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[34.76.113.97]) by smtp.gmail.com with ESMTPSA id 5b1f17b1804b1-493c636ea60sm326134815e9.3.2026.07.06.05.41.32 (version=TLS1_3 cipher=TLS_AES_256_GCM_SHA384 bits=256/256); Mon, 06 Jul 2026 05:41:32 -0700 (PDT) Date: Mon, 6 Jul 2026 12:41:29 +0000 From: Adrian =?utf-8?Q?Barna=C5=9B?= To: "Mike Rapoport (Microsoft)" Cc: linux-arm-kernel@lists.infradead.org, linux-mm@kvack.org, Catalin Marinas , Will Deacon , David Hildenbrand , "Mike Rapoport (Microsoft)" , Ard Biesheuvel , Christoph Lameter , Yang Shi , Brendan Jackman , Ryan Roberts Subject: Re: [RFC PATCH 6/6] arm64: mm: support PMD page coalescing in the linear map Message-ID: References: <20260611130144.1385343-1-abarnas@google.com> <20260611130144.1385343-7-abarnas@google.com> <799181c3-a1a1-4de7-bc6a-576d3282efb0@arm.com> MIME-Version: 1.0 Content-Type: text/plain; charset=us-ascii; format=flowed Content-Disposition: inline In-Reply-To: <799181c3-a1a1-4de7-bc6a-576d3282efb0@arm.com> X-Rspam-User: X-Rspamd-Server: rspam11 X-Rspamd-Queue-Id: 1B0C84000B X-Stat-Signature: ezkntf89ttbrhbjxzt6ye65sbhzogo6z X-HE-Tag: 1783341694-196814 X-HE-Meta: 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 osE6y0vd qMK2EzhWJU2hwpJJL4c6RP3fRFScq3r6RiPT8ez3mCUM6chq/tiEccECbdPKWqoKQHAnxceNgATF3LNQbwnzE3Bo3h0iFH/KqtKWLinlH9sRuYK6Ls3r62W/Tqdx3G0A93VILx+54Q0ac3hy0A+Im18lGirIzj56GSSCNw0oWiW8U+NVSfJS9N+fW8sLBFIq76lRrDBCkxMgiYmi6BzCJKLp00tTnPW72WATrWarCBBRtIfxsC5M1AZYYAHAWlo9ocqlQJpZAhtpd+5j/bRhKCUx1as9DPHcZh97KGJ/W2O8BqrksiGMrPyoesVu3WgxqBGlu9xZOEJXC644PuaZzj6wnmmZtL6F+4jJBQR1jpxQZcarwg7jbzMxbaplw/7a1dHzl Sender: owner-linux-mm@kvack.org Precedence: bulk X-Loop: owner-majordomo@kvack.org List-ID: List-Subscribe: List-Unsubscribe: Hi Mike, On Fri, Jun 19, 2026 at 02:40:40PM +0100, Ryan Roberts wrote: >I don't think this is safe in general. Let's say we have a 2M region split into >512 x 4K PTEs. It's possible that the first 1M is one object and the second 1M >is another object. Different CPUs could set_memory_*() on those 2 objects >concurrently. If one of them then calls this function, we could end up >collapsing the whole 2M while the other is trying to modify the PTEs and they >will race. > >Note that splitting _is_ safe (and protected by this lock) because you'd have 2 >objects backed by the same PMD, so they would both have to split before >modifying the PTEs. > >I think you'd need to ensure mutual exclusion at a higher level if doing this; >probably execmem is the place that can ensure that no objects within a 2M region >are racily trying to modify their permissions? > >Thanks, >Ryan I wonder what do you think about enforcing mutual exclusion for permission changes to the ROX_CACHE block as Ryan sugest? Would it be appropriate to add locks (over `&execmem_cache.mutex`) to `execmem_force_rw` and `execmem_restore_rox`? I see that x86 has a lock that wraps entire process of updating attributes (including spliting and collapsing pages): ``` spin_lock(&cpa_lock); ret = __change_page_attr(cpa, primary); spin_unlock(&cpa_lock); ``` As I understand it makes it safe already. However, on ARM64 there is a split lock (which works fine for spliting) but for collapsing at least on my implementation it is not sufficient as Ryan pointed out. Best regards Adrian