Re: [PATCH 1/3] mm/mlock.c: convert put_page() to put_user_page*()

From: Michal Hocko <mhocko@kernel.org>
To: John Hubbard <jhubbard@nvidia.com>
Cc: Vlastimil Babka <vbabka@suse.cz>,
	Andrew Morton <akpm@linux-foundation.org>,
	Christoph Hellwig <hch@infradead.org>,
	Ira Weiny <ira.weiny@intel.com>, Jan Kara <jack@suse.cz>,
	Jason Gunthorpe <jgg@ziepe.ca>,
	Jerome Glisse <jglisse@redhat.com>,
	LKML <linux-kernel@vger.kernel.org>,
	linux-mm@kvack.org, linux-fsdevel@vger.kernel.org,
	Dan Williams <dan.j.williams@intel.com>,
	Daniel Black <daniel@linux.ibm.com>,
	Matthew Wilcox <willy@infradead.org>,
	Mike Kravetz <mike.kravetz@oracle.com>
Subject: Re: [PATCH 1/3] mm/mlock.c: convert put_page() to put_user_page*()
Date: Fri, 9 Aug 2019 10:23:07 +0200	[thread overview]
Message-ID: <20190809082307.GL18351@dhcp22.suse.cz> (raw)
In-Reply-To: <420a5039-a79c-3872-38ea-807cedca3b8a@suse.cz>

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