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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> MIME-Version: 1.0 Content-Type: text/plain; charset=US-ASCII X-Rspamd-Server: rspam03 X-Rspam-User: X-Rspamd-Queue-Id: C041C20020 X-Stat-Signature: in8i9bwaj5eje9uyk33kduukgg8koh4c X-HE-Tag: 1724186056-897280 X-HE-Meta: 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 O7waEmJg br8WekjtdkaA2Ifj1ZLGYuX+C7Uy6xbOBYOve/PLocfboRWRViS26CEEEhuFvOE2KyBu3TiSFFUrBEHCBQnqFdIjkUDNoVdv0Tmt/NnidEIblsib2EcTpyikipFL6NQJlubzVis+EONuV0tIxfaKHqGXvy3Lx8l7EEkrk3HHjHYBN7dja8Ct4f/l1dJ0yCmELFZNUPJOtjdZxaK2bWVTTuTqr959dnvzR8NAYbAx70FfcxyHaFnR1Tb8+jGE2ZrtEurCh9HnuOv1B/Pnr0sAVLe+9e8A7Y0vsY59v7jAdhmIbZ9K/w/tjcIqewTCraI+jZMyItSMKu5/aH5GHWiM70I85DkfTmTC3otj84HczGicjyb1T77/uBDEpgBDGWrb+7ckD 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 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