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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 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 X-Stat-Signature: yke36kqzfd79pa7dezxdtfdrdkrkim3i X-Rspamd-Queue-Id: 6C2AA180010 X-Rspam-User: X-Rspamd-Server: rspam04 X-HE-Tag: 1770654038-618282 X-HE-Meta: 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 CEJyu9eX KfJiwvNMwxrxffu0129LUf4yUM7wcFS5GLK9OfKDV3+iMY5MtfftZaw+qVa5CfTA/w1Yc5QnK4VTkYkk= X-Bogosity: Ham, tests=bogofilter, spamicity=0.000000, version=1.2.4 Sender: owner-linux-mm@kvack.org Precedence: bulk X-Loop: owner-majordomo@kvack.org List-ID: List-Subscribe: List-Unsubscribe: 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