mm: ptep_get() conversion

Convert all instances of direct pte_t* dereferencing to instead use
ptep_get() helper. This means that by default, the accesses change from
a C dereference to a READ_ONCE(). This is technically the correct thing
to do since where pgtables are modified by HW (for access/dirty) they
are volatile and therefore we should always ensure READ_ONCE()
semantics.

But more importantly, by always using the helper, it can be overridden
by the architecture to fully encapsulate the contents of the pte. Arch
code is deliberately not converted, as the arch code knows best. It is
intended that arch code (arm64) will override the default with its own
implementation that can (e.g.) hide certain bits from the core code, or
determine young/dirty status by mixing in state from another source.

Conversion was done using Coccinelle:

----

// $ make coccicheck \
//          COCCI=ptepget.cocci \
//          SPFLAGS="--include-headers" \
//          MODE=patch

virtual patch

@ depends on patch @
pte_t *v;
@@

- *v
+ ptep_get(v)

----

Then reviewed and hand-edited to avoid multiple unnecessary calls to
ptep_get(), instead opting to store the result of a single call in a
variable, where it is correct to do so. This aims to negate any cost of
READ_ONCE() and will benefit arch-overrides that may be more complex.

Included is a fix for an issue in an earlier version of this patch that
was pointed out by kernel test robot. The issue arose because config
MMU=n elides definition of the ptep helper functions, including
ptep_get(). HUGETLB_PAGE=n configs still define a simple
huge_ptep_clear_flush() for linking purposes, which dereferences the
ptep. So when both configs are disabled, this caused a build error
because ptep_get() is not defined. Fix by continuing to do a direct
dereference when MMU=n. This is safe because for this config the arch
code cannot be trying to virtualize the ptes because none of the ptep
helpers are defined.

Reported-by: kernel test robot <lkp@intel.com>
Link: https://lore.kernel.org/oe-kbuild-all/202305120142.yXsNEo6H-lkp@intel.com/


Signed-off-by: Ryan Roberts <ryan.roberts@arm.com>