On Wed, Sep 22, 2021 at 04:18:09PM -0700, Hugh Dickins wrote:

On Wed, 22 Sep 2021, Peter Xu wrote:

quoted

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