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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 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 X-Rspam-User: X-Rspamd-Queue-Id: 68AB614000A X-Rspamd-Server: rspam07 X-Stat-Signature: ub1sq3xucb367eqdc56zrnwq564y5nxs X-HE-Tag: 1770658602-314308 X-HE-Meta: U2FsdGVkX1+r7VR9AaoLNi4YnafnU69G4ADcbdqUfQbgtRTet/3KRxxSYhwUQYhE2is/PP1QQZZVqqLvZabuctDvKpHLG5VOOJ3KTkDfcxVfmN6UIxEVT0N+wAGZDPERgYhYrlwxxBX+UZgRSkkZKjg5tRih/PkyaEyEhS5VTlAo8jFj1SMUxZhG9/SXDaoiMsRynupZaJvAu9x8ZaxCoyWhpjiB5UwzN3IvZ8KgvZ0P29GwbPYp95z9us4eNFyKHkvaFPC0LGtlzJps8jt4f1p6qEoqn6CBnSgs4nJObTdEK/0MFROeCBFSSYOKEFk4JXoXYtSpkRt2+SIrAA8FDEbG9uMjJAqaLbtXGZcJ5eeqWjiD4e/pzvWkcjqVGPLb+dDWfPSlriiRjsRTGxeHr3RLtGhfCeV6gB8whypb8xXrIU8JTFn9If/HpCG//roaW67vBUg1Imo4pISo4bToUFXQj1Ijgg5V2/2HXbfhTIZjT2P2GrmRbbYAN3MbMC9fwoLV5S0dK5PQo4FqlEpahT/QyQgYalnTwF6B9cjVpuLrhOL560oye1WVQGiqb/yA/ne4pCUBEGDi6WQMqYUCH1C94zvl/1c/MQ2GSmSAOQOyx7H7u2PzlMFC3TYWD6j8yDFRc2XuHLFKz8BzaPqAecmRMIr86dpGgwQ+74BezuJdhyDfaCQXBBpI1VZzVCBkaFPIGcfH5+HIifApOePF19XVRO7UfsE3efwybYw8lxNJH1gLFy3pfw7xIF1852gLNtI0vdPyEvAzon5l31VUf09Lpb+aMsRpx1k7YoQ7RQQjHGfw5fGigkIJN7CS0sBzir75I2aeKqZNcxR5EepsI175hEqLrTzaDpoCaf2H8QyxR7vSFUt5VmzP4Tal/214q2h80YzEJCuc06cGlQO6Dtlc1/SzmS57GUv9N1Uja8/fLXB3phl9YHr86HD7Zy7sBlnyLiIm99SiCdf7gbh 4yx74sue 2/lW/yxGx4gctGw0jevsIV0nyFx/YgSafYlnmGITzZ1OPl7zSb8gGCcgt+Hb4nXQOEaL4gvntMs2+SJdHEidHfQ0gG6wEcnFkZt1OsZR7JklHzgb0cWFUHa2K828VnAlYeFT06mUGjHBLBKTBvbXksQgHtFgUl1y7ERD7wpNrvDTS7r8LvMoW1NPFqbO9pkqDqqc8IvbwKhdapCC6+886MxJNj+cxHVyWXbUTQ20fVQjRBm0P0TOdgQwJprgeEMLGqAMwUu0qxfw5kzmOoNBGjPBFRljRQnd/oYtwjgnXmyq5UfymX40Ozm+Oeg6T/FCPrg0mmpiY+ZG+9vSZ8yZTCNOYVxbibFffDuWDzCsIWyu+4J2CvpD7colodaYrb0O0Om6zpU0/k/v26SC+KRehwNa34w== 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 17:33, 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