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Thu, 11 Mar 2021 09:22:39 -0800 (PST) X-Google-Smtp-Source: ABdhPJy8mshgg/0L30aXCuIEK/e/kasHDCVKsTgLCNdiIRDLs1wgoybQJrClHsg8u2sJjcTMIlmNUQ== X-Received: by 2002:a05:6214:1144:: with SMTP id b4mr8771834qvt.12.1615483359302; Thu, 11 Mar 2021 09:22:39 -0800 (PST) Received: from xz-x1 ([142.126.89.138]) by smtp.gmail.com with ESMTPSA id k126sm2473549qkb.4.2021.03.11.09.22.37 (version=TLS1_3 cipher=TLS_AES_256_GCM_SHA384 bits=256/256); Thu, 11 Mar 2021 09:22:38 -0800 (PST) Date: Thu, 11 Mar 2021 12:22:36 -0500 From: Peter Xu To: David Hildenbrand Subject: Re: [PATCH v3 02/12] softmmu/physmem: Fix ram_block_discard_range() to handle shared anonymous memory Message-ID: <20210311172236.GG194839@xz-x1> References: <20210308150600.14440-1-david@redhat.com> <20210308150600.14440-3-david@redhat.com> <20210311171153.GF194839@xz-x1> <26d2c57a-971d-3abd-6ec3-1a38fb47a398@redhat.com> MIME-Version: 1.0 In-Reply-To: <26d2c57a-971d-3abd-6ec3-1a38fb47a398@redhat.com> Authentication-Results: relay.mimecast.com; auth=pass smtp.auth=CUSA124A263 smtp.mailfrom=peterx@redhat.com X-Mimecast-Spam-Score: 0 X-Mimecast-Originator: redhat.com Content-Type: text/plain; charset=utf-8 Content-Disposition: inline Received-SPF: pass client-ip=216.205.24.124; envelope-from=peterx@redhat.com; helo=us-smtp-delivery-124.mimecast.com X-Spam_score_int: -29 X-Spam_score: -3.0 X-Spam_bar: --- X-Spam_report: (-3.0 / 5.0 requ) BAYES_00=-1.9, DKIMWL_WL_HIGH=-0.25, DKIM_SIGNED=0.1, DKIM_VALID=-0.1, DKIM_VALID_AU=-0.1, DKIM_VALID_EF=-0.1, RCVD_IN_DNSWL_LOW=-0.7, RCVD_IN_MSPIKE_H3=0.001, RCVD_IN_MSPIKE_WL=0.001, SPF_HELO_NONE=0.001, SPF_PASS=-0.001 autolearn=ham autolearn_force=no X-Spam_action: no action X-BeenThere: qemu-devel@nongnu.org X-Mailman-Version: 2.1.23 Precedence: list List-Id: List-Unsubscribe: , List-Archive: List-Post: List-Help: List-Subscribe: , Cc: Marcel Apfelbaum , Cornelia Huck , Eduardo Habkost , "Michael S. Tsirkin" , Stefan Weil , Murilo Opsfelder Araujo , Richard Henderson , "Dr. David Alan Gilbert" , Juan Quintela , qemu-devel@nongnu.org, Halil Pasic , Christian Borntraeger , Greg Kurz , Stefan Hajnoczi , Igor Mammedov , Thomas Huth , Paolo Bonzini , Philippe =?utf-8?Q?Mathieu-Daud=C3=A9?= , Igor Kotrasinski Errors-To: qemu-devel-bounces+qemu-devel=archiver.kernel.org@nongnu.org Sender: "Qemu-devel" On Thu, Mar 11, 2021 at 06:15:15PM +0100, David Hildenbrand wrote: > On 11.03.21 18:11, Peter Xu wrote: > > On Thu, Mar 11, 2021 at 05:45:46PM +0100, David Hildenbrand wrote: > > > On 11.03.21 17:39, Dr. David Alan Gilbert wrote: > > > > * David Hildenbrand (david@redhat.com) wrote: > > > > > We can create shared anonymous memory via > > > > > "-object memory-backend-ram,share=on,..." > > > > > which is, for example, required by PVRDMA for mremap() to work. > > > > > > > > > > Shared anonymous memory is weird, though. Instead of MADV_DONTNEED, we > > > > > have to use MADV_REMOVE. MADV_DONTNEED fails silently and does nothing. > > > > > > > > OK, I wonder how stable these rules are; is it defined anywhere that > > > > it's required? > > > > > > > > > > I had a look at the Linux implementation: it's essentially shmem ... but we > > > don't have an fd exposed, so we cannot use fallocate() ... :) > > > > > > MADV_REMOVE documents (man): > > > > > > "In the initial implementation, only tmpfs(5) was supported MADV_REMOVE; but > > > since Linux 3.5, any filesystem which supports the fallocate(2) > > > FALLOC_FL_PUNCH_HOLE mode also supports MADV_REMOVE." > > > > Hmm, I see that MADV_DONTNEED will still tear down all mappings even for > > anonymous shmem.. what did I miss? > > Where did you see that? I see madvise_dontneed_free() calls zap_page_range(). > > > > > MADV_DONTNEED only invalidates private copies in the pagecache. It's > essentially useless for any kind of shared mappings. Since it's about zapping page tables, then I don't understand why it won't work for shmem.. -- Peter Xu