Re: [PATCH] mm: page_mkclean vs MADV_DONTNEED race

From: Andrew Morton
Date: Thu Mar 21 2019 - 19:31:49 EST

On Thu, 21 Mar 2019 09:36:10 +0530 "Aneesh Kumar K.V" <aneesh.kumar@xxxxxxxxxxxxx> wrote:

> MADV_DONTNEED is handled with mmap_sem taken in read mode.
> We call page_mkclean without holding mmap_sem.
> MADV_DONTNEED implies that pages in the region are unmapped and subsequent
> access to the pages in that range is handled as a new page fault.
> This implies that if we don't have parallel access to the region when
> MADV_DONTNEED is run we expect those range to be unallocated.
> w.r.t page_mkclean we need to make sure that we don't break the MADV_DONTNEED
> semantics. MADV_DONTNEED check for pmd_none without holding pmd_lock.
> This implies we skip the pmd if we temporarily mark pmd none. Avoid doing
> that while marking the page clean.
> Keep the sequence same for dax too even though we don't support MADV_DONTNEED
> for dax mapping

What were the runtime effects of the bug?

Did you consider a -stable backport?