linux-kernel - Re: [Linux-kernel-mentees][PATCH v6 1/2] sgi-gru: Convert put_page() to put_user

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Date:   Tue, 20 Aug 2019 13:50:18 -0700
From:   John Hubbard <jhubbard@...dia.com>
To:     Michal Hocko <mhocko@...nel.org>
CC:     Bharath Vedartham <linux.bhar@...il.com>,
        Dimitri Sivanich <sivanich@....com>,
        Andrew Morton <akpm@...ux-foundation.org>,
        <jglisse@...hat.com>, <ira.weiny@...el.com>,
        <gregkh@...uxfoundation.org>, <arnd@...db.de>,
        <william.kucharski@...cle.com>, <hch@....de>,
        <inux-kernel@...r.kernel.org>, <linux-mm@...ck.org>,
        <linux-kernel-mentees@...ts.linuxfoundation.org>,
        <linux-kernel@...r.kernel.org>
Subject: Re: [Linux-kernel-mentees][PATCH v6 1/2] sgi-gru: Convert put_page()
 to put_user_page*()

On 8/20/19 1:18 AM, Michal Hocko wrote:
> On Mon 19-08-19 12:30:18, John Hubbard wrote:
>> On 8/19/19 12:06 PM, Bharath Vedartham wrote:
>>> On Mon, Aug 19, 2019 at 07:56:11AM -0500, Dimitri Sivanich wrote:
...
>> Conversion of gup/put_page sites:
>>
>> Before:
>>
>> 	get_user_pages(...);
>> 	...
>> 	for each page:
>> 		put_page();
>>
>> After:
>> 	
>> 	gup_flags |= FOLL_PIN; (maybe FOLL_LONGTERM in some cases)
>> 	vaddr_pin_user_pages(...gup_flags...)
> 
> I was hoping that FOLL_PIN would be handled by vaddr_pin_user_pages.
> 

Good point: now that we've got the 4 cases summarized, it turns out
that either FOLL_PIN is required, or there is no need to call
vaddr_pin_user_pages() at all.  So we can go back to setting FOLL_PIN
inside it, which is of course much better for maintenance. Great!


>> 	...
>> 	vaddr_unpin_user_pages(); /* which invokes put_user_page() */
>>
>> Fortunately, it's not harmful for the simpler conversion from put_page()
>> to put_user_page() to happen first, and in fact those have usually led
>> to simplifications, paving the way to make it easier to call
>> vaddr_unpin_user_pages(), once it's ready. (And showing exactly what
>> to convert, too.)
> 
> If that makes the later conversion easier then no real objections from
> me. Assuming that the current put_user_page conversions are correct of
> course (I have the mlock one and potentials that falls into the same
> category in mind).
> 

Agreed: only correct conversions should be done. Not the incorrect
ones. ahem.  :)

thanks,
-- 
John Hubbard
NVIDIA