mm/thp: fix page_vma_mapped_walk() if THP mapped by ptes

Running certain tests with a DEBUG_VM kernel would crash within hours,
on the total_mapcount BUG() in split_huge_page_to_list(), while trying
to free up some memory by punching a hole in a shmem huge page: split's
try_to_unmap() was unable to find all the mappings of the page (which,
on a !DEBUG_VM kernel, would then keep the huge page pinned in memory).

Crash dumps showed two tail pages of a shmem huge page remained mapped
by pte: ptes in a non-huge-aligned vma of a gVisor process, at the end
of a long unmapped range; and no page table had yet been allocated for
the head of the huge page to be mapped into.

Although designed to handle these odd misaligned huge-page-mapped-by-pte
cases, page_vma_mapped_walk() falls short by returning false prematurely
when !pmd_present or !pud_present or !p4d_present or !pgd_present: there
are cases when a huge page may span the boundary, with ptes present in
the next.

Restructure page_vma_mapped_walk() as a loop to continue in these cases,
while keeping its layout much as before.  Add a step_forward() helper to
advance pvmw->address across those boundaries: originally I tried to use
mm's standard p?d_addr_end() macros, but hit the same crash 512 times
less often: because of the way redundant levels are folded together, but
folded differently in different configurations, it was just too
difficult to use them correctly; and step_forward() is simpler anyway.

Link: https://lkml.kernel.org/r/fedb8632-1798-de42-f39e-873551d5bc81@google.com
Fixes: ace71a19 ("mm: introduce page_vma_mapped_walk()")
Signed-off-by: Hugh Dickins <hughd@google.com>
Acked-by: Kirill A. Shutemov <kirill.shutemov@linux.intel.com>
Cc: Alistair Popple <apopple@nvidia.com>
Cc: Matthew Wilcox <willy@infradead.org>
Cc: Peter Xu <peterx@redhat.com>
Cc: Ralph Campbell <rcampbell@nvidia.com>
Cc: Wang Yugui <wangyugui@e16-tech.com>
Cc: Will Deacon <will@kernel.org>
Cc: Yang Shi <shy828301@gmail.com>
Cc: Zi Yan <ziy@nvidia.com>
Cc: <stable@vger.kernel.org>
Signed-off-by: Andrew Morton <akpm@linux-foundation.org>
Signed-off-by: Linus Torvalds <torvalds@linux-foundation.org>

mm/page_vma_mapped.c

@@ -116,6 +116,13 @@ static bool check_pte(struct page_vma_mapped_walk *pvmw)
 	return pfn_is_match(pvmw->page, pfn);
 }
 
+static void step_forward(struct page_vma_mapped_walk *pvmw, unsigned long size)
+{
+	pvmw->address = (pvmw->address + size) & ~(size - 1);
+	if (!pvmw->address)
+		pvmw->address = ULONG_MAX;
+}
+
 /**
  * page_vma_mapped_walk - check if @pvmw->page is mapped in @pvmw->vma at
  * @pvmw->address
@@ -183,16 +190,22 @@ bool page_vma_mapped_walk(struct page_vma_mapped_walk *pvmw)
 	if (pvmw->pte)
 		goto next_pte;
 restart:
-	{
+	do {
 		pgd = pgd_offset(mm, pvmw->address);
-		if (!pgd_present(*pgd))
-			return false;
+		if (!pgd_present(*pgd)) {
+			step_forward(pvmw, PGDIR_SIZE);
+			continue;
+		}
 		p4d = p4d_offset(pgd, pvmw->address);
-		if (!p4d_present(*p4d))
-			return false;
+		if (!p4d_present(*p4d)) {
+			step_forward(pvmw, P4D_SIZE);
+			continue;
+		}
 		pud = pud_offset(p4d, pvmw->address);
-		if (!pud_present(*pud))
-			return false;
+		if (!pud_present(*pud)) {
+			step_forward(pvmw, PUD_SIZE);
+			continue;
+		}
 
 		pvmw->pmd = pmd_offset(pud, pvmw->address);
 		/*
@@ -239,7 +252,8 @@ bool page_vma_mapped_walk(struct page_vma_mapped_walk *pvmw)
 
 				spin_unlock(ptl);
 			}
-			return false;
+			step_forward(pvmw, PMD_SIZE);
+			continue;
 		}
 		if (!map_pte(pvmw))
 			goto next_pte;
@@ -269,7 +283,9 @@ bool page_vma_mapped_walk(struct page_vma_mapped_walk *pvmw)
 			spin_lock(pvmw->ptl);
 		}
 		goto this_pte;
-	}
+	} while (pvmw->address < end);
+
+	return false;
 }
 
 /**