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[172.10.233.147]) by smtp.gmail.com with ESMTPSA id d2e1a72fcca58-7127aef5457sm8629985b3a.127.2024.08.20.13.34.13 (version=TLS1_3 cipher=TLS_AES_256_GCM_SHA384 bits=256/256); Tue, 20 Aug 2024 13:34:13 -0700 (PDT) Date: Tue, 20 Aug 2024 13:33:59 -0700 (PDT) From: Hugh Dickins To: Vincent Donnefort cc: Matthew Wilcox , Andrew Morton , David Hildenbrand , Andrew Bresticker , linux-mm@kvack.org, linux-kernel@vger.kernel.org, maz@kernel.org, seanjc@google.com Subject: Re: [PATCH v2] mm/memory: Don't require head page for do_set_pmd() In-Reply-To: Message-ID: <0d4f99bd-609a-8c76-b665-adcf942e6a74@google.com> References: <20240611153216.2794513-1-abrestic@rivosinc.com> <8040793f-e9e9-4a2e-807c-afcb310a48f5@redhat.com> <20240611110622.8e9892e92618ddc36bca11b7@linux-foundation.org> Precedence: bulk X-Mailing-List: linux-kernel@vger.kernel.org List-Id: List-Subscribe: List-Unsubscribe: MIME-Version: 1.0 Content-Type: text/plain; charset=US-ASCII On Mon, 19 Aug 2024, Vincent Donnefort wrote: > On Tue, Jun 11, 2024 at 07:22:03PM +0100, Matthew Wilcox wrote: > > On Tue, Jun 11, 2024 at 11:06:22AM -0700, Andrew Morton wrote: ... > > > You know what I'm going to ask ;) I'm assuming that the runtime effects > > > are "small performance optimization" and that "should we backport the > > > fix" is "no". > > > > We're going to stop using PMDs to map large folios unless the fault is > > within the first 4KiB of the PMD. No idea how many workloads that > > affects, but it only needs to be backported as far as v6.8, so we > > may as well backport it. > > Hi, I am reviving this thread after noticing this comment attached > to the fix. > > If you intend to install PTE level mappings for faults that happen outside of > the first 4KiB, I believe this will make THP support for KVM ineffective. You can relax, it's okay: where Matthew wrote "We're going to stop...", he was describing the runtime effects of the bug (now fixed) to Andrew, not proposing to make a change to mess up THP support. The fix was backported to v6.9.N, but was too late for v6.8.N EOL. Hugh