linux-kernel - Re: [PATCH 1/3] mm/mlock.c: convert put_page() to put_user

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Date:   Fri, 9 Aug 2019 02:05:15 -0700
From:   John Hubbard <jhubbard@...dia.com>
To:     Michal Hocko <mhocko@...nel.org>
CC:     Vlastimil Babka <vbabka@...e.cz>,
        Andrew Morton <akpm@...ux-foundation.org>,
        Christoph Hellwig <hch@...radead.org>,
        Ira Weiny <ira.weiny@...el.com>, Jan Kara <jack@...e.cz>,
        Jason Gunthorpe <jgg@...pe.ca>,
        Jerome Glisse <jglisse@...hat.com>,
        LKML <linux-kernel@...r.kernel.org>, <linux-mm@...ck.org>,
        <linux-fsdevel@...r.kernel.org>,
        Dan Williams <dan.j.williams@...el.com>,
        Daniel Black <daniel@...ux.ibm.com>,
        Matthew Wilcox <willy@...radead.org>,
        Mike Kravetz <mike.kravetz@...cle.com>
Subject: Re: [PATCH 1/3] mm/mlock.c: convert put_page() to put_user_page*()

On 8/9/19 1:23 AM, Michal Hocko wrote:
> On Fri 09-08-19 10:12:48, Vlastimil Babka wrote:
>> On 8/9/19 12:59 AM, John Hubbard wrote:
>>>>> That's true. However, I'm not sure munlocking is where the
>>>>> put_user_page() machinery is intended to be used anyway? These are
>>>>> short-term pins for struct page manipulation, not e.g. dirtying of page
>>>>> contents. Reading commit fc1d8e7cca2d I don't think this case falls
>>>>> within the reasoning there. Perhaps not all GUP users should be
>>>>> converted to the planned separate GUP tracking, and instead we should
>>>>> have a GUP/follow_page_mask() variant that keeps using get_page/put_page?
>>>>>   
>>>>
>>>> Interesting. So far, the approach has been to get all the gup callers to
>>>> release via put_user_page(), but if we add in Jan's and Ira's vaddr_pin_pages()
>>>> wrapper, then maybe we could leave some sites unconverted.
>>>>
>>>> However, in order to do so, we would have to change things so that we have
>>>> one set of APIs (gup) that do *not* increment a pin count, and another set
>>>> (vaddr_pin_pages) that do.
>>>>
>>>> Is that where we want to go...?
>>>>
>>
>> We already have a FOLL_LONGTERM flag, isn't that somehow related? And if
>> it's not exactly the same thing, perhaps a new gup flag to distinguish
>> which kind of pinning to use?
> 
> Agreed. This is a shiny example how forcing all existing gup users into
> the new scheme is subotimal at best. Not the mention the overal
> fragility mention elsewhere. I dislike the conversion even more now.
> 
> Sorry if this was already discussed already but why the new pinning is
> not bound to FOLL_LONGTERM (ideally hidden by an interface so that users
> do not have to care about the flag) only?
> 

Oh, it's been discussed alright, but given how some of the discussions have gone,
I certainly am not surprised that there are still questions and criticisms!
Especially since I may have misunderstood some of the points, along the way.
It's been quite a merry go round. :)

Anyway, what I'm hearing now is: for gup(FOLL_LONGTERM), apply the pinned tracking.
And therefore only do put_user_page() on pages that were pinned with
FOLL_LONGTERM. For short term pins, let the locking do what it will:
things can briefly block and all will be well.

Also, that may or may not come with a wrapper function, courtesy of Jan
and Ira.

Is that about right? It's late here, but I don't immediately recall any
problems with doing it that way...

thanks,
-- 
John Hubbard
NVIDIA