linux-kernel - Re: [PATCH] mm/khugepaged: Convert VM_BUG

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Message-Id: <20180315160453.dff17cfe3dca056dabc98b9e@linux-foundation.org>
Date:   Thu, 15 Mar 2018 16:04:53 -0700
From:   Andrew Morton <akpm@...ux-foundation.org>
To:     "Kirill A. Shutemov" <kirill.shutemov@...ux.intel.com>
Cc:     Laura Abbott <labbott@...hat.com>, linux-mm@...ck.org,
        linux-kernel@...r.kernel.org
Subject: Re: [PATCH] mm/khugepaged: Convert VM_BUG_ON() to collapse fail

On Thu, 15 Mar 2018 18:23:53 +0300 "Kirill A. Shutemov" <kirill.shutemov@...ux.intel.com> wrote:

> 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.
> 
> Signed-off-by: Kirill A. Shutemov <kirill.shutemov@...ux.intel.com>
> Fixes: b1caa957ae6d ("khugepaged: ignore pmd tables with THP mapped with ptes")

Jan 2016.  Do we need a cc:stable?