From mboxrd@z Thu Jan 1 00:00:00 1970 Authentication-Results: smtp.subspace.kernel.org; dkim=none Received: from foss.arm.com (foss.arm.com [217.140.110.172]) by lindbergh.monkeyblade.net (Postfix) with ESMTP id 88FF610EC; Tue, 28 Nov 2023 09:17:33 -0800 (PST) Received: from usa-sjc-imap-foss1.foss.arm.com (unknown [10.121.207.14]) by usa-sjc-mx-foss1.foss.arm.com (Postfix) with ESMTP id 99F93C15; Tue, 28 Nov 2023 09:18:20 -0800 (PST) Received: from raptor (unknown [172.31.20.19]) by usa-sjc-imap-foss1.foss.arm.com (Postfix) with ESMTPSA id 3F3773F6C4; Tue, 28 Nov 2023 09:17:28 -0800 (PST) Date: Tue, 28 Nov 2023 17:17:25 +0000 From: Alexandru Elisei To: David Hildenbrand 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 Subject: Re: [PATCH RFC v2 05/27] mm: page_alloc: Add an arch hook to allow prep_new_page() to fail Message-ID: References: <20231119165721.9849-1-alexandru.elisei@arm.com> <20231119165721.9849-6-alexandru.elisei@arm.com> <0a0f9345-3138-4e89-80cd-c7edaf2ff62d@redhat.com> Precedence: bulk X-Mailing-List: linux-fsdevel@vger.kernel.org List-Id: List-Subscribe: List-Unsubscribe: MIME-Version: 1.0 Content-Type: text/plain; charset=us-ascii Content-Disposition: inline In-Reply-To: <0a0f9345-3138-4e89-80cd-c7edaf2ff62d@redhat.com> Hi, On Tue, Nov 28, 2023 at 05:57:31PM +0100, David Hildenbrand wrote: > 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. Great, thank you for the feedback, I will use this approach for the next iteration of the series. Thanks, Alex > > -- > Cheers, > > David / dhildenb >