linux-kernel - Re: [PATCH 01/31] mm: use pmdp_get_lockless() without surplus barrier()

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Date:   Thu, 1 Jun 2023 19:31:05 -0700 (PDT)
From:   Hugh Dickins <hughd@...gle.com>
To:     Peter Xu <peterx@...hat.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 Zijlstra <peterz@...radead.org>,
        Will Deacon <will@...nel.org>, Yu Zhao <yuzhao@...gle.com>,
        Alistair Popple <apopple@...dia.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 01/31] mm: use pmdp_get_lockless() without surplus
 barrier()

On Fri, 26 May 2023, Peter Xu wrote:
> 
> The other confusing thing on this _lockless trick on PAE is, I think it
> _might_ go wrong with devmap..
> 
> The problem is here we assumed even if high & low may not match, we still
> can rely on most pte/pmd checks are done only on low bits (except _none()
> check) to guarantee at least the checks are still atomic on low bits.
> 
> But it seems to me it's not true anymore if with pmd_trans_huge() after
> devmap introduced, e.g.:

I agree that there would likely be a problem for p??_devmap() on 32-bit
PAE: but (I hope I followed the chain correctly!) I had earlier found
that pmd_devmap() can only return true when CONFIG_ZONE_DEVICE=y, and
config ZONE_DEVICE depends on ARCH_HAS_PTE_DEVMAP, and ARCH_HAS_PTE_DEVMAP
is only selected (in some cases) by arm64, powerpc if PPC_BOOK3S_64, and
x86 if X86_64.

So I stopped worrying about devmap.

Hugh