Commit fece2029 authored Mar 22, 2018 by Kirill A. Shutemov Committed by Linus Torvalds Mar 22, 2018

mm/khugepaged.c: convert VM_BUG_ON() to collapse fail

khugepaged is not yet able to convert PTE-mapped huge pages back to PMD
mapped.  We do not collapse such pages.  See check
khugepaged_scan_pmd().

But if between khugepaged_scan_pmd() and __collapse_huge_page_isolate()
somebody managed to instantiate THP in the range and then split the PMD
back to PTEs we would have a problem --
VM_BUG_ON_PAGE(PageCompound(page)) will get triggered.

It's possible since we drop mmap_sem during collapse to re-take for
write.

Replace the VM_BUG_ON() with graceful collapse fail.

Link: http://lkml.kernel.org/r/20180315152353.27989-1-kirill.shutemov@linux.intel.com


Fixes: b1caa957 ("khugepaged: ignore pmd tables with THP mapped with ptes")
Signed-off-by: Kirill A. Shutemov <kirill.shutemov@linux.intel.com>
Cc: Laura Abbott <labbott@redhat.com>
Cc: Jerome Marchand <jmarchan@redhat.com>
Cc: Vlastimil Babka <vbabka@suse.cz>
Cc: <stable@vger.kernel.org>
Signed-off-by: Andrew Morton <akpm@linux-foundation.org>
Signed-off-by: Linus Torvalds <torvalds@linux-foundation.org>

parent 28ee90fe

mm/khugepaged.c

+6 −1

Original line number	Diff line number	Diff line
		@@ -530,7 +530,12 @@ static int __collapse_huge_page_isolate(struct vm_area_struct *vma,
		goto out;
		}

		VM_BUG_ON_PAGE(PageCompound(page), page);
		/* TODO: teach khugepaged to collapse THP mapped with pte */
		if (PageCompound(page)) {
		result = SCAN_PAGE_COMPOUND;
		goto out;
		}

		VM_BUG_ON_PAGE(!PageAnon(page), page);

		/*

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