Re: [PATCH] ib/core: not to set page dirty bit if it's already set.

[Qing Huang]

On 5/23/2017 12:42 AM, Christoph Hellwig wrote:
> On Mon, May 22, 2017 at 04:43:57PM -0700, Qing Huang wrote:
> > On 5/19/2017 6:05 AM, Christoph Hellwig wrote:
> > > On Thu, May 18, 2017 at 04:33:53PM -0700, Qing Huang wrote:
> > > > This change will optimize kernel memory deregistration operations.
> > > > __ib_umem_release() used to call set_page_dirty_lock() against every
> > > > writable page in its memory region. Its purpose is to keep data
> > > > synced between CPU and DMA device when swapping happens after mem
> > > > deregistration ops. Now we choose not to set page dirty bit if it's
> > > > already set by kernel prior to calling __ib_umem_release(). This
> > > > reduces memory deregistration time by half or even more when we ran
> > > > application simulation test program.
> > > As far as I can tell this code doesn't even need set_page_dirty_lock
> > > and could just use set_page_dirty
> > It seems that set_page_dirty_lock has been used here for more than 10 years.
> > Don't know the original purpose. Maybe it was used to prevent races between
> > setting dirty bits and swapping out pages?
> I suspect copy & paste.  Or maybe I don't actually understand the
> explanation of set_page_dirty vs set_page_dirty_lock enough.  But
> I'd rather not hack around the problem.
> --
I think there are two parts here. First part is that we don't need to 
set the dirty bit if it's already set. Second part is whether we use 
set_page_dirty or set_page_dirty_lock to set dirty bits.