linux-kernel - Re: [PATCH 3/3] mm: rmap: Fix CONT-PTE/PMD size hugetlb issue when unmapping

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Date:   Mon, 9 May 2022 12:41:38 -0400
From:   Peter Xu <peterx@...hat.com>
To:     Mike Kravetz <mike.kravetz@...cle.com>
Cc:     Gerald Schaefer <gerald.schaefer@...ux.ibm.com>,
        Baolin Wang <baolin.wang@...ux.alibaba.com>,
        akpm@...ux-foundation.org, catalin.marinas@....com,
        will@...nel.org, tsbogend@...ha.franken.de,
        James.Bottomley@...senPartnership.com, deller@....de,
        mpe@...erman.id.au, benh@...nel.crashing.org, paulus@...ba.org,
        hca@...ux.ibm.com, gor@...ux.ibm.com, agordeev@...ux.ibm.com,
        borntraeger@...ux.ibm.com, svens@...ux.ibm.com,
        ysato@...rs.sourceforge.jp, dalias@...c.org, davem@...emloft.net,
        arnd@...db.de, linux-arm-kernel@...ts.infradead.org,
        linux-kernel@...r.kernel.org, linux-ia64@...r.kernel.org,
        linux-mips@...r.kernel.org, linux-parisc@...r.kernel.org,
        linuxppc-dev@...ts.ozlabs.org, linux-s390@...r.kernel.org,
        linux-sh@...r.kernel.org, sparclinux@...r.kernel.org,
        linux-arch@...r.kernel.org, linux-mm@...ck.org
Subject: Re: [PATCH 3/3] mm: rmap: Fix CONT-PTE/PMD size hugetlb issue when
 unmapping

On Fri, May 06, 2022 at 12:07:13PM -0700, Mike Kravetz wrote:
> On 5/3/22 03:03, Gerald Schaefer wrote:
> > On Tue, 3 May 2022 10:19:46 +0800
> > Baolin Wang <baolin.wang@...ux.alibaba.com> wrote:
> >> On 5/2/2022 10:02 PM, Gerald Schaefer wrote:

[...]

> >> Please see previous code, we'll use the original pte value to check if 
> >> it is uffd-wp armed, and if need to mark it dirty though the hugetlbfs 
> >> is set noop_dirty_folio().
> >>
> >> pte_install_uffd_wp_if_needed(vma, address, pvmw.pte, pteval);
> > 
> > Uh, ok, that wouldn't work on s390, but we also don't have
> > CONFIG_PTE_MARKER_UFFD_WP / HAVE_ARCH_USERFAULTFD_WP set, so
> > I guess we will be fine (for now).
> > 
> > Still, I find it a bit unsettling that pte_install_uffd_wp_if_needed()
> > would work on a potential hugetlb *pte, directly de-referencing it
> > instead of using huge_ptep_get().
> > 
> > The !pte_none(*pte) check at the beginning would be broken in the
> > hugetlb case for s390 (not sure about other archs, but I think s390
> > might be the only exception strictly requiring huge_ptep_get()
> > for de-referencing hugetlb *pte pointers).

We could have used is_vm_hugetlb_page(vma) within the helper so as to
properly use either generic pte or hugetlb version of pte fetching.  We may
want to conditionally do set_[huge_]pte_at() too at the end.

I could prepare a patch for that even if it's not really anything urgently
needed. I assume that won't need to block this patchset since we need the
pteval for pte_dirty() check anyway and uffd-wp definitely needs it too.

Thanks,

-- 
Peter Xu