linux-kernel - Re: [PATCH v3 0/2] Align mmap address for DAX pmd mappings

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Date:	Fri, 15 Apr 2016 22:05:31 -0700
From:	Andrew Morton <akpm@...ux-foundation.org>
To:	Toshi Kani <toshi.kani@....com>
Cc:	dan.j.williams@...el.com, viro@...iv.linux.org.uk,
	willy@...ux.intel.com, ross.zwisler@...ux.intel.com,
	kirill.shutemov@...ux.intel.com, david@...morbit.com, jack@...e.cz,
	tytso@....edu, adilger.kernel@...ger.ca, linux-nvdimm@...1.01.org,
	linux-fsdevel@...r.kernel.org, linux-mm@...ck.org,
	linux-kernel@...r.kernel.org
Subject: Re: [PATCH v3 0/2] Align mmap address for DAX pmd mappings

On Thu, 14 Apr 2016 10:48:29 -0600 Toshi Kani <toshi.kani@....com> 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?