linux-kernel - Re: [PATCH v4 RESEND 0/2] Align mmap address for DAX pmd mappings

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Date:   Mon, 29 Aug 2016 21:32:32 +0000
From:   "Kani, Toshimitsu" <toshi.kani@....com>
To:     "kirill@...temov.name" <kirill@...temov.name>
CC:     "hughd@...gle.com" <hughd@...gle.com>,
        "kirill.shutemov@...ux.intel.com" <kirill.shutemov@...ux.intel.com>,
        "linux-kernel@...r.kernel.org" <linux-kernel@...r.kernel.org>,
        "linux-mm@...ck.org" <linux-mm@...ck.org>,
        "adilger.kernel@...ger.ca" <adilger.kernel@...ger.ca>,
        "mike.kravetz@...cle.com" <mike.kravetz@...cle.com>,
        "dan.j.williams@...el.com" <dan.j.williams@...el.com>,
        "mawilcox@...rosoft.com" <mawilcox@...rosoft.com>,
        "akpm@...ux-foundation.org" <akpm@...ux-foundation.org>,
        "linux-nvdimm@...ts.01.org" <linux-nvdimm@...ts.01.org>,
        "linux-fsdevel@...r.kernel.org" <linux-fsdevel@...r.kernel.org>,
        "ross.zwisler@...ux.intel.com" <ross.zwisler@...ux.intel.com>,
        "tytso@....edu" <tytso@....edu>,
        "david@...morbit.com" <david@...morbit.com>,
        "jack@...e.cz" <jack@...e.cz>
Subject: Re: [PATCH v4 RESEND 0/2] Align mmap address for DAX pmd mappings

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()
> > [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.
> 
> +Hugh
> 
> 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).
https://patchwork.kernel.org/patch/8916741/

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
!SHMEM_HUGE_FORCE.

Thanks,
-Toshi