On Mon, 2016-08-29 at 15:31 -0600, Kani, Toshimitsu wrote:
On Mon, 2016-08-29 at 23:48 +0300, Kirill A. Shutemov wrote:
> On Mon, Aug 29, 2016 at 01:11:19PM -0600, Toshi Kani 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()
> > . 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.
> Can we get it used for shmem/tmpfs too?
> I don't think we should duplicate essentially the same
> functionality in multiple places.
Here is my brief analysis when I had looked at the Hugh's patch last
time (before shmem_get_unmapped_area() was accepted).
Besides some differences in the logic, ex. shmem_get_unmapped_area()
always calls current->mm->get_unmapped_area twice, yes, they
basically provide the same functionality.
I think one issue is that shmem_get_unmapped_area() checks with its
static flag 'shmem_huge', and additinally deals with SHMEM_HUGE_DENY
and SHMEM_HUGE_FORCE cases. It also handles non-file case for
Looking further, these shmem_huge handlings only check pre-conditions.
So, we should be able to make shmem_get_unmapped_area() as a wrapper,
which checks such shmem-specific conitions, and then
call __thp_get_unmapped_area() for the actual work. All DAX-specific
checks are performed in thp_get_unmapped_area() as well. We can make
__thp_get_unmapped_area() as a common function.
I'd prefer to make such change as a separate item, but I can include it
to this patch series if needed.