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

[Kirill A. Shutemov]

On Thu, Mar 15, 2018 at 11:04:53PM +0000, Andrew Morton wrote:
> 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?

Yes, please. I forgot to put it.

-- 
 Kirill A. Shutemov