On Thu, Sep 05, 2019 at 05:44:00PM +0200, Dominique Martinet wrote: > > You shouldn't be calling vmf_insert_pfn_pmd() from a regular ->fault > > handler, as by then the fault handler has already inserted a PMD. > > The ->huge_fault handler is the place to call it from. > > > > You may need to force PMD-alignment for your call to mmap(). > > I was missing setting the VM_HUGE_FAULT vm_flags2 bit in the vma - the > huge_fault handler is now called, and I no longer have the pre-existing > pmd problem; that's a much better solution than manually fiddling with > flags :) > > Question though - is it ok to insert small pages if the huge_fault > handler is called with PE_SIZE_PMD ? > (I think the pte insertion will automatically create the pmd, but would > be good to confirm) No, you need to return VM_FAULT_FALLBACK, at which point the generic code will create a PMD for you and then call your ->fault handler which can insert PTEs. It works the same way from PUDs to PMDs by the way, in case you ever have a 1GB mapping ;-) > Now I've got this I'm back to where I stood with my kludge though, > programs work until they exit, and the zap_huge_pmd() function tries to > withdraw the pagetable from some magic field that was never set in my > case... I realize this is old code no longer upstream, but my new > workaround for this (looking at the zap_huge_pmd function) was to > pretend my file is dax. > Now that I've set it as dax I think it actually makes sense as in > "there's memory here that points to something linux no longer manages > directly, just let it be" and we might benefit from the other exceptions > dax have, I'll need to look at what this implies in more details... I think that should be fine, but I don't really know RHEL 7.3 all that well ;-) > > Hope these pointers are slightly more useful than a rubber duck ;-) > > Much appreciated, thank you for taking the time! :) No problem ... these APIs are relatively new and not necessarily all that intuitive.
