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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 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 X-Stat-Signature: xd5ttfa4dsate5rpk3qnawcpicx9ypbr X-Rspamd-Queue-Id: EAEA8180006 X-Rspam-User: X-Rspamd-Server: rspam04 X-HE-Tag: 1770653791-515191 X-HE-Meta: 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 mQBATRgE 698P/Jbx1ys2wt9IZ3QoWmTwpiprzuu/gxT/tpiqFlZBOY8uSoR6ny2lUvYO9ZZ8ZCCzkSLYoRhSunwpHdGEGzGeMkAej3kiJ50gMzlHv+FT39V11VDTfldEUD6EcoA3BkyWg0KFTXw2qR4VEiqHmszkQTch4U56rJ2IrK9rI3tdg25Ijbo9JrhYsLTFlCsenO+3EwmOBzI8N/Bl66CYHICs75r1yTE+r2sUFML7Ah+unuz4Zv5ShCQR3uJvYy0EpLVtWlaVtfvQU/rJEddQ5tO2sHum69D/424F0wSvlLJy81i0LmQoj/GastlVCWLmoVnZ/iYifcQ2jxdqlV8pSDOGzQRpP+wKspwRocCcWAwQfdLMCDW/CmIJUGfndcATpW++7 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: >> 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