From: John Hubbard <jhubbard@nvidia.com>
To: Ira Weiny <ira.weiny@intel.com>
Cc: Jason Gunthorpe <jgg@mellanox.com>,
Andrew Morton <akpm@linux-foundation.org>,
"linux-mm@kvack.org" <linux-mm@kvack.org>,
LKML <linux-kernel@vger.kernel.org>,
"linux-rdma@vger.kernel.org" <linux-rdma@vger.kernel.org>,
"linux-fsdevel@vger.kernel.org" <linux-fsdevel@vger.kernel.org>,
Doug Ledford <dledford@redhat.com>,
"Mike Marciniszyn" <mike.marciniszyn@intel.com>,
Dennis Dalessandro <dennis.dalessandro@intel.com>,
Christian Benvenuti <benve@cisco.com>, "Jan Kara" <jack@suse.cz>
Subject: Re: [PATCH 1/1] infiniband/mm: convert put_page() to put_user_page*()
Date: Thu, 23 May 2019 15:50:14 -0700 [thread overview]
Message-ID: <050f56d0-1dda-036e-e508-3a7255ac7b59@nvidia.com> (raw)
In-Reply-To: <20190523223701.GA15048@iweiny-DESK2.sc.intel.com>
On 5/23/19 3:37 PM, Ira Weiny wrote:
[...]
> I've dug in further and I see now that release_pages() implements (almost the
> same thing, see below) as put_page().
>
> However, I think we need to be careful here because put_page_testzero() calls
>
> page_ref_dec_and_test(page);
>
> ... and after your changes it will need to call ...
>
> page_ref_sub_return(page, GUP_PIN_COUNTING_BIAS);
>
> ... on a GUP page:
>
> So how do you propose calling release_pages() from within put_user_pages()? Or
> were you thinking this would be temporary?
I was thinking of it as a temporary measure, only up until, but not including the
point where put_user_pages() becomes active. That is, the point when put_user_pages
starts decrementing GUP_PIN_COUNTING_BIAS, instead of just forwarding to put_page().
(For other readers, that's this patch:
"mm/gup: debug tracking of get_user_pages() references"
...in https://github.com/johnhubbard/linux/tree/gup_dma_core )
>
> That said, there are 2 differences I see between release_pages() and put_page()
>
> 1) release_pages() will only work for a MEMORY_DEVICE_PUBLIC page and not all
> devmem pages...
> I think this is a bug, patch to follow shortly.
>
> 2) release_pages() calls __ClearPageActive() while put_page() does not
>
> I have no idea if the second difference is a bug or not. But it smells of
> one...
>
> It would be nice to know if the open coding of put_page is really a performance
> benefit or not. It seems like an attempt to optimize the taking of the page
> data lock.
>
> Does anyone have any information about the performance advantage here?
>
> Given the changes above it seems like it would be a benefit to merge the 2 call
> paths more closely to make sure we do the right thing.
>
Yes, it does. Maybe best to not do the temporary measure, then, while this stuff
gets improved. I'll look at your other patch...
thanks,
--
John Hubbard
NVIDIA
next prev parent reply other threads:[~2019-05-23 22:50 UTC|newest]
Thread overview: 13+ messages / expand[flat|nested] mbox.gz Atom feed top
2019-05-23 7:25 [PATCH 0/1] infiniband/mm: convert put_page() to put_user_page*() john.hubbard
2019-05-23 7:25 ` [PATCH 1/1] " john.hubbard
2019-05-23 15:31 ` Jerome Glisse
2019-05-23 17:56 ` John Hubbard
2019-05-23 17:28 ` Ira Weiny
2019-05-23 17:32 ` Jason Gunthorpe
2019-05-23 17:46 ` John Hubbard
2019-05-23 19:04 ` Ira Weiny
2019-05-23 19:13 ` John Hubbard
2019-05-23 22:37 ` Ira Weiny
2019-05-23 22:50 ` John Hubbard [this message]
2019-05-23 22:54 ` John Hubbard
2019-05-23 19:17 ` Jason Gunthorpe
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