[PATCH 0/19] get rid of superfluous __GFP_REPEAT

[Michal Hocko <mhocko@kernel.org>]

Hi,
this is the thrid version of the patchset previously sent [1]. I have
basically only rebased it on top of next-20160428 tree and dropped
"crypto: get rid of superfluous __GFP_REPEAT" which went through crypto
tree. I have added two more md patches as I couldn't resist more alloc
related cleanups at that area.

Motivation:
While working on something unrelated I've checked the current usage
of __GFP_REPEAT in the tree. It seems that a majority of the usage is
and always has been bogus because __GFP_REPEAT has always been about
costly high order allocations while we are using it for order-0 or very
small orders very often. It seems that a big pile of them is just a
copy&paste when a code has been adopted from one arch to another.

I think it makes some sense to get rid of them because they are just
making the semantic more unclear. Please note that GFP_REPEAT is
documented as
 * __GFP_REPEAT: Try hard to allocate the memory, but the allocation attempt
 *   _might_ fail.  This depends upon the particular VM implementation.
while !costly requests have basically nofail semantic. So one could
reasonably expect that order-0 request with __GFP_REPEAT will not loop
for ever. This is not implemented right now though.

I would like to move on with __GFP_REPEAT and define a better
semantic for it. One thought was to rename it to __GFP_BEST_EFFORT
which would behave consistently for all orders and guarantee that the
allocation would try as long as it seem feasible or fail eventually.
!costly request would then finally get a request context which neiter
fails too early (GFP_NORETRY) nor endlessly loops in the allocator for
ever (default behavior). Costly high order requests would keep the
current semantic.
We have discussed that at LSF/MM this year and another suggestion was
to introduce __GFP_TRYHARD instead which would be implicit for all
orders and users would opt out by ~__GFP_TRYHARD instead. I am not
sure which way is better right now but I plan to do the clean up first
before going further with semantic changes.

$ git grep __GFP_REPEAT next/master | wc -l
109
$ git grep __GFP_REPEAT | wc -l
35

So we are down to the third after this patch series. The remaining places
really seem to be relying on __GFP_REPEAT due to large allocation requests.
This still needs some double checking which I will do later after all the
simple ones are sorted out.

I am touching a lot of arch specific code here and I hope I got it right
but as a matter of fact I even didn't compile test for some archs as I
do not have cross compiler for them. Patches should be quite trivial to
review for stupid compile mistakes though. The tricky parts are usually
hidden by macro definitions and thats where I would appreciate help from
arch maintainers.

[1] http://lkml.kernel.org/r/1460372892-8157-1-git-send-email-mhocko@kernel.org

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