On Fri 09-08-19 02:05:15, John Hubbard wrote: > On 8/9/19 1:23 AM, 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? > > > > Oh, it's been discussed alright, but given how some of the discussions have gone, > I certainly am not surprised that there are still questions and criticisms! > Especially since I may have misunderstood some of the points, along the way. > It's been quite a merry go round. :) Yeah, I've tried to follow them but just gave up at some point. > Anyway, what I'm hearing now is: for gup(FOLL_LONGTERM), apply the pinned tracking. > And therefore only do put_user_page() on pages that were pinned with > FOLL_LONGTERM. For short term pins, let the locking do what it will: > things can briefly block and all will be well. > > Also, that may or may not come with a wrapper function, courtesy of Jan > and Ira. > > Is that about right? It's late here, but I don't immediately recall any > problems with doing it that way... Yes that makes more sense to me. Whoever needs that tracking should opt-in for it. Otherwise you just risk problems like the one discussed in the mlock path (because we do a strange stuff in the name of performance) and a never ending whack a mole where new users do not follow the new API usage and that results in all sorts of weird issues. Thanks! -- Michal Hocko SUSE Labs
