Re: [PATCH] mm/khugepaged: Detecting uffd-wp vma more efficiently

Peter Xu
2021-09-22T23:44:49Z

On Wed, Sep 22, 2021 at 04:18:09PM -0700, Hugh Dickins wrote:
> On Wed, 22 Sep 2021, Peter Xu wrote:
> >
> > Not installing pmd means uffd-minor can still trap any further faults just like
> > before, afaiu.
> >
> > There's a very trivial detail that the pmd missing case will have a very slight
> > code path change when the next page fault happens: in __handle_mm_fault() we'll
> > first try to go into create_huge_pmd() once, however since shmem didn't provide
> > huge_fault(), we'll go the VM_FAULT_FALLBACK path, and things will go like
> > before when faulting on a small pte. The next UFFDIO_CONTINUE will allocate
> > that missing pmd again, however it'll install a 4K page only.
>
> I think you're mistaken there.
>
> I can't tell you much about ->huge_fault(), something introduced for
> DAX I believe; but shmem has managed pmd mappings without it, since
> before ->huge_fault() was ever added.

Right, I wanted to express we didn't go into there, hence no way to allocate
pmd there.

>
> Look for the call to do_set_pmd() in finish_fault(): I think you'll
> find that is the way shmem's huge pmds get in.
>
> Earlier in the thread you suggested "shmem_getpage() only returns
> small pages": but it can very well return PageTransCompound pages,
> head or tail, which arrive at this do_set_pmd().

But note that uffd-minor will trap the shmem fault() even if pmd_none:

page = pagecache_get_page(mapping, index,
FGP_ENTRY | FGP_HEAD | FGP_LOCK, 0);

if (page && vma && userfaultfd_minor(vma)) {
if (!xa_is_value(page)) {
unlock_page(page);
put_page(page);
}
*fault_type = handle_userfault(vmf, VM_UFFD_MINOR);
return 0;
}

That's why I think it'll be fine, because it should only be UFFDIO_CONTINUE
that installs the pte (alongside with allocating the pmd).

Or did I miss something?

Thanks,

--
Peter Xu