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b=HGzUzC6f4aroci2M61+q5sGquB80q1nj1gcRlPhIu3eax9X+7c56OCAu+6wYNlABB 7faeIbFwpTgogflebupr6NZIIoQ6Ed5xcHLe9KnguCADZngZw5didjCfxOfHhGTkwc A/tCJvLgMnxirehIyE8R713SJyX7TDlQi+ORzqyzmI/GaidGiPI0Vj6KVBlgp74uzO nRW0ZurqANeYHJFHSzfUUtiFbOZgWLRQhAePeMTghm9LA+XjKQwfwkMSvvbJk66cvg IeH7HBQfVbJk+a+ZYDld9E6T7GdnygpLZGGjC86A57vELVeTEYw8hqrJ9zNuFiVW8+ ejLoBmkUWQlXg== Message-ID: <580fbfd5-2db7-4c46-9335-003ac99d1db9@kernel.org> Date: Fri, 2 Oct 2026 20:53:07 +0200 MIME-Version: 1.0 User-Agent: Mozilla Thunderbird Subject: Re: [PATCH v2 1/2] mm/migrate: walk runs of consecutive pages in do_pages_stat_array() To: Qiliang Yuan , Andrew Morton , Zi Yan , Matthew Brost , Joshua Hahn , Byungchul Park , Gregory Price , Ying Huang , Alistair Popple Cc: linux-mm@kvack.org, linux-kernel@vger.kernel.org References: <20261002-bug-mm-move-pages-stat-batch-v2-0-f73b5d20519f@gmail.com> <20261002-bug-mm-move-pages-stat-batch-v2-1-f73b5d20519f@gmail.com> From: "David Hildenbrand (Arm)" Content-Language: en-US Autocrypt: addr=david@kernel.org; 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charset=UTF-8 Content-Transfer-Encoding: 7bit X-Rspamd-Server: rspam05 X-Rspamd-Queue-Id: DED5D18000D X-Stat-Signature: bnth6h14ssbonocmtaaiwix3z95yh1se X-Rspam-User: X-HE-Tag: 1790967194-400553 X-HE-Meta: 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 /CyuH0Bk sxqVBBD9Ws0lf3Bl7K8Qh4LOepMenKlfHabQmXe4Ir3KQ2VN7sg1luQ35R/lUYYPsqw41cSHotdwITJVFdy6U7hXPmz+/Kn523/WCnwM+YB5KCvNZDK6uhW672ufOULznx1k+ruBJ88Y53I4BksnRZqZhcNe3RQ2cao8RGHc8vBbX7m2gkWDuE22xvs7Jp+ILiaEczzHPib/1ra2DDl4eBuvfFMI4iVnk9CdmRcN428VXk1s9DJreOvq53r+cyzVQC9MTwEPj7mOwn294Pk09aEyJD2O8mBoADSZ2fhbeODT/k1o2B69x8Q0O8Jv79pdNwPyiptSfuuLGm2LG9Jx84R4sb4+uLkoFfQcNv+tUMGcN+jV49tbIRjDkOs9VGGCBCEIYQaVVs+QVpIQVHPWcgJAfhewLT/5X8FeWGZFflURwEGk= Sender: owner-linux-mm@kvack.org Precedence: bulk X-Loop: owner-majordomo@kvack.org List-ID: List-Subscribe: List-Unsubscribe: On 10/2/26 03:25, Qiliang Yuan wrote: > move_pages() with a NULL node list reports the node of each page. RDMA > and KV-cache transfer engines use it to find where large registered > buffers live, querying every 4K page of buffers that span hundreds of > gigabytes. > > do_pages_stat_array() looks up the VMA and walks the page tables from > the top for every address, taking and dropping the PTE lock each time. > That costs about 105 ns per page, so a 16 GiB buffer takes 486 ms. > > Callers almost always pass consecutive addresses. Group them into runs > and walk each run with walk_page_range(), which looks up each VMA and > PTE table once and answers every page under it while holding the lock. > Report pages as folio_walk_start() with FW_ZEROPAGE found them: the node > of a normal folio, -EFAULT for the zero page or an address outside any > VMA, and -ENOENT otherwise. Handle PUD and hugetlb leaves in their own > callbacks so that the walk never splits them. > > On 7.3-rc5 in a 16-vCPU VM, querying every page of a populated 4 GiB > buffer: > > before after > 4K pages 105 ns 31.9 ns > THP 90 ns 23.1 ns > > Suggested-by: Zi Yan > Signed-off-by: Qiliang Yuan > --- > mm/migrate.c | 188 ++++++++++++++++++++++++++++++++++++++++++++++++++--------- > 1 file changed, 162 insertions(+), 26 deletions(-) That's a lot of churn ... which is really a shame, because all we want to walk is folio ranges. Oscar was working on a better page table walker API (but I was too busy to provide review so far :( ), which sounds strategically like the better long-term solution. Is there particular need to optimize this in the ns range for 4 KiB of memory immediately? -- Cheers, David