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

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Date:   Fri, 9 Aug 2019 10:23:07 +0200
From:   Michal Hocko <mhocko@...nel.org>
To:     John Hubbard <jhubbard@...dia.com>
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 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?
-- 
Michal Hocko
SUSE Labs