UPSTREAM: arm64: pgtable: Fix pte_accessible()

pte_accessible() is used by ptep_clear_flush() to figure out whether TLB
invalidation is necessary when unmapping pages for reclaim. Although our
implementation is correct according to the architecture, returning true
only for valid, young ptes in the absence of racing page-table
modifications, this is in fact flawed due to lazy invalidation of old
ptes in ptep_clear_flush_young() where we elide the expensive DSB
instruction for completing the TLB invalidation.

Rather than penalise the aging path, adjust pte_accessible() to return
true for any valid pte, even if the access flag is cleared.

Bug: 176475096
Change-Id: Ifd7919b4e9b790a94802b0115f93a9a260757036
Cc: <stable@vger.kernel.org>
Fixes: 76c714be ("arm64: pgtable: implement pte_accessible()")
Reported-by: Yu Zhao <yuzhao@google.com>
Acked-by: Yu Zhao <yuzhao@google.com>
Reviewed-by: Minchan Kim <minchan@kernel.org>
Reviewed-by: Catalin Marinas <catalin.marinas@arm.com>
Link: https://lore.kernel.org/r/20201120143557.6715-2-will@kernel.org


Signed-off-by: Will Deacon <will@kernel.org>
(cherry picked from commit 07509e10dcc77627f8b6a57381e878fe269958d3)
Signed-off-by: Isaac J. Manjarres <isaacm@codeaurora.org>