On 2/12/21 12:47 PM, Axel Rasmussen wrote:

On Fri, Feb 12, 2021 at 12:40 PM Peter Xu [off-list ref] wrote:

quoted

On Thu, Feb 11, 2021 at 04:19:55PM -0800, Mike Kravetz wrote:

quoted

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