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b=AYqSbQO9SsD0PTRC2TIpIFdHQQB9bhI3th75y2dTSqrRStFPbjkueqYtrJUfGwIYp ERgQa79fBLPPuomyN7RPX1i04FpqdpK+SRtiN7+b5r9g2JsMWEBWqWN8foFmiQah2O JPuSy4yj2UPmlNlHy5lCLc0wuoRGsga9T9OJxif9qq6ubosSyCCFjL4TmSTyI1aeqA T1ZZ5oFqvVL34Pid2ZnNd0XLRMRTmQuO1UWvXONw19FReuRxWaymShqM+/bCRlh4vQ 7MCX78Sk7PhEdb3wqfvmCgMoGnZWP9sUEr28tldBUEha+i+Ffq/XHaHIlTda3hh5MP 5FUu7lO8/T6Gg== Message-ID: <4ed1b111-f2f1-4f89-9308-fdd9d706ca37@kernel.org> Date: Mon, 9 Feb 2026 20:45:02 +0100 MIME-Version: 1.0 User-Agent: Mozilla Thunderbird Subject: Re: [PATCH] mm: thp: Deny THP for guest_memfd and secretmem in file_thp_enabled() To: Ackerley Tng , Deepanshu Kartikey Cc: akpm@linux-foundation.org, lorenzo.stoakes@oracle.com, baolin.wang@linux.alibaba.com, Liam.Howlett@oracle.com, npache@redhat.com, ryan.roberts@arm.com, dev.jain@arm.com, baohua@kernel.org, seanjc@google.com, pbonzini@redhat.com, michael.roth@amd.com, vannapurve@google.com, ziy@nvidia.com, linux-mm@kvack.org, linux-kernel@vger.kernel.org, syzbot+33a04338019ac7e43a44@syzkaller.appspotmail.com References: <20260209033558.22943-1-kartikey406@gmail.com> <0d9cada8-7148-4a5c-a09d-120ef54559d7@kernel.org> 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-Stat-Signature: 5x9wuzxz7konwic9m6ikop37xhuw9oxh X-Rspam-User: X-Rspamd-Server: rspam08 X-Rspamd-Queue-Id: 505F640010 X-HE-Tag: 1770666311-266932 X-HE-Meta: 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 Lp0p65Eh W8Tt+Eu+jyHfoXr/4cOXuyeZu1iQfXQcKm/Pu1JFcjiHTMUA09BaNHss9AoWKE/yrYW8G 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 19:22, Ackerley Tng wrote: > Deepanshu Kartikey writes: > >> On Mon, Feb 9, 2026 at 4:12 PM David Hildenbrand (Arm) wrote: >>> >>> >>> On second thought, why do we pass the >>> >>> !inode_is_open_for_write(inode) >>> >>> in file_thp_enabled()? >>> >>> Isn't that the main problem for these memfd things? >>> >>> Maybe a get_write_access() is missing somewhere? >>> >> >> Hi David, >> >> Thanks for the suggestion. I looked into the get_write_access() path. >> >> Both guest_memfd and secretmem use alloc_file_pseudo() which skips >> calling get_write_access(), so i_writecount stays 0. That's why >> file_thp_enabled() sees them as read-only files. >> >> We could add get_write_access() after alloc_file_pseudo() in both, but >> I think that would be a hack rather than a proper fix: >> >> - i_writecount has a specific semantic: tracking how many fds have the >> file open for writing. We'd be bumping it just to influence >> file_thp_enabled() behavior. >> > > I agree re-using i_writecount feels odd since it is abusing the idea of > being written to. I might have misunderstood the full context of > i_writecount though. i_writecount means "the file is open with write access" IIUC. So one can mmap(PROT_WRITE) it etc. And that's kind of the thing: the virtual file is open with write access. That's why I am still wondering whether mimicking that is actually the right fix. > >> - It doesn't express the actual intent. The real issue is that >> CONFIG_READ_ONLY_THP_FOR_FS was never meant for pseudo-filesystem >> backed files. >> >> I think the AS_NO_READ_ONLY_THP_FOR_FS flag you suggested earlier is >> the cleaner approach. It is explicit, has no side effects, and is easy >> to rip out when CONFIG_READ_ONLY_THP_FOR_FS goes away. >> > > I was considering other address space flags and I think the best might > be to make khugepaged respect AS_FOLIO_ORDER_MAX and have somewhere in > __vma_thp_allowable_orders() check the maximum allowed order for the > address space. The thing is that CONFIG_READ_ONLY_THP_FOR_FS explicitly bypasses these folio order checks. Changing it would degrade filesystems that do not support large folios yet. IOW, it would be similar to ripping out CONFIG_READ_ONLY_THP_FOR_FS. Which we plan for one of the next releases :) > > khugepaged is about consolidating memory to huge pages, so if the > address space doesn't allow a larger folio order, then khugepaged should > not operate on that memory. > > The other options are > > + AS_UNEVICTABLE: Sounds like khugepaged should respect AS_UNEVICTABLE, > but IIUC evictability is more closely related to swapping and > khugepaged might operate on swappable memory? Right, it does not really make sense > + AS_INACCESSIBLE: This is only used by guest_memfd, and is mostly used > to block migration. khugepaged kind of migrates the memory contents > too, but someday we want guest_memfd to support migration, and at that > time we would still want to block khugepaged, so I don't think we want > to reuse a flag that couples khugepaged to migration. It could be used at least for the time being and to fix the issue. -- Cheers, David