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b=NNDOELm4Hz4N7CmdMhpohkpVr08q/PWYgov7+1gVAQr7+hSjjyLzwt+gc5JDB9zG5 EhQmvVv8/sV5hwSbQvW0eWj5i/tlTCKYhTXrjtBL9j/Pfg3nNa/7kieX07YgfOp0Oo LZzp3hshTb3g3POqFSESr/VE7XC+DpFbf9BW/5f4rsslUVmQ2N4aoyOtnIPOE+OzAq VfV0DVzFkIDjNw333I4qs6PsuH9SoLTSVH3+4+EwX5G6mLFMcvcj6S0xziDqU02Awk 4NoEyPx8h0+b1TBbewzfPHjJ1UavJLvFlKiB3/T2Uo0tSH0Clevgj2BnKuR8WUuKaQ r6DiIWtj4SmIg== Message-ID: <8a3827cb-187d-4c4f-84aa-5ad811abef20@kernel.org> Date: Thu, 5 Nov 2026 20:28:57 +0100 X-Archived-By: MIME-Version: 1.0 User-Agent: Mozilla Thunderbird Subject: Re: [PATCH v2 1/2] mm/huge_memory: take a reference on a folio PUD copied by fork To: Konstantin Khorenko , Andrew Morton , Zi Yan , Lorenzo Stoakes , Alistair Popple , Dan Williams Cc: linux-mm@kvack.org, nvdimm@lists.linux.dev, linux-kselftest@vger.kernel.org, linux-kernel@vger.kernel.org, stable@vger.kernel.org References: <20261105-vstor-150143-copy_huge_pud-folio-refcount-v2-0-2828fe5a51ed@virtuozzo.com> <20261105-vstor-150143-copy_huge_pud-folio-refcount-v2-1-2828fe5a51ed@virtuozzo.com> From: "David Hildenbrand (Arm)" Content-Language: en-US Autocrypt: addr=david@kernel.org; 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charset=UTF-8 Content-Transfer-Encoding: 7bit X-Stat-Signature: 5h6ukk8w9hodj1xc6fw3kgfuktke4r6q X-Rspamd-Server: rspam01 X-Rspam-User: X-Spam-Status: No, score=-12.3 required=3.0 tests=BAYES_00,DKIMWL_WL_HIGH, DKIM_SIGNED,DKIM_VALID,DKIM_VALID_AU,HEADER_FROM_DIFFERENT_DOMAINS, INCLUDES_CR_TRAILER,MAILING_LIST_MULTI,SPF_HELO_PASS,SPF_PASS, USER_AGENT_SANE_1 autolearn=ham autolearn_force=no version=4.0.1 X-Rspamd-Queue-Id: E173612000B X-HE-Tag: 1793906943-100317 X-HE-Meta: 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 1uaX3sYS 9ZGoTBZyImafEdPLWIjl32vMxcMp1MGaPtCVWKu2xnLOPjtxcll1hXZyZbzjTDvFTkRy0JUYGAs0E5i6wGjTaLWlekFjmBbU/FEK4yMkHZkso75L1ugouHVtLQrwObw2VRsV+kpnxzARiiY6jYCM1aKe+oALYk5+LRKyUB5Lv80BxTt83ujlZiEUqa0ZozdvYrjVB73o5VKVvjX0XOIqwWmKOopTegc5/xsjC0dKC25x9Z9+BmPG5GKP9TZb3siVw6h4Ys2lYUXFI+KeT5CHGY5LhExq9HM6OwBZZPWbQ/7nr9tDle4GoXGS0M4rV2hHZXGgMlDhKb4WvPPXy9TM8SIniRgxEITqirbdKtYBYMizC0IzZzMyrsm44X5TQrq1zPc52ctANUfFaEaI= Sender: owner-linux-mm@kvack.org Precedence: bulk X-Loop: owner-majordomo@kvack.org List-ID: List-Subscribe: X-Spam-Checker-Version: SpamAssassin 4.0.1 (2024-03-26) on aws-us-west-2-korg-listserv-alma10-1 On 11/5/26 18:00, Konstantin Khorenko wrote: > Device DAX maps its memory with vmf_insert_folio_pud(), which takes a > folio reference, adds a PUD rmap and accounts the pages to the mm. > zap_huge_pud() drops all three, but copy_huge_pud() copies the PUD into > the child without taking any of them. Each child's unmap then drops > what the parent took: > > - the folio is a reference short, so with two children its refcount > drops to zero and free_zone_device_folio() runs while the parent > still maps it; > - the child's file counter goes negative, "Bad rss-counter state" when > its mm is freed; > - the parent's unmap drives the entire mapcount below zero. > > fork copies the page tables of a device DAX mapping that has a guard > region, and commit f807123d578d ("mm: allow guard regions in file-backed > and read-only mappings") allowed MADV_GUARD_INSTALL there. > > Take the reference, the rmap and the counter for a PUD that > vm_normal_page_pud() resolves to a page, as copy_present_ptes() does for > a PTE, with a new folio_dup_file_rmap_pud() helper. copy_huge_pud() now > needs both VMAs, so pass only them and take the mms from them. > > Fixes: aed877c2b425 ("device/dax: properly refcount device dax pages when mapping") > Suggested-by: David Hildenbrand (Arm) > Cc: stable@vger.kernel.org > Signed-off-by: Konstantin Khorenko > --- Thanks, I'll wait a couple of days and then queue this. Reviewed-by: David Hildenbrand (Arm) -- Cheers, David