linux-kernel - cma_alloc(), add sleep-and-retry for temporary page pinning

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Message-Id: <1596682582-29139-1-git-send-email-cgoldswo@codeaurora.org>
Date:   Wed,  5 Aug 2020 19:56:21 -0700
From:   Chris Goldsworthy <cgoldswo@...eaurora.org>
To:     akpm@...ux-foundation.org
Cc:     linux-mm@...ck.org, linux-arm-msm@...r.kernel.org,
        linux-kernel@...r.kernel.org, pratikp@...eaurora.org,
        pdaly@...eaurora.org, sudraja@...eaurora.org,
        iamjoonsoo.kim@....com
Subject: cma_alloc(), add sleep-and-retry for temporary page pinning

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).
This patch addresses this issue by adding a sleep-and-retry loop in
cma_alloc() . There's another example we know of similar to the above that
occurs during exit_mmap() (in zap_pte_range() specifically), but I need to
determine if this is still relevant today.