Re: [PATCH 3/3] mm, thp: make swapin readahead under down_read of mmap_sem

[Kirill A. Shutemov]

On Mon, May 23, 2016 at 02:49:09PM -0400, Rik van Riel wrote:
> On Mon, 2016-05-23 at 20:42 +0200, Michal Hocko wrote:
> > On Mon 23-05-16 20:14:11, Ebru Akagunduz wrote:
> > > 
> > > Currently khugepaged makes swapin readahead under
> > > down_write. This patch supplies to make swapin
> > > readahead under down_read instead of down_write.
> > You are still keeping down_write. Can we do without it altogether?
> > Blocking mmap_sem of a remote proces for write is certainly not nice.
> 
> Maybe Andrea can explain why khugepaged requires
> a down_write of mmap_sem?
> 
> If it were possible to have just down_read that
> would make the code a lot simpler.

You need a down_write() to retract page table. We need to make sure that
nobody sees the page table before we can replace it with huge pmd.

-- 
 Kirill A. Shutemov