On Fri, Sep 23, 2022 at 05:14:11PM -0600, Logan Gunthorpe wrote: > > > On 2022-09-23 17:07, Jason Gunthorpe wrote: > > On Fri, Sep 23, 2022 at 05:01:26PM -0600, Logan Gunthorpe wrote: > >> > >> > >> > >> On 2022-09-23 16:58, Jason Gunthorpe wrote: > >>> On Fri, Sep 23, 2022 at 02:11:03PM -0600, Logan Gunthorpe wrote: > >>>> > >>>> > >>>> On 2022-09-23 13:53, Jason Gunthorpe wrote: > >>>>> On Fri, Sep 23, 2022 at 01:08:31PM -0600, Logan Gunthorpe wrote: > >>>>> I'm encouraging Dan to work on better infrastructure in pgmap core > >>>>> because every pgmap implementation has this issue currently. > >>>>> > >>>>> For that reason it is probably not so relavent to this series. > >>>>> > >>>>> Perhaps just clarify in the commit message that the FOLL_LONGTERM > >>>>> restriction is to copy DAX until the pgmap page refcounts are fixed. > >>>> > >>>> Ok, I'll add that note. > >>>> > >>>> Per the fix for the try_grab_page(), to me it doesn't fit well in > >>>> try_grab_page() without doing a bunch of cleanup to change the > >>>> error handling, and the same would have to be added to try_grab_folio(). > >>>> So I think it's better to leave it where it was, but move it below the > >>>> respective grab calls. Does the incremental patch below look correct? > >>> > >>> Oh? I was thinking of just a very simple thing: > >> > >> Really would like it to return -EREMOTEIO instead of -ENOMEM as that's the > >> error used for bad P2PDMA page everywhere. > > > > I'd rather not see GUP made more fragile just for that.. > > Not sure how that's more fragile... You're way seems more dangerous given > the large number of call sites we are adding it to when it might not > apply. No, that is the point, it *always* applies. A devmap struct page of the wrong type should never exit gup, from any path, no matter what. We have two central functions that validate a page is OK to return, that *everyone* must call. If you don't put it there then we will probably miss copying it into a call site eventually. > > try_grab_page() calls folio_ref_inc(), that is only legal if it knows > > the page is already a valid pointer under the PTLs, so it is safe to > > check the pgmap as well. > > My point is it doesn't get a reference or a pin unless FOLL_PIN or FOLL_GET is > set and the documentation states that neither might be set, in which case > folio_ref_inc() will not be called... That isn't how GUP is structured, all the calls to try_grab_page() are in places where PIN/GET might be set and are safe for that usage. If we know PIN/GET is not set then we don't even need to call the function because it is a NOP. Jason
