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([2a09:80c0:192:0:5dac:bf3d:c41:c3e7]) by smtp.gmail.com with ESMTPSA id r4-20020adff104000000b003313e4dddecsm1698979wro.108.2023.11.30.06.39.35 (version=TLS1_3 cipher=TLS_AES_128_GCM_SHA256 bits=128/128); Thu, 30 Nov 2023 06:39:36 -0800 (PST) Message-ID: <92833873-cd70-44b0-9f34-f4ac11b9e498@redhat.com> Date: Thu, 30 Nov 2023 15:39:35 +0100 MIME-Version: 1.0 User-Agent: Mozilla Thunderbird Subject: Re: [PATCH RFC v2 19/27] mm: mprotect: Introduce PAGE_FAULT_ON_ACCESS for mprotect(PROT_MTE) To: Alexandru Elisei Cc: Hyesoo Yu , catalin.marinas@arm.com, will@kernel.org, oliver.upton@linux.dev, maz@kernel.org, james.morse@arm.com, suzuki.poulose@arm.com, yuzenghui@huawei.com, arnd@arndb.de, akpm@linux-foundation.org, mingo@redhat.com, peterz@infradead.org, juri.lelli@redhat.com, vincent.guittot@linaro.org, dietmar.eggemann@arm.com, rostedt@goodmis.org, bsegall@google.com, mgorman@suse.de, bristot@redhat.com, vschneid@redhat.com, mhiramat@kernel.org, rppt@kernel.org, hughd@google.com, pcc@google.com, steven.price@arm.com, anshuman.khandual@arm.com, vincenzo.frascino@arm.com, eugenis@google.com, kcc@google.com, linux-arm-kernel@lists.infradead.org, linux-kernel@vger.kernel.org, kvmarm@lists.linux.dev, linux-fsdevel@vger.kernel.org, linux-arch@vger.kernel.org, linux-mm@kvack.org, linux-trace-kernel@vger.kernel.org References: <20231119165721.9849-1-alexandru.elisei@arm.com> <20231119165721.9849-20-alexandru.elisei@arm.com> <20231129092725.GD2988384@tiffany> <4e7a4054-092c-4e34-ae00-0105d7c9343c@redhat.com> From: David Hildenbrand Autocrypt: addr=david@redhat.com; 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charset=UTF-8; format=flowed Content-Transfer-Encoding: 7bit X-Rspamd-Queue-Id: 25537180026 X-Rspam-User: X-Stat-Signature: uydpo7ozirmipx1rdi183ia74uyxzdw4 X-Rspamd-Server: rspam01 X-HE-Tag: 1701355180-973483 X-HE-Meta: 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 hXh9WFLJ +zd3nYrtm2NyQ5u9PDzEUfdJ+MRkTE0gkWdsQN5WI181LK7CbxuhMvCKnBu9mUBm1pUspmXMXLz/7jWzEoBe2N8E/9W0sO7U1+ueo73en4fA2XQSYqAgah/L+XuSw9rB41QsYX/HdxFpkZIXAEBSVSpQmuLZdiiokyi3humN/ZzrAFPeDelFAGM+AHrR9EG3y6NPaU35vGHzFigwehBV4fT7jo4504TSs3GfhpSxVU2yIJZeAk24Xif4tW6glomgUmrfrPwjG4N7cy1bAeHnWrQKB3FEthVgPTw5MzSHDlFBbRUOw7KtBQoebYg+Hn6++9Uiy5VjE4lQaS3gXxpg0Rj+aNkEltL28Ry4TooTtZA6GTXkxLxD7djhoFgG+n2JK5q9hG+Q98sl5hyZCQd/mP7bG4MOmcWtjQrZquyUi3DXNBMM1tnufC6lKUu2Bx7xduFNtA3FwREMB+lN8M5r4r8T6yFe1LKCDJdNuBzM3AFsABctMcHrS1x8KffbOB70PIeMAAOBZ5+rWaWXewZQ4CDdIzQ== 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 30.11.23 15:33, Alexandru Elisei wrote: > On Thu, Nov 30, 2023 at 02:43:48PM +0100, David Hildenbrand wrote: >> On 30.11.23 14:32, Alexandru Elisei wrote: >>> Hi, >>> >>> On Thu, Nov 30, 2023 at 01:49:34PM +0100, David Hildenbrand wrote: >>>>>>> + >>>>>>> +out_retry: >>>>>>> + put_page(page); >>>>>>> + if (vmf->flags & FAULT_FLAG_VMA_LOCK) >>>>>>> + vma_end_read(vma); >>>>>>> + if (fault_flag_allow_retry_first(vmf->flags)) { >>>>>>> + err = VM_FAULT_RETRY; >>>>>>> + } else { >>>>>>> + /* Replay the fault. */ >>>>>>> + err = 0; >>>>>> >>>>>> Hello! >>>>>> >>>>>> Unfortunately, if the page continues to be pinned, it seems like fault will continue to occur. >>>>>> I guess it makes system stability issue. (but I'm not familiar with that, so please let me know if I'm mistaken!) >>>>>> >>>>>> How about migrating the page when migration problem repeats. >>>>> >>>>> Yes, I had the same though in the previous iteration of the series, the >>>>> page was migrated out of the VMA if tag storage couldn't be reserved. >>>>> >>>>> Only short term pins are allowed on MIGRATE_CMA pages, so I expect that the >>>>> pin will be released before the fault is replayed. Because of this, and >>>>> because it makes the code simpler, I chose not to migrate the page if tag >>>>> storage couldn't be reserved. >>>> >>>> There are still some cases that are theoretically problematic: vmsplice() >>>> can pin pages forever and doesn't use FOLL_LONGTERM yet. >>>> >>>> All these things also affect other users that rely on movability (e.g., CMA, >>>> memory hotunplug). >>> >>> I wasn't aware of that, thank you for the information. Then to ensure that the >>> process doesn't hang by replying the loop indefinitely, I'll migrate the page if >>> tag storage cannot be reserved. Looking over the code again, I think I can reuse >>> the same function that migrates tag storage pages out of the MTE VMA (added in >>> patch #21), so no major changes needed. >> >> It's going to be interesting if migrating that page fails because it is >> pinned :/ > > I imagine that having both the page **and** its tag storage pinned longterm > without FOLL_LONGTERM is going to be exceedingly rare. Yes. I recall that the rule of thumb is that some O_DIRECT I/O can take up to 10 seconds, although extremely rare (and maybe not applicable on arm64). > > Am I mistaken in believing that the problematic vmsplice() behaviour is > recognized as something that needs to be fixed? Yes, for a couple of years I'm hoping this will actually get fixed now that O_DIRECT mostly uses FOLL_PIN instead of FOLL_GET. -- Cheers, David / dhildenb