> 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
