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b=feMhQtw2K2zdChSgSLstFJd3bNy6uSEy99k1mtXz/kwtBgV0LHgd76MCWex8UmUaU J30waBA4qqFt0LfuZ9tX1AiS4HPnftbJsHySSaQMBZFgHqf9nTA+5ZUw6rXhdZT/4G a02Jn8IsCJqunmYz/v71ueP17hLxHodCT71U4nixoIioRKUoLK2g95v/iQFBJhzqo+ p40LpQg/kOO7qu51oF1kL4tPWdDZjT/jsrpAeow2jWjQr0s1pthhzyBUNvsVM0VJt4 DM4/zS4GSXUEmvbYAXIuZA0W/JpGEVe+/bnuGLsoM7BeHhVzVo25bHLqWKU5NVRzTt YgvxQ0OadhQgw== Message-ID: <1dfa0bd6-9658-4d29-ae5d-baad0a7a9572@kernel.org> Date: Wed, 11 Feb 2026 20:55:37 +0100 MIME-Version: 1.0 User-Agent: Mozilla Thunderbird Subject: Re: [RFC 1/2] mm: thp: allocate PTE page tables lazily at split time To: Matthew Wilcox , Usama Arif Cc: Andrew Morton , lorenzo.stoakes@oracle.com, linux-mm@kvack.org, fvdl@google.com, hannes@cmpxchg.org, riel@surriel.com, shakeel.butt@linux.dev, kas@kernel.org, baohua@kernel.org, dev.jain@arm.com, baolin.wang@linux.alibaba.com, npache@redhat.com, Liam.Howlett@oracle.com, ryan.roberts@arm.com, vbabka@suse.cz, lance.yang@linux.dev, linux-kernel@vger.kernel.org, kernel-team@meta.com References: <20260211125507.4175026-1-usama.arif@linux.dev> <20260211125507.4175026-2-usama.arif@linux.dev> From: "David Hildenbrand (Arm)" Content-Language: en-US Autocrypt: addr=david@kernel.org; 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charset=UTF-8; format=flowed Content-Transfer-Encoding: 7bit X-Stat-Signature: apyu8fpxfg3f9oous8y5arzgzuyo889j X-Rspam-User: X-Rspamd-Server: rspam08 X-Rspamd-Queue-Id: BD0A140006 X-HE-Tag: 1770839747-413737 X-HE-Meta: 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 GJc3Vcj8 KqLG33sbbMg71gKoPNa/slKZQy0X8NWEZlGio0N4fDxkivYKzlnH5XdFBV/o6BMNeHnvZKvFXxYcXN7oB+wvyZY0VDC6v3oWYLttpyNi4+JqASzXPLdXUuWSnnA9HWVCYWIBfCmlZDDuh5rZJEoxEu366LcIFt1Jj1fQFcKbXN5B9Zv/kvjtmZRkd35BBvq2r4C383pqRuOR73wFOSVVDkVNjaFDpn1sVTeQSmGbATRFf0ohs8RVjA3UCI4pi7G3AnoMO 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 2/11/26 20:28, Matthew Wilcox wrote: > On Wed, Feb 11, 2026 at 04:49:45AM -0800, Usama Arif wrote: >> - Memory reclaim (try_to_unmap()): Returns false, folio rotated back >> LRU, retried in next reclaim cycle. > > I was advised to ask my stupid question ... > > Why do we still try to split the PMD in reclaim? I understand we're > about to swap the folio out and we'll need to put a swap entry in the page > table so we can find it again. But can't we now store swap entries at the > PMD level, or are we still forced to store 512 entries at the PTE level? Yes. We don't support PMD swap entries yet. I don't know all historical details. I suspect there are some rough edges around swapin (assume we cannot swapin a 2M THP), and maybe it was just easier to not deal with splitting of PMD swap entries (which we would similarly have to support). For sure an interesting project to look into. -- Cheers, David