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Message-ID: <2ad8b6cf-692a-ff89-ecc-586c20c5e07f@google.com>
Date:   Tue, 20 Jun 2023 12:54:25 -0700 (PDT)
From:   Hugh Dickins <hughd@...gle.com>
To:     Jason Gunthorpe <jgg@...pe.ca>
cc:     Hugh Dickins <hughd@...gle.com>,
        Andrew Morton <akpm@...ux-foundation.org>,
        Gerald Schaefer <gerald.schaefer@...ux.ibm.com>,
        Vasily Gorbik <gor@...ux.ibm.com>,
        Mike Kravetz <mike.kravetz@...cle.com>,
        Mike Rapoport <rppt@...nel.org>,
        "Kirill A. Shutemov" <kirill.shutemov@...ux.intel.com>,
        Matthew Wilcox <willy@...radead.org>,
        David Hildenbrand <david@...hat.com>,
        Suren Baghdasaryan <surenb@...gle.com>,
        Qi Zheng <zhengqi.arch@...edance.com>,
        Yang Shi <shy828301@...il.com>,
        Mel Gorman <mgorman@...hsingularity.net>,
        Peter Xu <peterx@...hat.com>,
        Peter Zijlstra <peterz@...radead.org>,
        Will Deacon <will@...nel.org>, Yu Zhao <yuzhao@...gle.com>,
        Alistair Popple <apopple@...dia.com>,
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        SeongJae Park <sj@...nel.org>,
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        Vishal Moola <vishal.moola@...il.com>,
        Vlastimil Babka <vbabka@...e.cz>,
        linux-arm-kernel@...ts.infradead.org, sparclinux@...r.kernel.org,
        linuxppc-dev@...ts.ozlabs.org, linux-s390@...r.kernel.org,
        linux-kernel@...r.kernel.org, linux-mm@...ck.org
Subject: Re: [PATCH v2 05/12] powerpc: add pte_free_defer() for pgtables
 sharing page

On Tue, 20 Jun 2023, Jason Gunthorpe wrote:
> On Tue, Jun 20, 2023 at 12:47:54AM -0700, Hugh Dickins wrote:
> > Add powerpc-specific 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 precedes
> > the generic version to avoid build breakage from incompatible pgtable_t.
> > 
> > This is awkward because the struct page contains only one rcu_head, but
> > that page may be shared between PTE_FRAG_NR pagetables, each wanting to
> > use the rcu_head at the same time: account concurrent deferrals with a
> > heightened refcount, only the first making use of the rcu_head, but
> > re-deferring if more deferrals arrived during its grace period.
> 
> You didn't answer my question why we can't just move the rcu to the
> actual free page?

I thought that I had answered it, perhaps not to your satisfaction:

https://lore.kernel.org/linux-mm/9130acb-193-6fdd-f8df-75766e663978@google.com/

My conclusion then was:
Not very good reasons: good enough, or can you supply a better patch?

Hugh

> 
> Since PPC doesn't recycle the frags, we don't need to carefully RCU
> free each frag, we just need to RCU free the entire page when it
> becomes eventually free?
> 
> Jason

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