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

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Date:   Fri, 9 Aug 2019 18:14:56 +0000
From:   "Weiny, Ira" <ira.weiny@...el.com>
To:     Michal Hocko <mhocko@...nel.org>, Jan Kara <jack@...e.cz>
CC:     John Hubbard <jhubbard@...dia.com>,
        Vlastimil Babka <vbabka@...e.cz>,
        Andrew Morton <akpm@...ux-foundation.org>,
        Christoph Hellwig <hch@...radead.org>,
        Jason Gunthorpe <jgg@...pe.ca>,
        Jerome Glisse <jglisse@...hat.com>,
        LKML <linux-kernel@...r.kernel.org>,
        "linux-mm@...ck.org" <linux-mm@...ck.org>,
        "linux-fsdevel@...r.kernel.org" <linux-fsdevel@...r.kernel.org>,
        "Williams, Dan J" <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 15:58:13, Jan Kara wrote:
> > 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.
> 
> OK, I see. Thanks for the clarification.

Not to beat a dead horse but FOLL_LONGTERM also has implications now for CMA pages which may or may not (I'm not an expert on those pages) need special tracking. 

> 
> > 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.
> 
> Yes, going with a dedicated API sounds much better to me. Whether a
> dedicated FOLL flag is used internally is not that important. I am also for
> making the underlying gup to be really internal to the core kernel.

+1

I think GUP is too confusing.  I've been working with the details for many months now and it continues to confuse me.  :-(

My patches should be posted soon (based on mmotm) and I'll have my flame suit on so we can debate the interface.

Ira