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

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Date:   Wed, 22 Sep 2021 16:18:09 -0700 (PDT)
From:   Hugh Dickins <hughd@...gle.com>
To:     Peter Xu <peterx@...hat.com>
cc:     Axel Rasmussen <axelrasmussen@...gle.com>,
        LKML <linux-kernel@...r.kernel.org>,
        Linux MM <linux-mm@...ck.org>,
        Andrew Morton <akpm@...ux-foundation.org>,
        Andrea Arcangeli <aarcange@...hat.com>,
        Hugh Dickins <hughd@...gle.com>,
        Nadav Amit <nadav.amit@...il.com>
Subject: Re: [PATCH] mm/khugepaged: Detecting uffd-wp vma more efficiently

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.

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().

Hugh