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[49.181.91.157]) by smtp.gmail.com with ESMTPSA id p13-20020a17090a348d00b002311ae14a01sm4907325pjb.11.2023.03.27.16.02.09 (version=TLS1_3 cipher=TLS_AES_256_GCM_SHA384 bits=256/256); Mon, 27 Mar 2023 16:02:09 -0700 (PDT) Received: from dave by dread.disaster.area with local (Exim 4.92.3) (envelope-from ) id 1pgvqo-00DwxK-SK; Tue, 28 Mar 2023 10:02:06 +1100 Date: Tue, 28 Mar 2023 10:02:06 +1100 From: Dave Chinner To: "Matthew Wilcox (Oracle)" Cc: Andrew Morton , linux-xfs@vger.kernel.org, linux-afs@lists.infradead.org, linux-fsdevel@vger.kernel.org, linux-mm@kvack.org Subject: Re: [PATCH v2 3/3] mm: Hold the RCU read lock over calls to ->map_pages Message-ID: <20230327230206.GB3223426@dread.disaster.area> References: <20230327174515.1811532-1-willy@infradead.org> <20230327174515.1811532-4-willy@infradead.org> MIME-Version: 1.0 Content-Type: text/plain; charset=us-ascii Content-Disposition: inline In-Reply-To: <20230327174515.1811532-4-willy@infradead.org> Precedence: bulk List-ID: X-Mailing-List: linux-xfs@vger.kernel.org On Mon, Mar 27, 2023 at 06:45:15PM +0100, Matthew Wilcox (Oracle) wrote: > Prevent filesystems from doing things which sleep in their map_pages > method. This is in preparation for a pagefault path protected only > by RCU. > > Signed-off-by: Matthew Wilcox (Oracle) > --- > Documentation/filesystems/locking.rst | 4 ++-- > mm/memory.c | 11 ++++++++--- > 2 files changed, 10 insertions(+), 5 deletions(-) > > diff --git a/Documentation/filesystems/locking.rst b/Documentation/filesystems/locking.rst > index 922886fefb7f..8a80390446ba 100644 > --- a/Documentation/filesystems/locking.rst > +++ b/Documentation/filesystems/locking.rst > @@ -645,7 +645,7 @@ ops mmap_lock PageLocked(page) > open: yes > close: yes > fault: yes can return with page locked > -map_pages: yes > +map_pages: read > page_mkwrite: yes can return with page locked > pfn_mkwrite: yes > access: yes > @@ -661,7 +661,7 @@ locked. The VM will unlock the page. > > ->map_pages() is called when VM asks to map easy accessible pages. > Filesystem should find and map pages associated with offsets from "start_pgoff" > -till "end_pgoff". ->map_pages() is called with page table locked and must > +till "end_pgoff". ->map_pages() is called with the RCU lock held and must > not block. If it's not possible to reach a page without blocking, > filesystem should skip it. Filesystem should use set_pte_range() to setup > page table entry. Pointer to entry associated with the page is passed in > diff --git a/mm/memory.c b/mm/memory.c > index 8071bb17abf2..a7edf6d714db 100644 > --- a/mm/memory.c > +++ b/mm/memory.c > @@ -4461,6 +4461,7 @@ static vm_fault_t do_fault_around(struct vm_fault *vmf) > /* The page offset of vmf->address within the VMA. */ > pgoff_t vma_off = vmf->pgoff - vmf->vma->vm_pgoff; > pgoff_t from_pte, to_pte; > + vm_fault_t ret; > > /* The PTE offset of the start address, clamped to the VMA. */ > from_pte = max(ALIGN_DOWN(pte_off, nr_pages), > @@ -4476,9 +4477,13 @@ static vm_fault_t do_fault_around(struct vm_fault *vmf) > return VM_FAULT_OOM; > } > > - return vmf->vma->vm_ops->map_pages(vmf, > - vmf->pgoff + from_pte - pte_off, > - vmf->pgoff + to_pte - pte_off); > + rcu_read_lock(); > + ret = vmf->vma->vm_ops->map_pages(vmf, > + vmf->pgoff + from_pte - pte_off, > + vmf->pgoff + to_pte - pte_off); > + rcu_read_unlock(); > + > + return ret; Doesn't this mean that the rcu_read_lock/unlock can be removed from filemap_map_pages()? i.e. all callers are now already under rcu_read_lock(). Maybe WARN_ON_ONCE(!rcu_read_lock_held()) could be put in filemap_map_pages() if you are worried about callers not holding it... Otherwise it looks fine. -Dave. -- Dave Chinner david@fromorbit.com