[PATCH v2] cma_alloc(), indefinitely retry allocations for -EBUSY failures

[Chris Goldsworthy]

On mobile devices, failure to allocate from a CMA area constitutes a
functional failure.  Sometimes during CMA allocations, we have observed
that pages in a CMA area allocated through alloc_pages(), that we're trying
to migrate away to make room for a CMA allocation, are temporarily pinned.
This temporary pinning can occur when a process that owns the pinned page
is being forked (the example is explained further in the commit text), or it is
exiting.  This patch addresses this issue by indefinitely retrying allocations
that fail due to a return of -EBUSY.

** This change log was re-sent due to threading issues **

Change log: 

v1: We were performing retries of the allocation a fixed number of times.
Andrew Morton disliked this, as it didn't guarantee that the allocation would
succeed.

https://lkml.org/lkml/2020/8/5/1096
https://lkml.org/lkml/2020/8/21/1490

v2: To address this concern, we switched to retrying indefinitely, as opposed to
doing to retrying  the allocation a limited number of times.

Chris Goldsworthy (1):
  mm: cma: indefinitely retry allocations in cma_alloc

 mm/cma.c | 25 +++++++++++++++++++++++--
 1 file changed, 23 insertions(+), 2 deletions(-)

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