Hello, currently we never clear dirty bits in the radix tree of a DAX inode. Thus fsync(2) or even periodical writeback flush all the dirty pfns again and again. This patches implement clearing of the dirty tag in the radix tree so that we issue flush only when needed. The difficulty with clearing the dirty tag is that we have to protect against a concurrent page fault setting the dirty tag and writing new data into the page. So we need a lock serializing page fault and clearing of the dirty tag and write-protecting PTEs (so that we get another pagefault when pfn is written to again and we have to set the dirty tag again). The effect of the patch set is easily visible: Writing 1 GB of data via mmap, then fsync twice. Before this patch set both fsyncs take ~205 ms on my test machine, after the patch set the first fsync takes ~283 ms (the additional cost of walking PTEs, clearing dirty bits etc. is very noticeable), the second fsync takes below 1 us. As a bonus, these patches make filesystem freezing for DAX filesystems reliable because mappings are now properly writeprotected while freezing the fs. Patches have passed xfstests for both xfs and ext4. So far the patches don't work with PMD pages - that's next on my todo list. Honza -- To unsubscribe from this list: send the line "unsubscribe linux-fsdevel" in the body of a message to majordomo@xxxxxxxxxxxxxxx More majordomo info at http://vger.kernel.org/majordomo-info.html
