Re: [PATCH v5 02/10] hugetlb/userfaultfd: Forbid huge pmd sharing when uffd enabled

[Axel Rasmussen]

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?
>
> Btw, Axel, it seems there will still be some respins on the pmd sharing
> patches.  Since it turns out it'll be shared by multiple tasks now, do you mind
> I pick those out and send them separately?  Then we can consolidate this part
> to move on with either the rest of the tasks we've got on hand.

Sounds good to me. :) Thanks Peter + Mike for working on this!

>
> Thanks,
>
> --
> Peter Xu
>