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b=XQc39hnvCdkTlJIa8y9YfgGkIJQb1cHVaoOkJ7pbNHFepqyzWDGV4Wxa5w7n7maGc X+b9cmNIXD1NNy46RiDLbrSNZKSvWZkVvzOf5pjYkd9GkeQ8uWitOK7rbabbOpRsMR cU2Dzy2Z/oHVXA1L7GHzimHqACLluo7QZV4M3RKQ0VSW0gAsm/ALa1Jg6hl3P+lCuJ 2OikXfZRe0W52IcmQMM/se/XYc5GpkRUpXmiQI7394jKgEDAK8kQbIZmBYmRb/oRNk R3bs2GEMBeAjc+i/0X3bVd97RkREIJ/H5nw8+YdfehKMnRuPgqE+f2PNVK0PTE1sZc eCBD5LIKuDCGA== Message-ID: <210d140d-c78a-47be-8046-caae359db6db@kernel.org> Date: Wed, 20 May 2026 23:29:07 +0200 MIME-Version: 1.0 User-Agent: Mozilla Thunderbird Subject: Re: [PATCH 1/8] mm: Add ptep_try_install() for lockless empty-slot installs To: Tejun Heo Cc: Alexei Starovoitov , David Vernet , Andrea Righi , Changwoo Min , Alexei Starovoitov , Andrii Nakryiko , Daniel Borkmann , Martin KaFai Lau , Kumar Kartikeya Dwivedi , Catalin Marinas , Will Deacon , Thomas Gleixner , Ingo Molnar , Borislav Petkov , Dave Hansen , Andrew Morton , Mike Rapoport , Emil Tsalapatis , sched-ext@lists.linux.dev, bpf , X86 ML , linux-arm-kernel , linux-mm , LKML References: <20260517211232.1670594-2-tj@kernel.org> <9ba50fd2-077e-4291-9276-9adb18186873@kernel.org> <2f02d90d-cdc9-48ef-abe3-99e00f22595f@kernel.org> <297658c4ae2d6e7103f5968efc936224@kernel.org> <04b03066-86d3-4c0b-b077-307fd0f3bc9c@kernel.org> <5590fd3d-dae1-4070-b52f-bc40982ac678@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-Rspam-User: X-Rspamd-Queue-Id: 62CB58000E X-Stat-Signature: wx4ked8jan8faf3ey8td4yqjkjk37dd6 X-Rspamd-Server: rspam06 X-HE-Tag: 1779312557-825320 X-HE-Meta: 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 6qs8o70c lPMUBug6N7biySER201Ph89TCdyYgD78nd+k7RaAu0k8NuFFs7jw2YN2duM/gCfcE8vcwt92DLOA4hE1V+jJyWh/D27M5+MRNlDV6BlzKFPU+k6sNy4PPaz7w4uRmdTZXy+x7OaelPRWHJdl9uWn1k/vE+Q8ePCSOXmKznFDkuHpeZOUjlJuwwuADvxCXSv6rwDSAPk4EaGQ5l1v5hGIUhRN09Rj8Kh61DotfyZ6AvyggqarvTpz1+ig7i3JT60AcTwV6R+SgGBt9yTaAFAAM/eQddNz0tpeebIupNeNEH6ASbTpo0kEZaTuPBmrpqET7Xlm1QS3rsidIW8AX7qwYBtZ6qjykKa8uusD1HykbTRBFP6PZYBKU8puIr11OFSkU97Cj Sender: owner-linux-mm@kvack.org Precedence: bulk X-Loop: owner-majordomo@kvack.org List-ID: List-Subscribe: List-Unsubscribe: On 5/20/26 20:38, Tejun Heo wrote: > On Wed, May 20, 2026 at 10:41:15AM +0200, David Hildenbrand (Arm) wrote: >> And that should be carefully documented. > > Will do. > >>> The fault handler has to install _some_ page and let kernel continue. >>> Scratch page or arena page doesn't matter. Potentially different CPUs >>> will see different page. It's not a concern at all. >>> bpf prog is buggy, but the kernel will continue to work without a glitch. >>> bpf runtime will disable and unload misbehaving prog. >> >> Having one page table walker overwrite a scratch page on race is just rather ... >> questionable locking design, that just screams for problems long-term, really. >> >> At least in apply_range_clear_cb() one could similarly switch to >> ptep_try_install() to at least have both these paths handle races in a >> reasonable way. (having to handle when ptep_try_install() is not really implemented) > > Hmm... wouldn't that be more confusing on apply_range_clear_cb() side? > Whether it maintains the current behavior (if collide with scratch, try > again with scratch as the original value) or flip the behavior (fail if > scratch), that extra logic is spurious, and those tend to confuse people as > they force asking why it has to be that specific way. If the goal is > documenting the subtlety, wouldn't a detailed comment serve the purpose > better? I would let all operations that can happen concurrently work with atomics, not just a single one. Alexei correctly concluded that ptep_get_and_clear() might be one option that should do on x86 and arm64 from a quick glimpse. > >> Anyhow, the documentation of ptep_try_install() must clearly spell out that this >> must be used very carefully, and only in special kernel page tables, never user >> page tables. There are likely other scenarios we should document (caller must >> prevent concurrent page table teardown somehow, and must be prepared to handle >> races if other code is not using atomics). >> >> To highlight that, we should likely consider adding a "kernel" in the name, like >> "ptep_try_install_kernel()". >> >> I am also not sure if "install" is the right terminology and whether it should >> instead be "ptep_try_set()". (set_pte_at is the non-atomic interface right now) > > So, ptep_try_set_kernel()? I guess we could do that, but we don't have a lot of existing functions that are specific to kernel page tables ... only pte_offset_kernel() and pte_alloc_one_kernel()/pte_free_kernel(). The most important part is getting the documentation right. We should probably document that code concurrently clearing PTEs should be using ptep_get_and_clear(). -- Cheers, David