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[2003:cb:c710:f600:634b:35f:ffa8:475b]) by smtp.gmail.com with ESMTPSA id t2-20020a5d4602000000b0032da4c98ab2sm18135574wrq.35.2023.11.29.05.13.51 (version=TLS1_3 cipher=TLS_AES_128_GCM_SHA256 bits=128/128); Wed, 29 Nov 2023 05:13:52 -0800 (PST) Message-ID: <930b6fba-43bf-4784-9bc9-1c83c1adc30c@redhat.com> Date: Wed, 29 Nov 2023 14:13:50 +0100 MIME-Version: 1.0 User-Agent: Mozilla Thunderbird Subject: Re: [PATCH RFC v2 18/27] arm64: mte: Reserve tag block for the zero page 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> <20231119165721.9849-19-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 X-Rspamd-Queue-Id: 55CD018001F X-Rspam-User: X-Stat-Signature: jr5ujxm74sb9nq54of7kjkgbnrz3np46 X-Rspamd-Server: rspam01 X-HE-Tag: 1701263636-733379 X-HE-Meta: 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 ZvtA416Q +d7UGCaWdTMbTVzhyZGoghSaYNO1bldCj5alqZIQKZ3qbkqUGhz4sJ9rFrLakAByN916sBKAHhOEby372lFyhMFhHUIW+PCD4Ij1fGCezRXY5ItIHAIQ7OK1wg+sdX/ZyxNRWkva1RqW93l0tAXPJE4q+Ny9tNh2Ay+aftt//EapXjC2NXAvzzxVzSqPpJteoB6aLHoPqUAqbfTQl9fjsf1Adh1EzDjMlYAQMo2huBXavBHvoQCka7Gix9dPfD00wdmrNJ0jv23osh05I+puY5WqJQQ1bzTl26VlN1/xBwbOjxObtd/sZ+DRGtOyy3QfDMFG3AYY2hMiEfqqhGXqPfNP+n8f+Fg8YNNMrfeXBW2SvqU6GbUSSPJoJaqSVxGSA3Hk/FivS3Rw3dYm8VJfhTzc1aa2W8gWIwvOqHd6fhbdaHYc= 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 29.11.23 12:30, Alexandru Elisei wrote: > On Tue, Nov 28, 2023 at 06:06:54PM +0100, David Hildenbrand wrote: >> On 19.11.23 17:57, Alexandru Elisei wrote: >>> On arm64, the zero page receives special treatment by having the tagged >>> flag set on MTE initialization, not when the page is mapped in a process >>> address space. Reserve the corresponding tag block when tag storage >>> management is being activated. >> >> Out of curiosity: why does the shared zeropage require tagged storage? What >> about the huge zeropage? > > There are two different tags that are used for tag checking: the logical > tag, the tag embedded in bits 59:56 of an address, and the physical tag > corresponding to the address. This tag is stored in a separate memory > location, called tag storage. When an access is performed, hardware > compares the logical tag (from the address) with the physical tag (from the > tag storage). If they match, the access is permitted. Ack, matches my understanding. > > The physical tag is set with special instructions. > > Userspace pointers have bits 59:56 zero. If the pointer is in a VMA with > MTE enabled, then for userspace to be able to access this address, the > physical tag must also be 0b0000. > > To make it easier on userspace, when a page is first mapped as tagged, its > tags are cleared by the kernel; this way, userspace can access the address > immediately, without clearing the physical tags beforehand. Another reason > for clearing the physical tags when a page is mapped as tagged would be to > avoid leaking uninitialized tags to userspace. Make sense. Zero it just like we zero page content. > > The zero page is special, because the physical tags are not zeroed every > time the page is mapped in a process; instead, the zero page is marked as > tagged (by setting a page flag) and the physical tags are zeroed only once, > when MTE is enabled at boot. Makes sense. > > All of this means that when tag storage is enabled, which happens after MTE > is enabled, the tag storage corresponding to the zero page is already in > use and must be rezerved, and it can never be used for data allocations. > > I hope all of the above makes sense. I can also put it in the commit > message :) Yes, makes sense! > > As for the zero huge page, the MTE code in the kernel treats it like a > regular page, and it zeroes the tags when it is mapped as tagged in a > process. I agree that this might not be the best solution from a > performance perspective, but it has worked so far. What if user space were to change the tag of that shared resource? Having a tag != 0 doesn't make sense for such a shared resource, so I suspect modifying the tag is like a write event: trigger write-fault -> COW. > > With tag storage management enabled, set_pte_at()->mte_sync_tags() will > discover that the huge zero page doesn't have tag storage reserved, the > table entry will be mapped as invalid to use the page fault-on-access > mechanism that I introduce later in the series [1] to reserve tag storage, I assume (without looking at the code) that you took proper care of possible races. Thanks for goind into detail! -- Cheers, David / dhildenb