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Howlett" , Nico Pache , Ryan Roberts , Dev Jain , Barry Song , Lance Yang , Usama Arif , Peter Xu , Alex Williamson , kvm@vger.kernel.org, =?utf-8?B?TG/Dr2M=?= Molinari , Lorenzo Stoakes , Shuah Khan , linux-kselftest@vger.kernel.org, dri-devel@lists.freedesktop.org, Madhavan Srinivasan , "Ritesh Harjani (IBM)" , linuxppc-dev@lists.ozlabs.org, linux-mm@kvack.org, linux-kernel@vger.kernel.org, Jason Gunthorpe Subject: Re: [PATCH v4 4/4] selftests/mm: test fork of PMD-mapped shmem THPs Message-ID: References: <20261021-b4-prep-vstor-149847-copy_huge_pmd-recheck-special-pmd-v4-0-66f00b7a564c@virtuozzo.com> <20261021-b4-prep-vstor-149847-copy_huge_pmd-recheck-special-pmd-v4-4-66f00b7a564c@virtuozzo.com> X-Mailing-List: linux-kernel@vger.kernel.org List-Id: List-Post: List-Help: List-Subscribe: List-Unsubscribe: List-Owner: Precedence: list MIME-Version: 1.0 Content-Type: text/plain; charset=us-ascii Content-Disposition: inline In-Reply-To: <20261021-b4-prep-vstor-149847-copy_huge_pmd-recheck-special-pmd-v4-4-66f00b7a564c@virtuozzo.com> On Wed, Oct 21, 2026 at 10:36:03PM +0200, Konstantin Khorenko wrote: > + /* The child's exit took the parent's share off the counter. */ > + if (*after < own || *before - *after >= own) > + return FAIL_COUNTER; > + if (*after != *before) > + return CHANGED; > + if (mem[pmdsize] != (shared ? 'c' : 2) || !data_ok(mem, size, pmdsize) || > + !check_huge_shmem(mem, size - pmdsize, NR_PMDS - 1, pmdsize)) > + return FAIL_PARENT; ShmemPmdMapped is system-wide. A drop of 'own' or more from another process between the two reads fails the test instead of being retried as CHANGED. The parent's THPs can also lose their PMDs during the test: reclaim can swap one out, and NUMA balancing can migrate one, which leaves the counter low while the migration entry is in place. That gives FAIL_COUNTER, or FAIL_PARENT from the exact count in check_huge_shmem(). -- Kiryl Shutsemau / Kirill A. Shutemov