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[2003:cb:c708:1d00:99ec:9656:7475:678d]) by smtp.gmail.com with ESMTPSA id m6-20020a5d4a06000000b003330aede2aesm2717126wrq.112.2023.11.28.08.57.32 (version=TLS1_3 cipher=TLS_AES_128_GCM_SHA256 bits=128/128); Tue, 28 Nov 2023 08:57:33 -0800 (PST) Message-ID: <0a0f9345-3138-4e89-80cd-c7edaf2ff62d@redhat.com> Date: Tue, 28 Nov 2023 17:57:31 +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 05/27] mm: page_alloc: Add an arch hook to allow prep_new_page() to fail Content-Language: en-US To: Alexandru Elisei Cc: 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, hyesoo.yu@samsung.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> <20231119165721.9849-6-alexandru.elisei@arm.com> From: David Hildenbrand Autocrypt: addr=david@redhat.com; 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charset=UTF-8; format=flowed Content-Transfer-Encoding: 7bit On 27.11.23 13:09, Alexandru Elisei wrote: > Hi, > > Thank you so much for your comments, there are genuinely useful. > > On Fri, Nov 24, 2023 at 08:35:47PM +0100, David Hildenbrand wrote: >> On 19.11.23 17:56, Alexandru Elisei wrote: >>> Introduce arch_prep_new_page(), which will be used by arm64 to reserve tag >>> storage for an allocated page. Reserving tag storage can fail, for example, >>> if the tag storage page has a short pin on it, so allow prep_new_page() -> >>> arch_prep_new_page() to similarly fail. >> >> But what are the side-effects of this? How does the calling code recover? >> >> E.g., what if we need to populate a page into user space, but that >> particular page we allocated fails to be prepared? So we inject a signal >> into that poor process? > > When the page fails to be prepared, it is put back to the tail of the > freelist with __free_one_page(.., FPI_TO_TAIL). If all the allocation paths > are exhausted and no page has been found for which tag storage has been > reserved, then that's treated like an OOM situation. > > I have been thinking about this, and I think I can simplify the code by > making tag reservation a best effort approach. The page can be allocated > even if reserving tag storage fails, but the page is marked as invalid in > set_pte_at() (PAGE_NONE + an extra bit to tell arm64 that it needs tag > storage) and next time it is accessed, arm64 will reserve tag storage in > the fault handling code (the mechanism for that is implemented in patch #19 > of the series, "mm: mprotect: Introduce PAGE_FAULT_ON_ACCESS for > mprotect(PROT_MTE)"). > > With this new approach, prep_new_page() stays the way it is, and no further > changes are required for the page allocator, as there are already arch > callbacks that can be used for that, for example tag_clear_highpage() and > arch_alloc_page(). The downside is extra page faults, which might impact > performance. > > What do you think? That sounds a lot more robust, compared to intermittent failures to allocate pages. -- Cheers, David / dhildenb