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b=Ecu3Fk6+BBTt28XCj/0byPzPkF3oH2shcUtw4oGlJngrRwn1Kj8+1wkGu9qx2r/Dm EqZ7Ms6pF87RRV7v8j7pImt2PTOzQt9j2kIo7RHaXDzq8rDlmqAB9qpi2rb3C/4qCn Pb8wbIDWzOymVkDaUEBUeKNjSJICBfmbIb/dGwBOSluCbLl27Tb8HIyHSCKTSYfmjC CkUyGNTvklmlPMrIsljFGeMQko9k9oWWc1U+ky3Y6tzCPJ9BgT4oB1llYYNFFl/evs 2piDM0acF9bX0KpAjim+Cwelo79TjED+uOqgLLoTD7OvYLMKK8QiZ4vhQ6gwsM5SbX r5GMYWudfthVQ== Message-ID: Date: Fri, 2 Oct 2026 12:02:15 +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> 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: bwijumnjg5ayj13dyjz5mjsuq9hh1ifs X-Rspam-User: X-Rspamd-Server: rspam09 X-Rspamd-Queue-Id: 97BD7C000D X-HE-Tag: 1790935344-250244 X-HE-Meta: U2FsdGVkX1/JnnXgRwHwdr4r3OJFf6U/B5I/9PMpX58izczuBqXw7Qws79RmtuS2Al10A/UZhKSmSbKlUYB/x5RAQvLLpCkYnzqR+IPV4Jao8LBXByh4xUTQVyJTcfft+52kRzOumZ0WkJaKB4Os+ILJ/ua2JqglXKWrzgUoy9JzHlgB9eXEh3khxoUFiiRhyPtws1YDXinwS/Ybosgh04E2v71Wwz6Fv+rvnWHVSSEfnZlU04uTyiXozbAPX9m4ZNbfg/kO2mByuGjQMOHVLfLQfhbEPaOJP1rIvugiKiXop48rzUoCvJTe8bdgSyP7um7CtOGJp2pITPsW0+xRqC8H/2ykap+dX3hIm5H+arklU9+PHJTiPGF37pKsSUfWIKBs/VdfgUH19QI5LCvFYMYm25P8F5PFIpscd/wWqWUL7v9Tcu59xtHWZWCJIXEgVWu6PpHF5PKhQA3vEJJTxPeKnUktJ0WA6nDlLL/ycdPx4sTRrZjI4NLF+H8L4MgLFLVp6AdXpR2BE1GTTDhyEQ0nzmngaYySHBvYIq+0pE0BZ6MhbpTQ/aUTDnGM6Uo5obHdO89SkIKHcytbBh3YgMCGPdpPIFh7SHuVDxyxttMuilWhBLL23NfauooeBbZKsc/r30Z02VuglkMJQLBSW5WIkkZzqrukf7QCYLcyhAvP7ASM7laL7DABN2vQFvDzuNAxwH6LnwOEYLuhAztsDNoWFDW2uQF/bTbAy9o1pKwGpm+5sUKA5KWQvv3PkIe0roOY/nw7oTuBvE8u9C8a3jY8SqV7J/OKce8VYztMOaFRO2eLd+dVKyYa8nWnh2MFxEfMjnz0eCtKzci9S0TlipcZ1c/A69MPe465aJg+yIIor4nl1k4r2TawdXmvXLi6UQKPTQ/m74Is0yn09dggiUAXNVJM9rIn2CA1s/6pnEBMKcqavmYNiAyz19aq6dcxegcq1jhBJlE/6IoUsRi nMWo26RO D5EfEywHmmMYqmAk9kKu9gQMMQjZxjvX5RO7bqXvt2XebeURBwXiObdBSp2z4/xUDUzoZ7zl4cUT8OwS+f/KHh/sgrlcXGzIXJBMC7/dl4EXJtqkPT3eF43rfCyNf3oAym7dInXpu8Q1d8mxaX49l84JKvgJlbiXmHjVx11vQLBkXQLDvFlIgadx8Yl5/U2aws4UVQHAcPPP4i7xgt5jdEte1/+a7h5vkOZdEobIZ+mPiRpUcv1+rp+y/1cILOcsO8bRtlzulxvbk/sR+XtCkzp3IZrphwGfPK6JJLZDfajZROPoNgQCUwQOMQFRPs4++/WzmIVzOVwQMmqdSl3wr4NdLowYyni+NC/GhNYLmnaYRukqyHOL0Z6LTsJ4y86NHroa4mX40TlAtI0gBkSfh2MDkaN6jb0kiODNi8uiZjM/GpbwkrkR77CXnH1QTItc3LVvH/ZNgPudUHy0evuIL0mLw+pLxB/ES0dd3SCoMS4oNdKw= Sender: owner-linux-mm@kvack.org Precedence: bulk X-Loop: owner-majordomo@kvack.org List-ID: List-Subscribe: List-Unsubscribe: On 10/2/26 10:38, Lorenzo Stoakes (ARM) wrote: > On Thu, Oct 01, 2026 at 10:22:29PM +0200, David Hildenbrand (Arm) wrote: >> On 10/1/26 17:25, Nguyen Ngoc Thang wrote: >>> A MAP_PRIVATE mapping of iomem (e.g. a PCI sysfs resourceN file) is a >>> COW pfnmap: a write fault replaces the pfn with an anonymous page, which >>> remap_pfn_range() allows by keeping vm_pgoff equal to the base pfn. >>> >>> generic_access_phys() does not tell those COWed pages apart from the >>> original pfns and ioremaps whatever the PTE points to. Reading such an >>> address via /proc/pid/mem or ptrace then ioremaps RAM: >>> >>> ioremap on RAM at 0x0000000045623000 - 0x0000000045623fff >>> WARNING: arch/x86/mm/ioremap.c:216 at __ioremap_caller.isra.0+0x4c2/0x5f0 >>> Call Trace: >>> generic_access_phys+0x130/0x4d0 mm/memory.c:7178 >>> kernfs_vma_access+0x1ce/0x280 fs/kernfs/file.c:437 >>> __access_remote_vm+0x58f/0x890 mm/memory.c:7256 >>> mem_rw+0x2a1/0x670 fs/proc/base.c:912 >>> >> > > Sorry to say this looks schlopped. > > This guy has sent 10 series across 6 subsystems over ~21 hrs: > > https://lore.kernel.org/all/?q=f%3Angocthang2710.1999%40gmail.com > > Nguyen - please do not flood the kernel with patches, and please use the > Assisted-by tag for generated content. > > The original code you submitted is really not great even if the issue may > be valid. > > So I'd say somebody from the core team should take over this if we want to > come up with a patch. Yes, I'll take care of it. [...] >> >> Signed-off-by: David Hildenbrand (Arm) > > This looks reasonable but I hate that we have 'special' CoW overrides like > this :) After sending this yesterday, I concluded that we can do this cleaner: just have bool normal_page; (naming suggestions?) that express that this is something refcounted with a struct page, like documented for vm_normal_page(). Then we can just refuse all of these. > >> --- >> include/linux/mm.h | 4 +++- >> mm/memory.c | 48 ++++++++++++++++++++++++++++++++++++++++------ >> 2 files changed, 45 insertions(+), 7 deletions(-) >> >> diff --git a/include/linux/mm.h b/include/linux/mm.h >> index c49ef99b4413..b90e547797a9 100644 >> --- a/include/linux/mm.h >> +++ b/include/linux/mm.h >> @@ -3193,6 +3193,8 @@ struct folio *vm_normal_folio_pmd(struct vm_area_struct *vma, >> unsigned long addr, pmd_t pmd); >> struct page *vm_normal_page_pmd(struct vm_area_struct *vma, unsigned long addr, >> pmd_t pmd); >> +struct folio *vm_normal_folio_pud(struct vm_area_struct *vma, >> + unsigned long addr, pud_t pud); > > Hmm, if not defined before why would this need a new PUD handler? Do we > even have PUD-leaf PFN mappings? Yes we do. In any case, good for consistency. [...] >> + args->cow = folio && folio_test_anon(folio); > > I actually wonder if this should be changed to folio_has_anon_rmap() at > some point :) Not a fan. > > Since a folio being 'anon' is vague, because we stupidly made 'anon' vague > in general. For folios it's an established term :) > > Anyway I was going to ask does this suffice for CoW but having an anon rmap > implies CoW so it does. The downside of using "bool normal_page;" is that we should check vm_normal_page() for any mapping, not just cow mappings. I suspect performance-wise we don't really care. -- Cheers, David