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

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Message-ID: <045c8fb5-fa64-c0e0-c5e4-2734f849a66a@oracle.com>
Date:   Tue, 23 May 2017 14:39:38 -0700
From:   Qing Huang <qing.huang@...cle.com>
To:     Christoph Hellwig <hch@...radead.org>
Cc:     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 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.