On Sat 06-02-16 17:33:07, Dmitry Monakhov wrote:

quoted

+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.

quoted

+	}
+	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 [off-list ref]
SUSE Labs, CR

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