From: Jan Kara Subject: Re: [PATCH v8 6/9] dax: add support for fsync/msync Date: Mon, 8 Feb 2016 10:44:30 +0100 Message-ID: <20160208094430.GA9451@quack.suse.cz> References: <1452230879-18117-1-git-send-email-ross.zwisler@linux.intel.com> <1452230879-18117-7-git-send-email-ross.zwisler@linux.intel.com> <878u2xrjrw.fsf@openvz.org> Mime-Version: 1.0 Content-Type: text/plain; charset=us-ascii Cc: Ross Zwisler , linux-kernel@vger.kernel.org, "H. Peter Anvin" , "J. Bruce Fields" , Theodore Ts'o , Alexander Viro , Andreas Dilger , Andrew Morton , Dan Williams , Dave Chinner , Dave Hansen , Ingo Molnar , Jan Kara , Jeff Layton , Matthew Wilcox , Matthew Wilcox , Thomas Gleixner , linux-ext4@vger.kernel.org, linux-fsdevel@vger.kernel.org, linux-mm@kvack.org, linux-nvdimm@lists.01.org, x86@kernel.org, xfs@oss.sgi.com To: Dmitry Monakhov Return-path: Content-Disposition: inline In-Reply-To: <878u2xrjrw.fsf@openvz.org> Sender: owner-linux-mm@kvack.org List-Id: linux-ext4.vger.kernel.org On Sat 06-02-16 17:33:07, Dmitry Monakhov wrote: > > +int dax_writeback_mapping_range(struct address_space *mapping, loff_t start, > > + loff_t end) > > +{ > > + struct inode *inode = mapping->host; > > + struct block_device *bdev = inode->i_sb->s_bdev; > > + pgoff_t indices[PAGEVEC_SIZE]; > > + pgoff_t start_page, end_page; > > + struct pagevec pvec; > > + void *entry; > > + int i, ret = 0; > > + > > + if (WARN_ON_ONCE(inode->i_blkbits != PAGE_SHIFT)) > > + return -EIO; > > + > > + rcu_read_lock(); > > + entry = radix_tree_lookup(&mapping->page_tree, start & PMD_MASK); > > + rcu_read_unlock(); > > + > > + /* see if the start of our range is covered by a PMD entry */ > > + if (entry && RADIX_DAX_TYPE(entry) == RADIX_DAX_PMD) > > + start &= PMD_MASK; > > + > > + start_page = start >> PAGE_CACHE_SHIFT; > > + end_page = end >> PAGE_CACHE_SHIFT; > > + > > + tag_pages_for_writeback(mapping, start_page, end_page); > > + > > + pagevec_init(&pvec, 0); > > + while (1) { > > + pvec.nr = find_get_entries_tag(mapping, start_page, > > + PAGECACHE_TAG_TOWRITE, PAGEVEC_SIZE, > > + pvec.pages, indices); > > + > > + if (pvec.nr == 0) > > + break; > > + > > + for (i = 0; i < pvec.nr; i++) { > > + ret = dax_writeback_one(bdev, mapping, indices[i], > > + pvec.pages[i]); > > + if (ret < 0) > > + return ret; > > + } > I think it would be more efficient to use batched locking like follows: > spin_lock_irq(&mapping->tree_lock); > for (i = 0; i < pvec.nr; i++) { > struct blk_dax_ctl dax[PAGEVEC_SIZE]; > radix_tree_tag_clear(page_tree, indices[i], PAGECACHE_TAG_TOWRITE); > /* It is also reasonable to merge adjacent dax > * regions in to one */ > dax[i].sector = RADIX_DAX_SECTOR(entry); > dax[i].size = (type == RADIX_DAX_PMD ? PMD_SIZE : PAGE_SIZE); > > } > spin_unlock_irq(&mapping->tree_lock); > if (blk_queue_enter(q, true) != 0) > goto error; > for (i = 0; i < pvec.nr; i++) { > rc = bdev_direct_access(bdev, dax[i]); > wb_cache_pmem(dax[i].addr, dax[i].size); > } > ret = blk_queue_exit(q, true) We need to clear the radix tree tag only after flushing caches. But in principle I agree that some batching of radix tree tag manipulations should be doable. But frankly so far we have issues with correctness so speed is not our main concern. > > + } > > + wmb_pmem(); > > + return 0; > > +} > > +EXPORT_SYMBOL_GPL(dax_writeback_mapping_range); > > + > > static int dax_insert_mapping(struct inode *inode, struct buffer_head *bh, > > struct vm_area_struct *vma, struct vm_fault *vmf) > > { > > @@ -363,6 +532,11 @@ static int dax_insert_mapping(struct inode *inode, struct buffer_head *bh, > > } > > dax_unmap_atomic(bdev, &dax); > > > > + error = dax_radix_entry(mapping, vmf->pgoff, dax.sector, false, > > + vmf->flags & FAULT_FLAG_WRITE); > > + if (error) > > + goto out; > > + > > error = vm_insert_mixed(vma, vaddr, dax.pfn); > > > > out: > > @@ -487,6 +661,7 @@ int __dax_fault(struct vm_area_struct *vma, struct vm_fault *vmf, > > delete_from_page_cache(page); > > unlock_page(page); > > page_cache_release(page); > > + page = NULL; > > } > I've realized that I do not understand why dax_fault code works at all. > During dax_fault we want to remove page from mapping and insert dax-entry > Basically code looks like follows: > 0 page = find_get_page() > 1 lock_page(page) > 2 delete_from_page_cache(page); > 3 unlock_page(page); > 4 dax_insert_mapping(inode, &bh, vma, vmf); > > BUT what on earth protects us from other process to reinsert page again > after step(2) but before (4)? Nothing, it's a bug and Ross / Matthew are working on fixing it... Honza -- Jan Kara SUSE Labs, CR -- To unsubscribe, send a message with 'unsubscribe linux-mm' in the body to majordomo@kvack.org. For more info on Linux MM, see: http://www.linux-mm.org/ . Don't email: email@kvack.org