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

[Michal Hocko]

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