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Received: from willy by casper.infradead.org with local (Exim 4.97.1 #2 (Red Hat Linux)) id 1rzyRu-000000032Gx-0008; Thu, 25 Apr 2024 12:43:38 +0000 Date: Thu, 25 Apr 2024 13:43:37 +0100 From: Matthew Wilcox To: David Hildenbrand Cc: Andrew Morton , linux-mm@kvack.org Subject: Re: [PATCH 5/7] gup: Use folios for gup_devmap Message-ID: References: <20240424191914.361554-1-willy@infradead.org> <20240424191914.361554-6-willy@infradead.org> <3f317def-9690-44b5-8d17-9a31e6eee32a@redhat.com> MIME-Version: 1.0 Content-Type: text/plain; charset=us-ascii Content-Disposition: inline In-Reply-To: <3f317def-9690-44b5-8d17-9a31e6eee32a@redhat.com> X-Rspamd-Server: rspam01 X-Rspamd-Queue-Id: EB03E4000B X-Stat-Signature: szd4com9ijznmrm3rgtnpn64urbj8bqa X-Rspam-User: X-HE-Tag: 1714049020-442271 X-HE-Meta: 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 bvWFZEPF xt/uVVZq9hvNe8IrjZDPuhJ/hFjFV3RK5W802/+Y36FBJzraZtF2m369rVWNcAnt3OFbQQSGXnvLKRP4UiIlJ0X3wRdXCxVYKnF3Qhbi/cTdpyNTQGFjAFKKucVaN9Ue0cbjpEWO1GHyXkAQch5tMxjZRyrhq1eJ7f26iVHfR+8KqGYxp1tdVv7oHPg== 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: List-Subscribe: List-Unsubscribe: On Thu, Apr 25, 2024 at 11:49:01AM +0200, David Hildenbrand wrote: > > while ((*nr) - nr_start) { > > - struct page *page = pages[--(*nr)]; > > + struct folio *folio = page_folio(pages[--(*nr)]); > > - ClearPageReferenced(page); > > I stumbled over that likely unwarranted ClearPageReferenced() recently as > well: what if the page was already referenced before we called > SetPageReferenced? Yes, it seems bizarre. I imagine the person who wrote this was trying to undo everything, but no other cleanup path calls folio_clear_referenced(). I don't object to taking it out. > > - if (flags & FOLL_PIN) > > - unpin_user_page(page); > > - else > > - put_page(page); > > + folio_clear_referenced(folio); > > + gup_put_folio(folio, 1, flags); > > For !FOLL_PIN, we wouldn't have done the > if (!put_devmap_managed_page_refs(&folio->page, refs)) > folio_put_refs(folio, refs); > > Magic in gup_put_folio() > > ... was that a BUG? I think so. But probably nobody noticed because it's an error path. Also nobody noticed that we shouldn't have called SetPageReferenced() before getting the refcount on the page. I didn't bother mentioning it in the changelog because I think it's harmless, just conceptually wrong.