mm: check against orig_pte for finish_fault()

This patch allows do_fault() to trigger on !pte_none() cases too.  This
prepares for the pte markers to be handled by do_fault() just like none
pte.

To achieve this, instead of unconditionally check against pte_none() in
finish_fault(), we may hit the case that the orig_pte was some pte marker
so what we want to do is to replace the pte marker with some valid pte
entry.  Then if orig_pte was set we'd want to check the current *pte
(under pgtable lock) against orig_pte rather than none pte.

Right now there's no solid way to safely reference orig_pte because when
pmd is not allocated handle_pte_fault() will not initialize orig_pte, so
it's not safe to reference it.

There's another solution proposed before this patch to do pte_clear() for
vmf->orig_pte for pmd==NULL case, however it turns out it'll break arm32
because arm32 could have assumption that pte_t* pointer will always reside
on a real ram32 pgtable, not any kernel stack variable.

To solve this, we add a new flag FAULT_FLAG_ORIG_PTE_VALID, and it'll be
set along with orig_pte when there is valid orig_pte, or it'll be cleared
when orig_pte was not initialized.

It'll be updated every time we call handle_pte_fault(), so e.g.  if a page
fault retry happened it'll be properly updated along with orig_pte.

[1] https://lore.kernel.org/lkml/710c48c9-406d-e4c5-a394-10501b951316@samsung.com/

[akpm@linux-foundation.org: coding-style cleanups]
[peterx@redhat.com: fix crash reported by Marek]
  Link: https://lkml.kernel.org/r/Ylb9rXJyPm8/ao8f@xz-m1.local
Link: https://lkml.kernel.org/r/20220405014836.14077-1-peterx@redhat.comSigned-off-by: Peter Xu <peterx@redhat.com>
Reviewed-by: Alistair Popple <apopple@nvidia.com>
Tested-by: Marek Szyprowski <m.szyprowski@samsung.com>
Cc: Andrea Arcangeli <aarcange@redhat.com>
Cc: Axel Rasmussen <axelrasmussen@google.com>
Cc: David Hildenbrand <david@redhat.com>
Cc: Hugh Dickins <hughd@google.com>
Cc: Jerome Glisse <jglisse@redhat.com>
Cc: "Kirill A . Shutemov" <kirill@shutemov.name>
Cc: Matthew Wilcox <willy@infradead.org>
Cc: Mike Kravetz <mike.kravetz@oracle.com>
Cc: Mike Rapoport <rppt@linux.vnet.ibm.com>
Cc: Nadav Amit <nadav.amit@gmail.com>
Signed-off-by: Andrew Morton <akpm@linux-foundation.org>

include/linux/mm_types.h

@@ -822,6 +822,8 @@ typedef struct {
  * @FAULT_FLAG_UNSHARE: The fault is an unsharing request to unshare (and mark
  *                      exclusive) a possibly shared anonymous page that is
  *                      mapped R/O.
+ * @FAULT_FLAG_ORIG_PTE_VALID: whether the fault has vmf->orig_pte cached.
+ *                        We should only access orig_pte if this flag set.
  *
  * About @FAULT_FLAG_ALLOW_RETRY and @FAULT_FLAG_TRIED: we can specify
  * whether we would allow page faults to retry by specifying these two
@@ -858,6 +860,7 @@ enum fault_flag {
 	FAULT_FLAG_INSTRUCTION =	1 << 8,
 	FAULT_FLAG_INTERRUPTIBLE =	1 << 9,
 	FAULT_FLAG_UNSHARE =		1 << 10,
+	FAULT_FLAG_ORIG_PTE_VALID =	1 << 11,
 };
 
 #endif /* _LINUX_MM_TYPES_H */

mm/memory.c

@@ -4183,6 +4183,14 @@ void do_set_pte(struct vm_fault *vmf, struct page *page, unsigned long addr)
 	set_pte_at(vma->vm_mm, addr, vmf->pte, entry);
 }
 
+static bool vmf_pte_changed(struct vm_fault *vmf)
+{
+	if (vmf->flags & FAULT_FLAG_ORIG_PTE_VALID)
+		return !pte_same(*vmf->pte, vmf->orig_pte);
+
+	return !pte_none(*vmf->pte);
+}
+
 /**
  * finish_fault - finish page fault once we have prepared the page to fault
  *
@@ -4241,7 +4249,7 @@ vm_fault_t finish_fault(struct vm_fault *vmf)
 				      vmf->address, &vmf->ptl);
 	ret = 0;
 	/* Re-check under ptl */
-	if (likely(pte_none(*vmf->pte)))
+	if (likely(!vmf_pte_changed(vmf)))
 		do_set_pte(vmf, page, vmf->address);
 	else
 		ret = VM_FAULT_NOPAGE;
@@ -4709,6 +4717,7 @@ static vm_fault_t handle_pte_fault(struct vm_fault *vmf)
 		 * concurrent faults and from rmap lookups.
 		 */
 		vmf->pte = NULL;
+		vmf->flags &= ~FAULT_FLAG_ORIG_PTE_VALID;
 	} else {
 		/*
 		 * If a huge pmd materialized under us just retry later.  Use
@@ -4732,6 +4741,7 @@ static vm_fault_t handle_pte_fault(struct vm_fault *vmf)
 		 */
 		vmf->pte = pte_offset_map(vmf->pmd, vmf->address);
 		vmf->orig_pte = *vmf->pte;
+		vmf->flags |= FAULT_FLAG_ORIG_PTE_VALID;
 
 		/*
 		 * some architectures can have larger ptes than wordsize,