selftests/vm: anon_cow: prepare for non-anonymous COW tests

Patch series "mm/gup: remove FOLL_FORCE usage from drivers (reliable R/O
long-term pinning)".

For now, we did not support reliable R/O long-term pinning in COW
mappings.  That means, if we would trigger R/O long-term pinning in
MAP_PRIVATE mapping, we could end up pinning the (R/O-mapped) shared
zeropage or a pagecache page.

The next write access would trigger a write fault and replace the pinned
page by an exclusive anonymous page in the process page table; whatever
the process would write to that private page copy would not be visible by
the owner of the previous page pin: for example, RDMA could read stale
data.  The end result is essentially an unexpected and hard-to-debug
memory corruption.

Some drivers tried working around that limitation by using
"FOLL_FORCE|FOLL_WRITE|FOLL_LONGTERM" for R/O long-term pinning for now. 
FOLL_WRITE would trigger a write fault, if required, and break COW before
pinning the page.  FOLL_FORCE is required because the VMA might lack write
permissions, and drivers wanted to make that working as well, just like
one would expect (no write access, but still triggering a write access to
break COW).

However, that is not a practical solution, because
(1) Drivers that don't stick to that undocumented and debatable pattern
    would still run into that issue. For example, VFIO only uses
    FOLL_LONGTERM for R/O long-term pinning.
(2) Using FOLL_WRITE just to work around a COW mapping + page pinning
    limitation is unintuitive. FOLL_WRITE would, for example, mark the
    page softdirty or trigger uffd-wp, even though, there actually isn't
    going to be any write access.
(3) The purpose of FOLL_FORCE is debug access, not access without lack of
    VMA permissions by arbitrarty drivers.

So instead, make R/O long-term pinning work as expected, by breaking COW
in a COW mapping early, such that we can remove any FOLL_FORCE usage from
drivers and make FOLL_FORCE ptrace-specific (renaming it to FOLL_PTRACE).
More details in patch #8.


This patch (of 19):

Originally, the plan was to have a separate tests for testing COW of
non-anonymous (e.g., shared zeropage) pages.

Turns out, that we'd need a lot of similar functionality and that there
isn't a really good reason to separate it. So let's prepare for non-anon
tests by renaming to "cow".

Link: https://lkml.kernel.org/r/[email protected]
Link: https://lkml.kernel.org/r/[email protected]
Signed-off-by: David Hildenbrand <[email protected]>
Acked-by: Vlastimil Babka <[email protected]>
Cc: Alexander Shishkin <[email protected]>
Cc: Alexander Viro <[email protected]>
Cc: Alex Williamson <[email protected]>
Cc: Andrea Arcangeli <[email protected]>
Cc: Andy Walls <[email protected]>
Cc: Anton Ivanov <[email protected]>
Cc: Arnaldo Carvalho de Melo <[email protected]>
Cc: Arnd Bergmann <[email protected]>
Cc: Bernard Metzler <[email protected]>
Cc: Borislav Petkov <[email protected]>
Cc: Catalin Marinas <[email protected]>
Cc: Christian Benvenuti <[email protected]>
Cc: Christian Gmeiner <[email protected]>
Cc: Christophe Leroy <[email protected]>
Cc: Christoph Hellwig <[email protected]>
Cc: Daniel Vetter <[email protected]>
Cc: Daniel Vetter <[email protected]>
Cc: Dave Hansen <[email protected]>
Cc: David Airlie <[email protected]>
Cc: David S. Miller <[email protected]>
Cc: Dennis Dalessandro <[email protected]>
Cc: "Eric W . Biederman" <[email protected]>
Cc: Greg Kroah-Hartman <[email protected]>
Cc: Hans Verkuil <[email protected]>
Cc: "H. Peter Anvin" <[email protected]>
Cc: Hugh Dickins <[email protected]>
Cc: Ingo Molnar <[email protected]>
Cc: Inki Dae <[email protected]>
Cc: Ivan Kokshaysky <[email protected]>
Cc: James Morris <[email protected]>
Cc: Jason Gunthorpe <[email protected]>
Cc: Jiri Olsa <[email protected]>
Cc: Johannes Berg <[email protected]>
Cc: John Hubbard <[email protected]>
Cc: Kees Cook <[email protected]>
Cc: Kentaro Takeda <[email protected]>
Cc: Krzysztof Kozlowski <[email protected]>
Cc: Kyungmin Park <[email protected]>
Cc: Leon Romanovsky <[email protected]>
Cc: Leon Romanovsky <[email protected]>
Cc: Linus Torvalds <[email protected]>
Cc: Lucas Stach <[email protected]>
Cc: Marek Szyprowski <[email protected]>
Cc: Mark Rutland <[email protected]>
Cc: Matthew Wilcox <[email protected]>
Cc: Matt Turner <[email protected]>
Cc: Mauro Carvalho Chehab <[email protected]>
Cc: Michael Ellerman <[email protected]>
Cc: Mike Kravetz <[email protected]>
Cc: Muchun Song <[email protected]>
Cc: Nadav Amit <[email protected]>
Cc: Namhyung Kim <[email protected]>
Cc: Nelson Escobar <[email protected]>
Cc: Nicholas Piggin <[email protected]>
Cc: Oded Gabbay <[email protected]>
Cc: Oleg Nesterov <[email protected]>
Cc: Paul Moore <[email protected]>
Cc: Peter Xu <[email protected]>
Cc: Peter Zijlstra <[email protected]>
Cc: Richard Henderson <[email protected]>
Cc: Richard Weinberger <[email protected]>
Cc: Russell King <[email protected]>
Cc: Serge Hallyn <[email protected]>
Cc: Seung-Woo Kim <[email protected]>
Cc: Shuah Khan <[email protected]>
Cc: Tetsuo Handa <[email protected]>
Cc: Thomas Bogendoerfer <[email protected]>
Cc: Thomas Gleixner <[email protected]>
Cc: Tomasz Figa <[email protected]>
Cc: Will Deacon <[email protected]>
Signed-off-by: Andrew Morton <[email protected]>

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