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[99.254.121.117]) by smtp.gmail.com with ESMTPSA id 6a1803df08f44-6ac070c2d12sm45629346d6.14.2024.05.28.09.05.17 (version=TLS1_3 cipher=TLS_AES_256_GCM_SHA384 bits=256/256); Tue, 28 May 2024 09:05:17 -0700 (PDT) Date: Tue, 28 May 2024 12:05:16 -0400 From: Peter Xu To: Nicholas Piggin Cc: qemu-devel@nongnu.org, Fabiano Rosas , Thomas Huth , Laurent Vivier , Paolo Bonzini , qemu-ppc@nongnu.org Subject: Re: [PATCH] tests/qtest/migrate-test: Use regular file file for shared-memory tests Message-ID: References: <20240528042758.621589-1-npiggin@gmail.com> MIME-Version: 1.0 Content-Type: text/plain; charset=utf-8 Content-Disposition: inline In-Reply-To: Received-SPF: pass client-ip=170.10.133.124; envelope-from=peterx@redhat.com; helo=us-smtp-delivery-124.mimecast.com X-Spam_score_int: -20 X-Spam_score: -2.1 X-Spam_bar: -- X-Spam_report: (-2.1 / 5.0 requ) BAYES_00=-1.9, DKIMWL_WL_HIGH=-0.034, DKIM_SIGNED=0.1, DKIM_VALID=-0.1, DKIM_VALID_AU=-0.1, DKIM_VALID_EF=-0.1, RCVD_IN_DNSWL_NONE=-0.0001, RCVD_IN_MSPIKE_H4=0.001, RCVD_IN_MSPIKE_WL=0.001, SPF_HELO_NONE=0.001, SPF_PASS=-0.001, T_SCC_BODY_TEXT_LINE=-0.01 autolearn=unavailable autolearn_force=no X-Spam_action: no action X-BeenThere: qemu-devel@nongnu.org X-Mailman-Version: 2.1.29 Precedence: list List-Id: List-Unsubscribe: , List-Archive: List-Post: List-Help: List-Subscribe: , Errors-To: qemu-devel-bounces+qemu-devel=archiver.kernel.org@nongnu.org Sender: qemu-devel-bounces+qemu-devel=archiver.kernel.org@nongnu.org On Tue, May 28, 2024 at 09:35:22AM -0400, Peter Xu wrote: > On Tue, May 28, 2024 at 02:27:57PM +1000, Nicholas Piggin wrote: > > There is no need to use /dev/shm for file-backed memory devices, and > > it is too small to be usable in gitlab CI. Switch to using a regular > > file in /tmp/ which will usually have more space available. > > > > Signed-off-by: Nicholas Piggin > > --- > > Am I missing something? AFAIKS there is not even any point using > > /dev/shm aka tmpfs anyway, there is not much special about it as a > > filesystem. This applies on top of the series just sent, and passes > > gitlab CI qtests including aarch64. > > I think it's just that /dev/shm guarantees shmem usage, while the var > "tmpfs" implies g_dir_make_tmp() which may be another non-ram based file > system, while that'll be slightly different comparing to what a real user > would use - we don't suggest user to put guest RAM on things like btrfs. > > One real implication is if we add a postcopy test it'll fail with > g_dir_make_tmp() when it is not pointing to a shmem mount, as > UFFDIO_REGISTER will fail there. But that test doesn't yet exist as the > QEMU paths should be the same even if Linux will trigger different paths > when different types of mem is used (anonymous v.s. shmem). > > If the goal here is to properly handle the case where tmpfs doesn't have > enough space, how about what I suggested in the other email? > > https://lore.kernel.org/r/ZlSppKDE6wzjCF--@x1n > > IOW, try populate the shmem region before starting the guest, skip if > population failed. Would that work? Let me append some more info here.. I think madvise() isn't needed as fallocate() should do the population work already, afaiu, then it means we pass the shmem path to QEMU and QEMU should notice this memory-backend-file existed, open() directly. I quicked walk the QEMU memory code and so far it looks all applicable, so that QEMU should just start the guest with the pre-populated shmem page caches. There's one trick where qemu_ram_mmap() will map some extra pages, on x86 4k, and I don't yet know why we did that.. /* * Note: this always allocates at least one extra page of virtual address * space, even if size is already aligned. */ total = size + align; But that was only used in mmap_reserve() not mmap_activate(), so the real mmap() should still be exactly what we fallocate()ed. Thanks, -- Peter Xu