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Thu, 06 Feb 2025 13:06:31 -0800 (PST) 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: MIME-Version: 1.0 Content-Type: text/plain; charset=us-ascii Content-Disposition: inline In-Reply-To: X-Rspamd-Queue-Id: 6F0F440016 X-Stat-Signature: 7dho3gkmc74z187tx3wmped96moj3oyq X-Rspam-User: X-Rspamd-Server: rspam10 X-HE-Tag: 1738875993-578158 X-HE-Meta: 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 DZ3cCzEy 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 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 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