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Registered in England and Wales under Company Registration No. 3798903 From: David Howells In-Reply-To: <492558dc-1377-fc4b-126f-c358bb000ff7@redhat.com> References: <492558dc-1377-fc4b-126f-c358bb000ff7@redhat.com> <20230526214142.958751-1-dhowells@redhat.com> <20230526214142.958751-2-dhowells@redhat.com> <510965.1685522152@warthog.procyon.org.uk> To: David Hildenbrand Cc: dhowells@redhat.com, Christoph Hellwig , Lorenzo Stoakes , Jens Axboe , Al Viro , Matthew Wilcox , Jan Kara , Jeff Layton , Jason Gunthorpe , Logan Gunthorpe , Hillf Danton , Christian Brauner , Linus Torvalds , linux-fsdevel@vger.kernel.org, linux-block@vger.kernel.org, linux-kernel@vger.kernel.org, linux-mm@kvack.org, Andrew Morton Subject: Re: [PATCH v4 1/3] mm: Don't pin ZERO_PAGE in pin_user_pages() MIME-Version: 1.0 Content-Type: text/plain; charset="us-ascii" Content-ID: <703627.1685541335.1@warthog.procyon.org.uk> Content-Transfer-Encoding: quoted-printable Date: Wed, 31 May 2023 14:55:35 +0100 Message-ID: <703628.1685541335@warthog.procyon.org.uk> X-Scanned-By: MIMEDefang 3.1 on 10.11.54.8 X-Rspamd-Queue-Id: 23F8C1C0011 X-Rspam-User: X-Rspamd-Server: rspam02 X-Stat-Signature: x8ox4unz1c38gomwcjdkq3z5j8ehop7j X-HE-Tag: 1685541343-744724 X-HE-Meta: U2FsdGVkX1+rL21iRbjQPlVCiDVP0tdxsEqPXC41yRvZsMOVas/yAhEb2gBvaa8T3dQoi4e5NtKAA6foB0uIQEjYtMJl7Yg3QkdalTJcYj07nBQwXCJWBkGqDUePKI0gLg3h0aqnOYIEAIRE5DSHzWPS9KUK01zMFCsStYwoofVUpIxtzhBvNX/7mDens/c9nxpOCEMx5m8N/GPoyy1EDSJ7N6LiZMCqY+rvdkoVy5r0ChE8LR4GWhDZG6re3Kd5AstcAbirqoHuMPdTVV/pEbg19PUPJTtNPTU/lL8NXyGmuapIa5dKMMxjgp4g6mVB7GqHr8CPTmtBfbik4pyGIKj48GgQx2D75ZwcdmQ3ZKOXLQMF4MABqGABFFaEBsI9/QlqxBVpcoMc9IrWOvtJ0sAqpiAJu5rXQPXzStxVOXGkKgBUpf0UbPw8CQlLPcyuo2A+0fbeq1etxsQMZ3NvK7eOGfAbWI+ooE+agCfmPWl+LKc+DfyGVvwX21rcdKrn8LmDPBgtDrwf9FAIrY7OIYRfVnnZsjAGDpsjdX12hrDC/WcDCTD2bjlhjtssGhhQOMEOd2lLRM+e1Gg4XukjXqU1ioAH6KkwERew2alBvdFizMntMyNqcmaGDKbGq3FYTzC/fpBja73jogn/nUes8WJEpsNgzNvP7eVDNCXBrjaWZ0l863ULQ8ldynZlrCmxzo/xLINoZGVuXgprPMn5lPWNH6rwQxwaMg6ilW3GL/FxKlU05dPbi8B6nEasyivRZkXpU9zpWvt9u1bNgCbPoCdZgJLUQK9vBQ7LIt9+684BALSWgT22l6SDKxawhaHi+SD9k5DOJpr9OhhREjTv88Wq2TXqcyQOfTA5ckOJLLEGB29SAhI0qX16LX261SQ5DxxNo6fjI8DYnEyaaILU8GZveaE2Fi1kzLKezWkx6ZW0bCnWfsT0L2LZBxYowMPQBANmV+KhWAHsIrtnzPc mqQznwL7 otq8OS+K1NtVsQoNVV9stQ6LaGxQ6ACRkocW32+OC3EiHtX41UIsUA6RoCD+2jQyd7V45ueCGh6FQLFcyvOb88IFRsGOiUFGxXEMKzBVrYh/e3QTU0NJdq8B6pwMOYywprTmOVFxKnteReEpxNuJZ5nsqfm9/b1aMBBjGc/hBBBWAB0x4PSuaEet5HyKtObfZGtxT X-Bogosity: Ham, tests=bogofilter, spamicity=0.000000, version=1.2.4 Sender: owner-linux-mm@kvack.org Precedence: bulk X-Loop: owner-majordomo@kvack.org List-ID: David Hildenbrand wrote: > Yes, it would be clearer if we would be using "pinned" now only for FOLL= _PIN You're not likely to get that. "To pin" is too useful a verb that gets us= ed in other contexts too. For that reason, I think FOLL_PIN was a poor choic= e of name:-/. I guess the English language has got somewhat overloaded. Maybe FOLL_PEG? ;-) > and everything else is simply "taking a temporary reference on the page"= . Excluding refs taken with pins, many refs are more permanent than pins as,= so far as I'm aware, pins only last for the duration of an I/O operation. > >> "Note that the refcount of any zero_pages returned among the pinned p= ages will > >> not be incremented, and unpin_user_page() will similarly not decremen= t it." > > That's not really right (although it happens to be true), because we'r= e > > talking primarily about the pin counter, not the refcount - and they m= ay be > > separate. > = > In any case (FOLL_PIN/FOLL_GET) you increment/decrement the refcount. If= we > have a separate pincount, we increment/decrement the refcount by 1 when > (un)pinning. FOLL_GET isn't relevant here - only FOLL_PIN. Yes, as it happens, we coun= t a ref if we count a pin, but that's kind of irrelevant; what matters is that= the effect must be undone with un-PUP. It would be nice not to get a ref on the zero page in FOLL_GET, but I don'= t think we can do that yet. Too many places assume that GUP will give them = a ref they can release later via ordinary methods. David