linux-kernel - Re: [PATCH 02/31] mm/migrate: remove cruft from migration_entry

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Date:   Tue, 23 May 2023 18:57:17 -0700 (PDT)
From:   Hugh Dickins <hughd@...gle.com>
To:     Alistair Popple <apopple@...dia.com>
cc:     Hugh Dickins <hughd@...gle.com>,
        Andrew Morton <akpm@...ux-foundation.org>,
        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>,
        Ralph Campbell <rcampbell@...dia.com>,
        Ira Weiny <ira.weiny@...el.com>,
        Steven Price <steven.price@....com>,
        SeongJae Park <sj@...nel.org>,
        Naoya Horiguchi <naoya.horiguchi@....com>,
        Christophe Leroy <christophe.leroy@...roup.eu>,
        Zack Rusin <zackr@...are.com>, Jason Gunthorpe <jgg@...pe.ca>,
        Axel Rasmussen <axelrasmussen@...gle.com>,
        Anshuman Khandual <anshuman.khandual@....com>,
        Pasha Tatashin <pasha.tatashin@...een.com>,
        Miaohe Lin <linmiaohe@...wei.com>,
        Minchan Kim <minchan@...nel.org>,
        Christoph Hellwig <hch@...radead.org>,
        Song Liu <song@...nel.org>,
        Thomas Hellstrom <thomas.hellstrom@...ux.intel.com>,
        linux-kernel@...r.kernel.org, linux-mm@...ck.org
Subject: Re: [PATCH 02/31] mm/migrate: remove cruft from
 migration_entry_wait()s

On Tue, 23 May 2023, Alistair Popple wrote:
> 
> Hugh Dickins <hughd@...gle.com> writes:
> 
> > migration_entry_wait_on_locked() does not need to take a mapped pte
> > pointer, its callers can do the unmap first.  Annotate it with
> > __releases(ptl) to reduce sparse warnings.
> 
> Thanks Hugh, I debated doing something similar when I reworked this to
> not take a pageref. However I was unsure if the pte unmap/unlock
> ordering mattered as this reverses the order of operations (from unlock
> then unmap to unmap then unlock).
> 
> I never found any reason why that would be a problem though so please
> add:
> 
> Reviewed-by: Alistair Popple <apopple@...dia.com>

Thanks Alistair.  Right, the ordering doesn't matter: you might briefly
think that because I'm going to put the rcu_read_unlock() into the
pte_unmap(), that there's something wrong with doing that early; but
you'd quickly realize that holding on to the spinlock is good enough.

Hugh