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

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Message-ID: <20190809135813.GF17568@quack2.suse.cz>
Date:   Fri, 9 Aug 2019 15:58:13 +0200
From:   Jan Kara <jack@...e.cz>
To:     Michal Hocko <mhocko@...nel.org>
Cc:     John Hubbard <jhubbard@...dia.com>,
        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 10:23:07, 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?

The new tracking cannot be bound to FOLL_LONGTERM. Anything that gets page
reference and then touches page data (e.g. direct IO) needs the new kind of
tracking so that filesystem knows someone is messing with the page data.
So what John is trying to address is a different (although related) problem
to someone pinning a page for a long time.

In principle, I'm not strongly opposed to a new FOLL flag to determine
whether a pin or an ordinary page reference will be acquired at least as an
internal implementation detail inside mm/gup.c. But I would really like to
discourage new GUP users taking just page reference as the most clueless
users (drivers) usually need a pin in the sense John implements. So in
terms of API I'd strongly prefer to deprecate GUP as an API, provide
vaddr_pin_pages() for drivers to get their buffer pages pinned and then for
those few users who really know what they are doing (and who are not
interested in page contents) we can have APIs like follow_page() to get a
page reference from a virtual address.

								Honza
-- 
Jan Kara <jack@...e.com>
SUSE Labs, CR