+ mm-huge_memory-splitting-set-mappingindex-before-unfreeze.patch added to -mm tree

+ mm-huge_memory-splitting-set-mappingindex-before-unfreeze.patch added to -mm tree

[akpm]

The patch titled
     Subject: mm/huge_memory: splitting set mapping+index before unfreeze
has been added to the -mm tree.  Its filename is
     mm-huge_memory-splitting-set-mappingindex-before-unfreeze.patch

This patch should soon appear at
    http://ozlabs.org/~akpm/mmots/broken-out/mm-huge_memory-splitting-set-mappingindex-before-unfreeze.patch
and later at
    http://ozlabs.org/~akpm/mmotm/broken-out/mm-huge_memory-splitting-set-mappingindex-before-unfreeze.patch

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------------------------------------------------------
From: Hugh Dickins <hughd@google.com>
Subject: mm/huge_memory: splitting set mapping+index before unfreeze

Huge tmpfs stress testing has occasionally hit shmem_undo_range()'s
VM_BUG_ON_PAGE(page_to_pgoff(page) != index, page).

Move the setting of mapping and index up before the page_ref_unfreeze() in
__split_huge_page_tail() to fix this: so that a page cache lookup cannot
get a reference while the tail's mapping and index are unstable.

In fact, might as well move them up before the smp_wmb(): I don't see an
actual need for that, but if I'm missing something, this way round is
safer than the other, and no less efficient.

You might argue that VM_BUG_ON_PAGE(page_to_pgoff(page) != index, page) is
misplaced, and should be left until after the trylock_page(); but left as
is has not crashed since, and gives more stringent assurance.

Link: http://lkml.kernel.org/r/alpine.LSU.2.11.1811261516380.2275@eggly.anvils
Fixes: e9b61f19858a5 ("thp: reintroduce split_huge_page()")
Requires: 605ca5ede764 ("mm/huge_memory.c: reorder operations in __split_huge_page_tail()")
Signed-off-by: Hugh Dickins <hughd@google.com>
Acked-by: Kirill A. Shutemov <kirill.shutemov@linux.intel.com>
Cc: Konstantin Khlebnikov <khlebnikov@yandex-team.ru>
Cc: Jerome Glisse <jglisse@redhat.com>
Cc: Matthew Wilcox <willy@infradead.org>
Cc: <stable@vger.kernel.org>	[4.8+]
Signed-off-by: Andrew Morton <akpm@linux-foundation.org>
---


--- a/mm/huge_memory.c~mm-huge_memory-splitting-set-mappingindex-before-unfreeze
+++ a/mm/huge_memory.c
@@ -2402,6 +2402,12 @@ static void __split_huge_page_tail(struc
 			 (1L << PG_unevictable) |
 			 (1L << PG_dirty)));
 
+	/* ->mapping in first tail page is compound_mapcount */
+	VM_BUG_ON_PAGE(tail > 2 && page_tail->mapping != TAIL_MAPPING,
+			page_tail);
+	page_tail->mapping = head->mapping;
+	page_tail->index = head->index + tail;
+
 	/* Page flags must be visible before we make the page non-compound. */
 	smp_wmb();
 
@@ -2422,12 +2428,6 @@ static void __split_huge_page_tail(struc
 	if (page_is_idle(head))
 		set_page_idle(page_tail);
 
-	/* ->mapping in first tail page is compound_mapcount */
-	VM_BUG_ON_PAGE(tail > 2 && page_tail->mapping != TAIL_MAPPING,
-			page_tail);
-	page_tail->mapping = head->mapping;
-
-	page_tail->index = head->index + tail;
 	page_cpupid_xchg_last(page_tail, page_cpupid_last(head));
 
 	/*
_

Patches currently in -mm which might be from hughd@google.com are

mm-huge_memory-rename-freeze_page-to-unmap_page.patch
mm-huge_memory-splitting-set-mappingindex-before-unfreeze.patch
mm-huge_memory-fix-lockdep-complaint-on-32-bit-i_size_read.patch
mm-khugepaged-collapse_shmem-stop-if-punched-or-truncated.patch
mm-khugepaged-fix-crashes-due-to-misaccounted-holes.patch
mm-khugepaged-collapse_shmem-remember-to-clear-holes.patch
mm-khugepaged-minor-reorderings-in-collapse_shmem.patch
mm-khugepaged-collapse_shmem-without-freezing-new_page.patch
mm-khugepaged-collapse_shmem-do-not-crash-on-compound.patch
mm-khugepaged-fix-the-xas_create_range-error-path.patch
mm-put_and_wait_on_page_locked-while-page-is-migrated.patch