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. > 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. Thanks! -- Michal Hocko SUSE Labs
