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Thu, 06 Feb 2025 13:06:32 -0800 (PST) Received: from server.roeck-us.net ([2600:1700:e321:62f0:da43:aeff:fecc:bfd5]) by smtp.gmail.com with ESMTPSA id 98e67ed59e1d1-2fa17618064sm1089562a91.41.2025.02.06.13.06.31 (version=TLS1_3 cipher=TLS_AES_256_GCM_SHA384 bits=256/256); Thu, 06 Feb 2025 13:06:31 -0800 (PST) Sender: Guenter Roeck Date: Thu, 6 Feb 2025 13:06:30 -0800 From: Guenter Roeck To: Alistair Popple Cc: akpm@linux-foundation.org, dan.j.williams@intel.com, linux-mm@kvack.org, Alison Schofield , lina@asahilina.net, zhang.lyra@gmail.com, gerald.schaefer@linux.ibm.com, vishal.l.verma@intel.com, dave.jiang@intel.com, logang@deltatee.com, bhelgaas@google.com, jack@suse.cz, jgg@ziepe.ca, catalin.marinas@arm.com, will@kernel.org, mpe@ellerman.id.au, npiggin@gmail.com, dave.hansen@linux.intel.com, ira.weiny@intel.com, willy@infradead.org, djwong@kernel.org, tytso@mit.edu, linmiaohe@huawei.com, david@redhat.com, peterx@redhat.com, linux-doc@vger.kernel.org, linux-kernel@vger.kernel.org, linux-arm-kernel@lists.infradead.org, linuxppc-dev@lists.ozlabs.org, nvdimm@lists.linux.dev, linux-cxl@vger.kernel.org, linux-fsdevel@vger.kernel.org, linux-ext4@vger.kernel.org, linux-xfs@vger.kernel.org, jhubbard@nvidia.com, hch@lst.de, david@fromorbit.com, chenhuacai@kernel.org, kernel@xen0n.name, loongarch@lists.linux.dev Subject: Re: [PATCH v7 19/20] fs/dax: Properly refcount fs dax pages Message-ID: References: Precedence: bulk X-Mailing-List: linux-kernel@vger.kernel.org List-Id: List-Subscribe: List-Unsubscribe: MIME-Version: 1.0 Content-Type: text/plain; charset=us-ascii Content-Disposition: inline In-Reply-To: On Wed, Feb 05, 2025 at 09:48:16AM +1100, Alistair Popple wrote: > Currently fs dax pages are considered free when the refcount drops to > one and their refcounts are not increased when mapped via PTEs or > decreased when unmapped. This requires special logic in mm paths to > detect that these pages should not be properly refcounted, and to > detect when the refcount drops to one instead of zero. > > On the other hand get_user_pages(), etc. will properly refcount fs dax > pages by taking a reference and dropping it when the page is > unpinned. > > Tracking this special behaviour requires extra PTE bits > (eg. pte_devmap) and introduces rules that are potentially confusing > and specific to FS DAX pages. To fix this, and to possibly allow > removal of the special PTE bits in future, convert the fs dax page > refcounts to be zero based and instead take a reference on the page > each time it is mapped as is currently the case for normal pages. > > This may also allow a future clean-up to remove the pgmap refcounting > that is currently done in mm/gup.c. > > Signed-off-by: Alistair Popple > Reviewed-by: Dan Williams > > --- ... > -static inline unsigned long dax_page_share_put(struct page *page) > +static inline unsigned long dax_folio_put(struct folio *folio) > { > - WARN_ON_ONCE(!page->share); > - return --page->share; > + unsigned long ref; > + > + if (!dax_folio_is_shared(folio)) > + ref = 0; > + else > + ref = --folio->share; > + > + WARN_ON_ONCE(ref < 0); Kind of unlikely for an unsigned long to ever be < 0. [ thanks to coverity for noticing ] Guenter