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

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Message-ID: <20170523074234.GE29525@infradead.org>
Date:   Tue, 23 May 2017 00:42:34 -0700
From:   Christoph Hellwig <hch@...radead.org>
To:     Qing Huang <qing.huang@...cle.com>
Cc:     Christoph Hellwig <hch@...radead.org>, linux-rdma@...r.kernel.org,
        linux-kernel@...r.kernel.org, dledford@...hat.com,
        sean.hefty@...el.com, artemyko@...lanox.com, linux-mm@...ck.org
Subject: Re: [PATCH] ib/core: not to set page dirty bit if it's already set.

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.