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From:   Jann Horn <jannh@google.com>
Date:   Thu, 1 Jun 2023 15:31:37 +0200
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Subject: Re: [PATCH 08/12] mm/pgtable: add pte_free_defer() for pgtable as page
To:     Hugh Dickins <hughd@google.com>
Cc:     Andrew Morton <akpm@linux-foundation.org>,
        Mike Kravetz <mike.kravetz@oracle.com>,
        Mike Rapoport <rppt@kernel.org>,
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On Mon, May 29, 2023 at 8:23=E2=80=AFAM Hugh Dickins <hughd@google.com> wro=
te:
> Add the generic pte_free_defer(), to call pte_free() via call_rcu().
> pte_free_defer() will be called inside khugepaged's retract_page_tables()
> loop, where allocating extra memory cannot be relied upon.  This version
> suits all those architectures which use an unfragmented page for one page
> table (none of whose pte_free()s use the mm arg which was passed to it).

Pages that have been scheduled for deferred freeing can still be
locked, right? So struct page's members "ptl" and "rcu_head" can now
be in use at the same time? If that's intended, it would probably be a
good idea to add comments in the "/* Page table pages */" part of
struct page to point out that the first two members can be used by the
rcu_head while the page is still used as a page table in some
contexts, including use of the ptl.