Hello, this is a second revision of my patches to clear dirty bits from radix tree of DAX inodes when caches for corresponding pfns have been flushed. This patch set is significantly larger than the previous version because I'm changing how ->fault, ->page_mkwrite, and ->pfn_mkwrite handlers may choose to handle the fault so that we don't have to leak details about DAX locking into the generic code. In principle, these patches enable handlers to easily update PTEs and do other work necessary to finish the fault without duplicating the functionality present in the generic code. I'd be really interested in feedback from mm folks whether such changes to fault handling code are fine or what they'd do differently... Changes since v1: * make sure all PTE updates happen under radix tree entry lock to protect against races between faults & write-protecting code * remove information about DAX locking from mm/memory.c * smaller updates based on Ross' feedback ---- Background information regarding the motivation: Currently we never clear dirty bits in the radix tree of a DAX inode. Thus fsync(2) flushes 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. Honza -- To unsubscribe, send a message with 'unsubscribe linux-mm' in the body to majordomo@xxxxxxxxx. For more info on Linux MM, see: http://www.linux-mm.org/ . Don't email: <a href=mailto:"dont@xxxxxxxxx";> email@xxxxxxxxx </a>
