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b=jKHPTuBmLFZipjd9P4kREgUhXNCvJCeC2JaAU0iC/JBffENTSJaTkCZu7s981u7Uk det8OEUxoTnC8iAQKoOaSIgcf6KGQXWjftcVGVZxviw8jds4C/5PHzbNy6VceuuE7z js+VQ/jlz6s2zAY5fYrPzab0LyQsPk5m6vjpIxzE1rFqG3mywMVnKL+kjsHd4xcISb h/k9nDaErnfi6zJjuqMY8rUGSUKpgwi6ruLawoqF61ZVJYp19sXUwQncH3+hYbfgEh UXFgb5mGrfeEc5HkZmqzQKUDuLBsQ4UwI2UBAQDWgNuknib9mIp1606vArV28lOrJB 7Y0Xht+9Z2FbQ== Date: Tue, 13 Oct 2026 13:33:16 +0300 From: Mike Rapoport To: "David Hildenbrand (Arm)" Cc: Zenghui Yu , linux-mm@kvack.org, linux-kernel@vger.kernel.org, akpm@linux-foundation.org, peterx@redhat.com, ljs@kernel.org, liam@infradead.org, vbabka@kernel.org, surenb@google.com, mhocko@suse.com Subject: Re: [PATCH] selftests/mm: fix uffd move-pmd test crash on 64K base page systems Message-ID: References: <20261011163527.95988-1-zenghui.yu@linux.dev> <67c30e4c-3576-4ded-a71f-b49075ed42e3@kernel.org> MIME-Version: 1.0 Content-Type: text/plain; charset=us-ascii Content-Disposition: inline In-Reply-To: <67c30e4c-3576-4ded-a71f-b49075ed42e3@kernel.org> X-Rspamd-Queue-Id: 5E4F6100005 X-Rspam-User: X-Rspamd-Server: rspam05 X-Stat-Signature: 7rg6okf8s63yep4773yj9fmpno9ftjt9 X-HE-Tag: 1791887604-658610 X-HE-Meta: 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 a7fnER9h dCNs+aY/qJPKuH1yunwT2KSuSboYJoCEvA0sdIM9vXQr/eGBONfaHL4mqMWOctfKL0f5fhz7sXliJWhMQW78hPl/aUs4TMmLw/vyqdT9AMpW8h9LFkCtXQjDY48DzbBqyxkArvg4V0kzzPNxqp9nzghUCIzkVwHwrMT12l97PAothOrWl5YX2FACXpc7ctBm387b0cv+/QnkK7Ob1fq1wt4l8501KltbsKJhYZ7vp2gCuQEpPSZ9o3Uzy1gyNbzGqN37r Sender: owner-linux-mm@kvack.org Precedence: bulk X-Loop: owner-majordomo@kvack.org List-ID: List-Subscribe: List-Unsubscribe: On Tue, Oct 13, 2026 at 09:39:54AM +0200, David Hildenbrand (Arm) wrote: > On 10/11/26 18:35, Zenghui Yu wrote: > > From: "Zenghui Yu (Huawei)" > > > > uffd_move_pmd_test()/uffd_move_test_common() splits the test area into > > chunks of chunk_size and moves them one by one. On arm64 with 64K base > > pages the chunk_size (read_pmd_pagesize()) is 512MB, which is larger than > > the whole 32MB (UFFD_TEST_MEM_SIZE) test area: > > > > step_count = nr_pages / (chunk_size / page_size) == 0 > > > > Because chunk_size > page_size, the test then aligns both areas up to > > 512MB, which points far outside of the 32MB mappings, and decrements > > step_count, which underflows to ULONG_MAX. The verification loop then > > runs away and the first dereference of area_src faults with a SIGSEGV. > > > > Skip the test when a single chunk cannot fit into the test area. > > That leaves 64k untested, though? Why not adjust the chunk size accordingly? To test anything PMD on arm64 with 64k pages we need to adjust UFFD_TEST_MEM_SIZE to have at least 4 pages, i.e 2G :) Maybe we want UFFD_TEST_MEM_SIZE to scale with the page size? > -- > Cheers, > > David -- Sincerely yours, Mike.