On 2022-09-23 17:21, Jason Gunthorpe wrote: > 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. Most of the call sites don't apply though, with huge pages and gate pages... >>> 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. That's not what the documentation for the function says: "Either FOLL_PIN or FOLL_GET (or neither) may be set... Return: true for success, or if no action was required (if neither FOLL_PIN nor FOLL_GET was set, nothing is done)." https://elixir.bootlin.com/linux/v6.0-rc6/source/mm/gup.c#L194 Logan
