Re: [PATCH v2 00/10] evacuate struct page from the block layer, introduce __pfn_t

From: Dave Chinner
Date: Sat May 09 2015 - 17:57:10 EST

On Fri, May 08, 2015 at 11:02:28PM -0400, Rik van Riel wrote:
> On 05/08/2015 09:14 PM, Linus Torvalds wrote:
> > On Fri, May 8, 2015 at 9:59 AM, Rik van Riel <riel@xxxxxxxxxx> wrote:
> >>
> >> However, for persistent memory, all of the files will be "in memory".
> >
> > Yes. However, I doubt you will find a very sane rw filesystem that
> > then also makes them contiguous and aligns them at 2MB boundaries.
> >
> > Anything is possible, I guess, but things like that are *hard*. The
> > fragmentation issues etc cause it to a really challenging thing.
> The TLB performance bonus of accessing the large files with
> large pages may make it worthwhile to solve that hard problem.

FWIW, for DAX ththe filesystem allocation side is already mostly
solved - this is just an allocation alignment hint, analogous to
RAID stripe alignment. We don't need to reinvent the wheel here.
i.e. On XFS, use a 2MB stripe unit for the fs, a 2MB extent size
hint for files you want to use large pages on and you'll get 2MB
sized and aligned allocations from the filesystem for as long as
there are such freespace regions available.


Dave Chinner
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