My application fallocates and mmaps (shared, writable) a lot (several GB) of data at startup. Those mappings are mlocked, and they live on ext4. The first write to any given page is slow because ext4_da_get_block_prep can block. This means that, to get decent performance, I need to write something to all of these pages at startup. This, in turn, causes a giant IO storm as several GB of zeros get pointlessly written to disk. This series is an attempt to add madvise(..., MADV_WILLWRITE) to signal to the kernel that I will eventually write to the referenced pages. It should cause any expensive operations that happen on the first write to happen immediately, but it should not result in dirtying the pages. madvice(addr, len, MADV_WILLWRITE) returns the number of bytes that the operation succeeded on or a negative error code if there was an actual failure. A return value of zero signifies that the kernel doesn't know how to "willwrite" the range and that userspace should implement a fallback. For now, it only works on shared writable ext4 mappings. Eventually it should support other filesystems as well as private pages (it should COW the pages but not cause swap IO) and anonymous pages (it should COW the zero page if applicable). The implementation leaves much to be desired. In particular, it generates dirty buffer heads on a clean page, and this scares me. Thoughts? Andy Lutomirski (3): mm: Add MADV_WILLWRITE to indicate that a range will be written to fs: Add block_willwrite ext4: Implement willwrite for the delalloc case fs/buffer.c | 57 ++++++++++++++++++++++++++++++++++ fs/ext4/ext4.h | 2 ++ fs/ext4/file.c | 1 + fs/ext4/inode.c | 22 +++++++++++++ include/linux/buffer_head.h | 3 ++ include/linux/mm.h | 12 +++++++ include/uapi/asm-generic/mman-common.h | 3 ++ mm/madvise.c | 28 +++++++++++++++-- 8 files changed, 126 insertions(+), 2 deletions(-) -- 1.8.3.1 -- To unsubscribe from this list: send the line "unsubscribe linux-ext4" in the body of a message to majordomo@xxxxxxxxxxxxxxx More majordomo info at http://vger.kernel.org/majordomo-info.html
