diff mbox series

[10/11] mm/thp: fix page_vma_mapped_walk() if THP mapped by ptes

Message ID fedb8632-1798-de42-f39e-873551d5bc81@google.com (mailing list archive)
State New
Headers show
Series mm: page_vma_mapped_walk() cleanup and THP fixes | expand

Commit Message

Hugh Dickins June 10, 2021, 6:52 a.m. UTC
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.

Fixes: ace71a19cec5 ("mm: introduce page_vma_mapped_walk()")
Signed-off-by: Hugh Dickins <hughd@google.com>
Cc: <stable@vger.kernel.org>
---
 mm/page_vma_mapped.c | 34 +++++++++++++++++++++++++---------
 1 file changed, 25 insertions(+), 9 deletions(-)

Comments

Kirill A. Shutemov June 10, 2021, 9:42 a.m. UTC | #1
On Wed, Jun 09, 2021 at 11:52:37PM -0700, Hugh Dickins wrote:
> 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.
> 
> Fixes: ace71a19cec5 ("mm: introduce page_vma_mapped_walk()")
> Signed-off-by: Hugh Dickins <hughd@google.com>
> Cc: <stable@vger.kernel.org>

Acked-by: Kirill A. Shutemov <kirill.shutemov@linux.intel.com>
diff mbox series

Patch

diff --git a/mm/page_vma_mapped.c b/mm/page_vma_mapped.c
index f6839f536645..6eb2f1863506 100644
--- a/mm/page_vma_mapped.c
+++ b/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);
 		/*
@@ -240,7 +253,8 @@  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;
@@ -270,7 +284,9 @@  bool page_vma_mapped_walk(struct page_vma_mapped_walk *pvmw)
 			spin_lock(pvmw->ptl);
 		}
 		goto this_pte;
-	}
+	} while (pvmw->address < end);
+
+	return false;
 }
 
 /**