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Date:   Wed, 22 Sep 2021 19:44:42 -0400
From:   Peter Xu <peterx@redhat.com>
To:     Hugh Dickins <hughd@google.com>
Cc:     Axel Rasmussen <axelrasmussen@google.com>,
        LKML <linux-kernel@vger.kernel.org>,
        Linux MM <linux-mm@kvack.org>,
        Andrew Morton <akpm@linux-foundation.org>,
        Andrea Arcangeli <aarcange@redhat.com>,
        Nadav Amit <nadav.amit@gmail.com>
Subject: Re: [PATCH] mm/khugepaged: Detecting uffd-wp vma more efficiently
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References: <20210922175156.130228-1-peterx@redhat.com>
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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