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b=D4NVIkYYfKTLHNAaRXc6SujIt9UCWspisz0EDi19o3anfRl2NF2C4Bh/TMg80rHRD mOuouIvnWBbxzYOHc4yrdqMK/LbJiC7SEf6HlhXVL5ftfFsm8KixrlV425ibAPDye+ /0H80aSuxIlUC9Go+cy6dB3TPQckDdxTvz2y7M5JjKWXMVVawvaZwQ8NNnN7vcFdbF 1qADLgx/87As509MttzR65d94uphJikgDugdPOJnsRuHcV6886KD4wONXuhTJgmjuj Nzf/uEKv5RpGe+86wcuTyn2Cqdahu0D2I8B3uLaX6rRlEjxTd94znGfU4izNM4pMLI JU6/j+Gw9QAAw== Message-ID: <6659669c-d5a0-42b1-809d-95d18937f662@kernel.org> Date: Fri, 2 Oct 2026 16:26:42 +0200 MIME-Version: 1.0 User-Agent: Mozilla Thunderbird Subject: Re: [PATCH] mm: don't ioremap COWed anon pages in generic_access_phys() To: "Lorenzo Stoakes (ARM)" Cc: Nguyen Ngoc Thang , akpm@linux-foundation.org, liam@infradead.org, vbabka@kernel.org, rppt@kernel.org, surenb@google.com, mhocko@suse.com, peterx@redhat.com, dave.hansen@linux.intel.com, linux-mm@kvack.org, linux-kernel@vger.kernel.org, syzbot+49b1021becba70c1f3f6@syzkaller.appspotmail.com References: <699ae98c.050a0220.340abe.0d31.GAE@google.com> <20261001152524.171115-1-ngocthang2710.1999@gmail.com> <32fe40e2-854a-47ec-9d95-93e23f9a7af4@kernel.org> <9d43234f-93cb-4c86-a2a6-a3cdebf48f3b@kernel.org> <494d436c-e3b3-4b65-8a6c-1059d9f468d4@kernel.org> 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: 3g84kmogf9o4rziuakk48jjjn6yhamyb X-Rspam-User: X-Rspamd-Server: rspam09 X-Rspamd-Queue-Id: 3CE984000C X-HE-Tag: 1790951210-194656 X-HE-Meta: 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 kygkOuiC 496P6NarvT6P0Usl+OhvSkwNIAwd5oy/qD/IWSDnCyAMWPwMK0dFdeGUsyr3ky477cKETLplWj3zgbkG9eUC4NITKMbesd7cEzwCnsG+YmfgjZC1WMclTcsHxjk47cNGEySFHWSfJrPdcMGAFlOJkrWmYnQCZmgdCNog4SauSUTLs5R1HL6aTNJp8lil7ScSrZsUdlEZiPb65zGx0Q03CydcVwEY5E54Tktl32204kDOPiVkWXlvVRz1zt61dc/kmQeCKwHjTXxI0qN3Ze8Stz2goOLX0HNxJRaY24Q+v9+e9KON6arSjga9uSG4kQwOgjLoNaf0agGP+uoQ4S3lGUQykALjhATew4w00pUDAefBhaIpMDyi/dCdZ9UalZiycKD84i8XZYapQtlVCn4AKnkaVt/4eUtE0fs8Gj+apOB/EpGeTpy77UyZTzTHJ9Wq4fW7Z Sender: owner-linux-mm@kvack.org Precedence: bulk X-Loop: owner-majordomo@kvack.org List-ID: List-Subscribe: List-Unsubscribe: On 10/2/26 16:03, Lorenzo Stoakes (ARM) wrote: > On Fri, Oct 02, 2026 at 02:19:00PM +0200, David Hildenbrand (Arm) wrote: >> On 10/2/26 12:09, Lorenzo Stoakes (ARM) wrote: >>> >>> Well, my series at least makes it easier to reason about VMA_IO_BIT! >>> >>> Though not sure if it really touches PFN map cases specifically. >> >> I'm more concerned about someone using this function on VM_MIXEDMAP | VM_IO with >> a memory page that has a struct page but is actually not memory. So we could get >> something that vm_normal_page() would flag but generic_access_phys() could >> actually read ... I'll have to explore the generic_access_phys() users once more. > > Hmm that could be a problem also for struct page's that are there but you're not > supposed to access for other reasons to as well? > > Definitely need to be careful about that. > >> >> All way to complicated (and you series improves things). > > :) well there's still a lot of complexity in there, one battle at a time... My conclusion so far is: indicating is_normal should work when using it in generic_access_phys(), as it is never used on non-VM_PFNMAP VMAs. follow_pfnmap_start(), however, can be called on some non-VM_PFNMAP-but-VM_IO VMAs from KVM as it seems. So we cannot easily restrict follow_pfnmap_start() to VM_PFNMAP only (weird, right?) without risking breaking some weird KVM use case. Gah. Anyhow, I'll prepare a fix for this one and send it out likely later today. -- Cheers, David