLNw+FxvZHojX8ryLVPAy3U5gCQbGmv9uX9Z6FB9EU=; h=Date:Subject:From:To:Cc:References:In-Reply-To:From; b=N+NxrIrM7kH02l5iDsHLo+ECn6R/kHcPbqfB3y3/J473sQscuaCu38HiYOxwaehtS gBAB+r+pW/ABoVOgdn41WKBSjqowY0Mlmf5bYxlSG4Cglc37eFaIByHTIt16NGCPAv SXwXoSDXptTF2nd1Htk3tzC9XZUt1nfBVazrug/1S2uwcVMjOq2UQJxfIwIto/mWa+ f174nOxtpsWId1rVGD5uJk1djaIvWFhj0/PR7JkdiD9lraBwWh07UDvnjOAkyjH5h/ GeoOLYe8AfLMf4ywXuagSMh4uYIeDHTqNjWxN5x/9CDlD9b4g7fLHhvIRa+9+UP4eT Hp6I+TqsjrAHw== Message-ID: <22431471-b569-4ade-9881-387debada00b@kernel.org> Date: Mon, 9 Feb 2026 17:20:28 +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() From: "David Hildenbrand (Arm)" 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> 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 17:16, David Hildenbrand (Arm) wrote: >>> 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); >                 } >         } 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. -- Cheers, David