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 E6EDE93; Thu, 30 Nov 2023 06:33:35 -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 357001042; Thu, 30 Nov 2023 06:34:22 -0800 (PST) Received: from raptor (unknown [172.31.20.19]) by usa-sjc-imap-foss1.foss.arm.com (Postfix) with ESMTPSA id AC1633F5A1; Thu, 30 Nov 2023 06:33:30 -0800 (PST) Date: Thu, 30 Nov 2023 14:33:28 +0000 From: Alexandru Elisei To: David Hildenbrand 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 Subject: Re: [PATCH RFC v2 19/27] mm: mprotect: Introduce PAGE_FAULT_ON_ACCESS for mprotect(PROT_MTE) Message-ID: 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> 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: 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. Am I mistaken in believing that the problematic vmsplice() behaviour is recognized as something that needs to be fixed? Thanks, Alex > > -- > Cheers, > > David / dhildenb >