[PATCH] extend prefault helpers to fault in more than PAGE_SIZE

[Daniel Vetter <daniel.vetter@ffwll.ch>]

Hi all,

drm/i915 has write/read paths to upload/download data to/from gpu buffer
objects. For a bunch of reasons we have special fastpaths with decent setup
costs, so when we fall back to the slow-path we don't fully recover to the
fastest fast-path when grabbing our locks again. This is also in parts due to
that we have multiple fallbacks to slower paths, so control-flow in our code
would get really ugly.

As part of a larger rewrite and re-tuning of these functions we've noticed that
the prefault helpers in pagemap.h only prefault up to PAGE_SIZE because that's
all the other users need. The follow-up patch extends this to abritary sizes so
that we can fully exploit our special fastpaths in more cases (we typically see
reads and writes of a few pages up to a few mb).

I'd like to get this in for 3.4. There's no functional dependency between this
patch and the drm/i915 rework (only performance effects), so this can go in
through -mm without causing merge issues.

If this or something similar isn't acceptable, plan B is to hand-roll these 2
functions in drm/i915. But I don't like nih these things in driver code much.

Comments highly welcome.

Yours, Daniel

For reference, the drm/i915 read/write rework is avaialable at:

http://cgit.freedesktop.org/~danvet/drm/log/?h=pwrite-pread

Unfortunately cgit.fd.o is currently on hiatus.

Daniel Vetter (1):
  mm: extend prefault helpers to fault in more than PAGE_SIZE

 include/linux/pagemap.h |   28 ++++++++++++++++++----------
 1 files changed, 18 insertions(+), 10 deletions(-)

-- 
1.7.7.5

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