When servicing mmap() reads from file holes the current DAX code allocates a page cache page of all zeroes and places the struct page pointer in the mapping->page_tree radix tree. This has three major drawbacks: 1) It consumes memory unnecessarily. For every 4k page that is read via a DAX mmap() over a hole, we allocate a new page cache page. This means that if you read 1GiB worth of pages, you end up using 1GiB of zeroed memory. 2) It is slower than using a common zero page because each page fault has more work to do. Instead of just inserting a common zero page we have to allocate a page cache page, zero it, and then insert it. 3) The fact that we had to check for both DAX exceptional entries and for page cache pages in the radix tree made the DAX code more complex. This series solves these issues by following the lead of the DAX PMD code and using a common 4k zero page instead. This reduces memory usage and decreases latencies for some workloads, and it simplifies the DAX code, removing over 100 lines in total. Andrew, I'm still hoping to get this merged for v4.13 if possible. I I have addressed all of Jan's feedback, but he is on vacation for the next few weeks so he may not be able to give me Reviewed-by tags. I think this series is relatively low risk with clear benefits, and I think we should be able to address any issues that come up during the v4.13 RC series. This series has passed my targeted testing and a full xfstests run on both XFS and ext4. --- Changes since v2: - If we call insert_pfn() with 'mkwrite' for an entry that already exists, don't overwrite the pte with a brand new one. Just add the appropriate flags. (Jan) - Keep put_locked_mapping_entry() as a simple wrapper for dax_unlock_mapping_entry() so it has naming parity with get_unlocked_mapping_entry(). (Jan) - Remove DAX special casing in page_cache_tree_insert(), move now-private definitions from dax.h to dax.c. (Jan) Ross Zwisler (5): mm: add vm_insert_mixed_mkwrite() dax: relocate some dax functions dax: use common 4k zero page for dax mmap reads dax: remove DAX code from page_cache_tree_insert() dax: move all DAX radix tree defs to fs/dax.c Documentation/filesystems/dax.txt | 5 +- fs/dax.c | 345 ++++++++++++++++---------------------- fs/ext2/file.c | 25 +-- fs/ext4/file.c | 32 +--- fs/xfs/xfs_file.c | 2 +- include/linux/dax.h | 45 ----- include/linux/mm.h | 2 + include/trace/events/fs_dax.h | 2 - mm/filemap.c | 13 +- mm/memory.c | 57 ++++++- 10 files changed, 205 insertions(+), 323 deletions(-) -- 2.9.4
