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From patchwork Thu Mar 15 15:23:53 2018
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Content-Type: text/plain; charset="utf-8"
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MIME-Version: 1.0
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Content-Transfer-Encoding: 7bit
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Subject: mm/khugepaged: Convert VM_BUG_ON() to collapse fail
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From: "Kirill A. Shutemov" <kirill.shutemov@linux.intel.com>
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X-Patchwork-Id: 10284785
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Message-Id: <20180315152353.27989-1-kirill.shutemov@linux.intel.com>
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To: Andrew Morton <akpm@linux-foundation.org>
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Cc: Laura Abbott <labbott@redhat.com>, linux-mm@kvack.org,
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linux-kernel@vger.kernel.org,
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"Kirill A. Shutemov" <kirill.shutemov@linux.intel.com>
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Date: Thu, 15 Mar 2018 18:23:53 +0300
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khugepaged is not yet able to convert PTE-mapped huge pages back to PMD
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mapped. We do not collapse such pages. See check khugepaged_scan_pmd().
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But if between khugepaged_scan_pmd() and __collapse_huge_page_isolate()
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somebody managed to instantiate THP in the range and then split the PMD
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back to PTEs we would have a problem -- VM_BUG_ON_PAGE(PageCompound(page))
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will get triggered.
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It's possible since we drop mmap_sem during collapse to re-take for
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write.
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Replace the VM_BUG_ON() with graceful collapse fail.
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Signed-off-by: Kirill A. Shutemov <kirill.shutemov@linux.intel.com>
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Fixes: b1caa957ae6d ("khugepaged: ignore pmd tables with THP mapped with ptes")
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---
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mm/khugepaged.c | 7 ++++++-
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1 file changed, 6 insertions(+), 1 deletion(-)
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diff --git a/mm/khugepaged.c b/mm/khugepaged.c
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index b7e2268dfc9a..c15da1ea7e63 100644
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--- a/mm/khugepaged.c
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+++ b/mm/khugepaged.c
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@@ -530,7 +530,12 @@ static int __collapse_huge_page_isolate(struct vm_area_struct *vma,
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goto out;
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}
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- VM_BUG_ON_PAGE(PageCompound(page), page);
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+ /* TODO: teach khugepaged to collapse THP mapped with pte */
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+ if (PageCompound(page)) {
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+ result = SCAN_PAGE_COMPOUND;
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+ goto out;
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+ }
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+
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VM_BUG_ON_PAGE(!PageAnon(page), page);
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/*
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