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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 Precedence: bulk X-Mailing-List: linux-fsdevel@vger.kernel.org List-Id: List-Subscribe: List-Unsubscribe: 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) Content-Language: en-US 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 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