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Thu, 19 Feb 2026 09:08:29 -0800 (PST) Date: Thu, 19 Feb 2026 12:08:28 -0500 From: Johannes Weiner To: Zi Yan Cc: "David Hildenbrand (Arm)" , Usama Arif , willy@infradead.org, Lorenzo Stoakes , Andrew Morton , lsf-pc@lists.linux-foundation.org, linux-mm@kvack.org, riel@surriel.com, Shakeel Butt , Kiryl Shutsemau , Barry Song , Dev Jain , Baolin Wang , Nico Pache , "Liam R . Howlett" , Ryan Roberts , Vlastimil Babka , Lance Yang , Frank van der Linden Subject: Re: [LSF/MM/BPF TOPIC] Beyond 2MB: Why Terabyte-Scale Machines Need 1GB Transparent Huge Pages Message-ID: References: <540c5c13-9cfb-44ea-b18f-8e4abff30a01@linux.dev> <3485c8c8-9bfc-4725-885a-626e79d0aebb@kernel.org> MIME-Version: 1.0 Content-Type: text/plain; charset=utf-8 Content-Disposition: inline Content-Transfer-Encoding: 8bit In-Reply-To: X-Rspam-User: X-Rspamd-Server: rspam07 X-Rspamd-Queue-Id: 97DC5100014 X-Stat-Signature: w7k5s8d8gcqmhi7ojabkpesyk8basqnk X-HE-Tag: 1771520911-339548 X-HE-Meta: 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 vHAEwcwP GN9GEHWw0LQWums1kYxuV1gnuzwqtQ/a4vLkaprv0MZQnRhuP0yrtY/CdI2gGf/NZQln9J0UwunPrE4zGttG79zYTExC+TSJBA1ZREAl4Yl8xaXDn52M635rRTxL006K1L1VVm0QXDKSbM/WfgpNzONUzdPM4KQWJ+tafdRkj+ntCrv35lXhuLmo5N5fpDb/XXGSLoHND89kQuNQx9ELBaGQCoNF5VBAq1oQx5dH4EspG4SLbr6mSPqEF7mNwqEIgm1rV1mXn3bdHuAfeJf/ajaRLcTY/iEoIS4ccDOvQTbp8JQM07MiyWWYC2uONEmLTHb7fBHgphk/85zCqPX74+uAzeq0083u0LnLvY+KBfw4hRkTh/0CgrKx1wIjleQ5trqBwbvKfrKo0BI91HbEd13ae7toAsydcEEO2Ad6/2sFbxuVWW1VhrTTwnw== 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 Thu, Feb 19, 2026 at 11:52:57AM -0500, Zi Yan wrote: > On 19 Feb 2026, at 11:48, Johannes Weiner wrote: > > > On Thu, Feb 19, 2026 at 05:00:19PM +0100, David Hildenbrand (Arm) wrote: > >> > >>> > >>> I see 1G THPs being opportunistically used ideally at the start of the application > >>> or by the allocator (jemalloc/tcmalloc) when there is plenty of free memory > >>> available and a greater chance of getting 1G THPs. > >>> > >>> Splitting strategy > >>> ================== > >>> > >>> When PUD THP must be break -- for COW after fork, partial munmap, mprotect on > >>> a subregion, or reclaim -- it splits directly from PUD to PTE level, converting > >>> 1 PUD entry into 262,144 PTE entries. The ideal solution would be to split to > >>> PMDs and only the necessary PMDs to PTEs. This is something that would hopefully > >>> be possible with Davids proposal [3]. > >> > >> There once was this proposal where we would, instead of splitting a THP, > >> migrate all memory away instead. That means, instead of splitting the 1 > >> GiB THP, you would instead return it to the page allocator where > >> somebody else could use it. > > > > With TLB coalescing, there is benefit in preserving contiguity. If you > > lop off the last 4k of a 2M-backed range, a split still gives you 511 > > contiguously mapped pfns that can be coalesced. > > Which CPU are you referring to? AMD’s PTE coalescing works up to 32KB > and ARM’s contig PTE supports larger sizes. BTW, do we have PMD level > ARM contiguous bit support? I'm not aware of a CPU that will coalesce the 511 entries into a single one. But *any* coalescing effects will be lost when the range is scattered into discontiguous 4k pagelets.