On Mon, Apr 18, 2016 at 10:26:10PM +0200, Jan Kara wrote: > On Fri 15-04-16 22:05:31, Andrew Morton wrote: > > On Thu, 14 Apr 2016 10:48:29 -0600 Toshi Kani <toshi.kani@xxxxxxx> wrote: > > > > > When CONFIG_FS_DAX_PMD is set, DAX supports mmap() using pmd page > > > size. This feature relies on both mmap virtual address and FS > > > block (i.e. physical address) to be aligned by the pmd page size. > > > Users can use mkfs options to specify FS to align block allocations. > > > However, aligning mmap address requires code changes to existing > > > applications for providing a pmd-aligned address to mmap(). > > > > > > For instance, fio with "ioengine=mmap" performs I/Os with mmap() [1]. > > > It calls mmap() with a NULL address, which needs to be changed to > > > provide a pmd-aligned address for testing with DAX pmd mappings. > > > Changing all applications that call mmap() with NULL is undesirable. > > > > > > This patch-set extends filesystems to align an mmap address for > > > a DAX file so that unmodified applications can use DAX pmd mappings. > > > > Matthew sounded unconvinced about the need for this patchset, but I > > must say that > > > > : The point is that we do not need to modify existing applications for using > > : DAX PMD mappings. > > : > > : For instance, fio with "ioengine=mmap" performs I/Os with mmap(). > > : https://github.com/caius/fio/blob/master/engines/mmap.c > > : > > : With this change, unmodified fio can be used for testing with DAX PMD > > : mappings. There are many examples like this, and I do not think we want > > : to modify all applications that we want to evaluate/test with. > > > > sounds pretty convincing? > > > > > > And if we go ahead with this, it looks like 4.7 material to me - it > > affects ABI and we want to get that stabilized asap. What do people > > think? > > So I think Mathew didn't question the patch set as a whole. I think we all > agree that we should align the virtual address we map to so that PMD > mappings can be used. What Mathew was questioning was whether we really > need to play tricks when logical offset in the file where mmap is starting > is not aligned (and similarly for map length). Whether allowing PMD > mappings for unaligned file offsets is worth the complication is IMO a > valid question. I was questioning the approach as a whole ... since we have userspace already doing this in the form of NVML, do we really need the kernel to do this for us? Now, a further wrinkle. We have two competing patch sets (from Kirill and Hugh) which are going to give us THP for page cache filesystems. I would suggest that this is not DAX functionality but rather VFS functionality to opportunistically align all mmaps on files which are reasonably likely to be able to use THP. I hadn't thought about this until earlier today, and I'm sorry I didn't raise it further. Perhaps we can do a lightning session on this later today at LSFMM since all six (Toshi, Andrew, Jan, Hugh, Kirill and myself) are here. -- 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
