On 10/20/21 18:12, Dan Williams wrote: > On Wed, Oct 20, 2021 at 10:09 AM Joao Martins <joao.m.martins@xxxxxxxxxx> wrote: >> On 10/19/21 20:21, Dan Williams wrote: >>> On Tue, Oct 19, 2021 at 9:02 AM Jason Gunthorpe <jgg@xxxxxxxx> wrote: >>>> On Tue, Oct 19, 2021 at 04:13:34PM +0100, Joao Martins wrote: >>>>> On 10/19/21 00:06, Jason Gunthorpe wrote: >>>>>> On Mon, Oct 18, 2021 at 12:37:30PM -0700, Dan Williams wrote: >>>>> Whats the benefit between preventing longterm at start >>>>> versus only after mounting the filesystem? Or is the intended future purpose >>>>> to pass more context into an holder potential future callback e.g. nack longterm >>>>> pins on a page basis? >>>> >>>> I understood Dan's remark that the device-dax path allows >>>> FOLL_LONGTERM and the FSDAX path does not ? >>>> >>>> Which, IIRC, today is signaled basd on vma properties and in all cases >>>> fast-gup is denied. >>> >>> Yeah, I forgot that 7af75561e171 eliminated any possibility of >>> longterm-gup-fast for device-dax, let's not disturb that status quo. >>> >> I am slightly confused by this comment -- the status quo is what we are >> questioning here -- And we talked about changing that in the past too >> (thread below), that longterm-gup-fast was an oversight that that commit >> was only applicable to fsdax. [Maybe this is just my english confusion] > > No, you have it correct. However that "regression" has gone unnoticed, > so unless there is data showing that gup-fast on device-dax is > critical for longterm page pinning workflows I'm ok for longterm to > continue to trigger gup-slow. > To be fair, it's not surprising that nobody noticed -- my general intent was just to special-case less for device-dax. Not many places use pin_user_pages_fast(FOLL_LONGTERM). This is only exposed on those few cases that do try to use it (e.g. RDMA/IB, vDPA), and specifically when the page fault occurs (that requires fallback-ing to gup-slow) at a higher level than the amount you're pinning e.g. pinning in 2M extents on a device-dax of 1G pagesize. Pin size is limited to a 2M extent at a time by the users I mentioned above -- regardless of the total size of the extent you will be pinning (i.e. 512 struct pages pointers fit one page). But even with all this, this [FOLL_LONGTERM on pin-fast] would still go unnoticed because gup-fast on devmap is just as slow as gup :)
