Re: [PATCH] mm: page_mkclean vs MADV_DONTNEED race

[Aneesh Kumar K.V]

Andrew Morton <akpm@xxxxxxxxxxxxxxxxxxxx> writes:

> 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?

The bug was noticed by code review and I didn't observe any failures
w.r.t test run. This is similar to 

commit 58ceeb6bec86d9140f9d91d71a710e963523d063
Author: Kirill A. Shutemov <kirill.shutemov@xxxxxxxxxxxxxxx>
Date:   Thu Apr 13 14:56:26 2017 -0700

    thp: fix MADV_DONTNEED vs. MADV_FREE race
    
commit ced108037c2aa542b3ed8b7afd1576064ad1362a
Author: Kirill A. Shutemov <kirill.shutemov@xxxxxxxxxxxxxxx>
Date:   Thu Apr 13 14:56:20 2017 -0700

    thp: fix MADV_DONTNEED vs. numa balancing race
    
>
> Did you consider a -stable backport?

Considering nobody reported issues w.r.t MADV_DONTNEED I was not sure.

-aneesh