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

[Kirill A. Shutemov]

On Mon, May 23, 2016 at 03:26:47PM -0400, Rik van Riel wrote:
> On Mon, 2016-05-23 at 22:01 +0300, Kirill A. Shutemov wrote:
> > 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.
> 
> Good point.
> 
> I guess the alternative is to have the page_table_lock
> taken by a helper function (everywhere) that can return
> failure if the page table was changed while the caller
> was waiting for the lock.

Not page table was changed, but pmd is now pointing to something else.
Basically, we would need to nest all pte-ptl's within pmd_lock().
That's not good for scalability.

-- 
 Kirill A. Shutemov