mm/debug_vm_pgtable: more pte_swp_exclusive() sanity checks

Patch series "mm: support __HAVE_ARCH_PTE_SWP_EXCLUSIVE on all
architectures with swap PTEs".

This is the follow-up on [1]:
	[PATCH v2 0/8] mm: COW fixes part 3: reliable GUP R/W FOLL_GET of
	anonymous pages

After we implemented __HAVE_ARCH_PTE_SWP_EXCLUSIVE on most prominent
enterprise architectures, implement __HAVE_ARCH_PTE_SWP_EXCLUSIVE on all
remaining architectures that support swap PTEs.

This makes sure that exclusive anonymous pages will stay exclusive, even
after they were swapped out -- for example, making GUP R/W FOLL_GET of
anonymous pages reliable.  Details can be found in [1].

This primarily fixes remaining known O_DIRECT memory corruptions that can
happen on concurrent swapout, whereby we can lose DMA reads to a page
(modifying the user page by writing to it).

To verify, there are two test cases (requiring swap space, obviously):
(1) The O_DIRECT+swapout test case [2] from Andrea. This test case tries
    triggering a race condition.
(2) My vmsplice() test case [3] that tries to detect if the exclusive
    marker was lost during swapout, not relying on a race condition.


For example, on 32bit x86 (with and without PAE), my test case fails
without these patches:
	$ ./test_swp_exclusive
	FAIL: page was replaced during COW
But succeeds with these patches:
	$ ./test_swp_exclusive
	PASS: page was not replaced during COW


Why implement __HAVE_ARCH_PTE_SWP_EXCLUSIVE for all architectures, even
the ones where swap support might be in a questionable state?  This is the
first step towards removing "readable_exclusive" migration entries, and
instead using pte_swp_exclusive() also with (readable) migration entries
instead (as suggested by Peter).  The only missing piece for that is
supporting pmd_swp_exclusive() on relevant architectures with THP
migration support.

As all relevant architectures now implement __HAVE_ARCH_PTE_SWP_EXCLUSIVE,,
we can drop __HAVE_ARCH_PTE_SWP_EXCLUSIVE in the last patch.

I tried cross-compiling all relevant setups and tested on x86 and sparc64
so far.

CCing arch maintainers only on this cover letter and on the respective
patch(es).

[1] https://lkml.kernel.org/r/20220329164329.208407-1-david@redhat.com
[2] https://gitlab.com/aarcange/kernel-testcases-for-v5.11/-/blob/main/page_count_do_wp_page-swap.c
[3] https://gitlab.com/davidhildenbrand/scratchspace/-/blob/main/test_swp_exclusive.c


This patch (of 26):

We want to implement __HAVE_ARCH_PTE_SWP_EXCLUSIVE on all architectures. 
Let's extend our sanity checks, especially testing that our PTE bit does
not affect:

* is_swap_pte() -> pte_present() and pte_none()
* the swap entry + type
* pte_swp_soft_dirty()

Especially, the pfn_pte() is dodgy when the swap PTE layout differs
heavily from ordinary PTEs.  Let's properly construct a swap PTE from swap
type+offset.

[david@redhat.com: fix build]
  Link: https://lkml.kernel.org/r/6aaad548-cf48-77fa-9d6c-db83d724b2eb@redhat.com
Link: https://lkml.kernel.org/r/20230113171026.582290-1-david@redhat.com
Link: https://lkml.kernel.org/r/20230113171026.582290-2-david@redhat.comSigned-off-by: David Hildenbrand <david@redhat.com>
Cc: Andrea Arcangeli <aarcange@redhat.com>
Cc: Anton Ivanov <anton.ivanov@cambridgegreys.com>
Cc: <aou@eecs.berkeley.edu>
Cc: Borislav Petkov (AMD) <bp@alien8.de>
Cc: Brian Cain <bcain@quicinc.com>
Cc: Christophe Leroy <christophe.leroy@csgroup.eu>
Cc: Chris Zankel <chris@zankel.net>
Cc: Dave Hansen <dave.hansen@linux.intel.com>
Cc: David S. Miller <davem@davemloft.net>
Cc: Dinh Nguyen <dinguyen@kernel.org>
Cc: Geert Uytterhoeven <geert@linux-m68k.org>
Cc: Greg Ungerer <gerg@linux-m68k.org>
Cc: Guo Ren <guoren@kernel.org>
Cc: Helge Deller <deller@gmx.de>
Cc: H. Peter Anvin (Intel) <hpa@zytor.com>
Cc: Huacai Chen <chenhuacai@kernel.org>
Cc: Hugh Dickins <hughd@google.com>
Cc: Ingo Molnar <mingo@redhat.com>
Cc: Ivan Kokshaysky <ink@jurassic.park.msu.ru>
Cc: James Bottomley <James.Bottomley@HansenPartnership.com>
Cc: Jason Gunthorpe <jgg@nvidia.com>
Cc: Johannes Berg <johannes@sipsolutions.net>
Cc: John Hubbard <jhubbard@nvidia.com>
Cc: Matt Turner <mattst88@gmail.com>
Cc: Max Filippov <jcmvbkbc@gmail.com>
Cc: Michael Ellerman <mpe@ellerman.id.au>
Cc: Michal Simek <monstr@monstr.eu>
Cc: Mike Rapoport <rppt@linux.ibm.com>
Cc: Nadav Amit <namit@vmware.com>
Cc: Nicholas Piggin <npiggin@gmail.com>
Cc: Palmer Dabbelt <palmer@dabbelt.com>
Cc: Paul Walmsley <paul.walmsley@sifive.com>
Cc: Peter Xu <peterx@redhat.com>
Cc: Richard Henderson <richard.henderson@linaro.org>
Cc: Richard Weinberger <richard@nod.at>
Cc: Rich Felker <dalias@libc.org>
Cc: Russell King <linux@armlinux.org.uk>
Cc: Stafford Horne <shorne@gmail.com>
Cc: Stefan Kristiansson <stefan.kristiansson@saunalahti.fi>
Cc: Thomas Bogendoerfer <tsbogend@alpha.franken.de>
Cc: Thomas Gleixner <tglx@linutronix.de>
Cc: Vineet Gupta <vgupta@kernel.org>
Cc: Vlastimil Babka <vbabka@suse.cz>
Cc: Xuerui Wang <kernel@xen0n.name>
Cc: Yang Shi <shy828301@gmail.com>
Cc: Yoshinori Sato <ysato@users.osdn.me>
Signed-off-by: Andrew Morton <akpm@linux-foundation.org>

mm/debug_vm_pgtable.c

@@ -811,13 +811,36 @@ static void __init pmd_swap_soft_dirty_tests(struct pgtable_debug_args *args) {
 static void __init pte_swap_exclusive_tests(struct pgtable_debug_args *args)
 {
 #ifdef __HAVE_ARCH_PTE_SWP_EXCLUSIVE
-	pte_t pte = pfn_pte(args->fixed_pte_pfn, args->page_prot);
+	unsigned long max_swap_offset;
+	swp_entry_t entry, entry2;
+	pte_t pte;
 
 	pr_debug("Validating PTE swap exclusive\n");
+
+	/* See generic_max_swapfile_size(): probe the maximum offset */
+	max_swap_offset = swp_offset(pte_to_swp_entry(swp_entry_to_pte(swp_entry(0, ~0UL))));
+
+	/* Create a swp entry with all possible bits set */
+	entry = swp_entry((1 << MAX_SWAPFILES_SHIFT) - 1, max_swap_offset);
+
+	pte = swp_entry_to_pte(entry);
+	WARN_ON(pte_swp_exclusive(pte));
+	WARN_ON(!is_swap_pte(pte));
+	entry2 = pte_to_swp_entry(pte);
+	WARN_ON(memcmp(&entry, &entry2, sizeof(entry)));
+
 	pte = pte_swp_mkexclusive(pte);
 	WARN_ON(!pte_swp_exclusive(pte));
+	WARN_ON(!is_swap_pte(pte));
+	WARN_ON(pte_swp_soft_dirty(pte));
+	entry2 = pte_to_swp_entry(pte);
+	WARN_ON(memcmp(&entry, &entry2, sizeof(entry)));
+
 	pte = pte_swp_clear_exclusive(pte);
 	WARN_ON(pte_swp_exclusive(pte));
+	WARN_ON(!is_swap_pte(pte));
+	entry2 = pte_to_swp_entry(pte);
+	WARN_ON(memcmp(&entry, &entry2, sizeof(entry)));
 #endif /* __HAVE_ARCH_PTE_SWP_EXCLUSIVE */
 }