Re: [PATCH] mm/khugepaged: Convert VM_BUG_ON() to collapse fail

[Andrew Morton]

On Thu, 15 Mar 2018 18:23:53 +0300 "Kirill A. Shutemov" <kirill.shutemov@xxxxxxxxxxxxxxx> 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@xxxxxxxxxxxxxxx>
> Fixes: b1caa957ae6d ("khugepaged: ignore pmd tables with THP mapped with ptes")

Jan 2016.  Do we need a cc:stable?