On 2/12/21 12:47 PM, Axel Rasmussen wrote: > On Fri, Feb 12, 2021 at 12:40 PM Peter Xu <peterx@xxxxxxxxxx> wrote: >> >> On Thu, Feb 11, 2021 at 04:19:55PM -0800, Mike Kravetz wrote: >>> want_pmd_share() is currently just a check for CONFIG_ARCH_WANT_HUGE_PMD_SHARE. >>> How about leaving that mostly as is, and adding the new vma checks to >>> vma_shareable(). vma_shareable() would then be something like: >>> >>> if (!(vma->vm_flags & VM_MAYSHARE)) >>> return false; >>> #ifdef CONFIG_USERFAULTFD >>> if (uffd_disable_huge_pmd_share(vma) >>> return false; >>> #endif >>> #ifdef /* XXX */ >>> /* add other checks for things like uffd wp and soft dirty here */ >>> #endif /* XXX */ >>> >>> if (range_in_vma(vma, base, end) >>> return true; >>> return false; >>> >>> Of course, this would require we leave the call to vma_shareable() at the >>> beginning of huge_pmd_share. It also means that we are always making a >>> function call into huge_pmd_share to determine if sharing is possible. >>> That is not any different than today. If we do not want to make that extra >>> function call, then I would suggest putting all that code in want_pmd_share. >>> It just seems that all the vma checks for sharing should be in one place >>> if possible. >> >> I don't worry a lot on that since we've already got huge_pte_alloc() which >> takes care of huge pmd sharing case, so I don't expect e.g. even most hugetlb >> developers to use want_pmd_share() at all, because huge_pte_alloc() will be the >> one that frequently got called. >> >> But yeah we can definitely put the check logic into huge_pmd_share() too. >> Looking at above code it looks still worth a helper like want_pmd_share() or >> with some other name. Then... instead of making this complicated, how about I >> mostly keep this patch but move want_pmd_share() call into huge_pmd_share() >> instead? When looking at this again, all I was suggesting was a single routine to check for the possibility of pmd sharing. That is what the version of want_pmd_share in this patch does. I have some patches for future optimizations that only take i_mmap_rwsem in the fault path if sharing is possible. This is before huge_pte_alloc. want_pmd_share as defined in this patch would work for that. Sorry for the noise. -- Mike Kravetz
