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

[Peter Xu <peterx@redhat.com>]

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.

Thanks,

-- 
Peter Xu