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Date:   Tue, 23 May 2023 19:44:40 -0700 (PDT)
From:   Hugh Dickins <hughd@google.com>
To:     Qi Zheng <qi.zheng@linux.dev>
cc:     Hugh Dickins <hughd@google.com>,
        Andrew Morton <akpm@linux-foundation.org>,
        Mike Kravetz <mike.kravetz@oracle.com>,
        Mike Rapoport <rppt@kernel.org>,
        "Kirill A. Shutemov" <kirill.shutemov@linux.intel.com>,
        Matthew Wilcox <willy@infradead.org>,
        David Hildenbrand <david@redhat.com>,
        Suren Baghdasaryan <surenb@google.com>,
        Qi Zheng <zhengqi.arch@bytedance.com>,
        Yang Shi <shy828301@gmail.com>,
        Mel Gorman <mgorman@techsingularity.net>,
        Peter Xu <peterx@redhat.com>,
        Peter Zijlstra <peterz@infradead.org>,
        Will Deacon <will@kernel.org>, Yu Zhao <yuzhao@google.com>,
        Alistair Popple <apopple@nvidia.com>,
        Ralph Campbell <rcampbell@nvidia.com>,
        Ira Weiny <ira.weiny@intel.com>,
        Steven Price <steven.price@arm.com>,
        SeongJae Park <sj@kernel.org>,
        Naoya Horiguchi <naoya.horiguchi@nec.com>,
        Christophe Leroy <christophe.leroy@csgroup.eu>,
        Zack Rusin <zackr@vmware.com>, Jason Gunthorpe <jgg@ziepe.ca>,
        Axel Rasmussen <axelrasmussen@google.com>,
        Anshuman Khandual <anshuman.khandual@arm.com>,
        Pasha Tatashin <pasha.tatashin@soleen.com>,
        Miaohe Lin <linmiaohe@huawei.com>,
        Minchan Kim <minchan@kernel.org>,
        Christoph Hellwig <hch@infradead.org>,
        Song Liu <song@kernel.org>,
        Thomas Hellstrom <thomas.hellstrom@linux.intel.com>,
        linux-kernel@vger.kernel.org, linux-mm@kvack.org
Subject: Re: [PATCH 08/31] mm/page_vma_mapped: pte_offset_map_nolock() not
 pte_lockptr()
In-Reply-To: <10e58e7e-a52e-751d-f693-cd4e05ac10ca@linux.dev>
Message-ID: <83ec98e0-d17-6d59-e276-557c13ac58ea@google.com>
References: <68a97fbe-5c1e-7ac6-72c-7b9c6290b370@google.com> <8fa3fb6e-2e39-cbea-c529-ee9e64c7d2d0@google.com> <10e58e7e-a52e-751d-f693-cd4e05ac10ca@linux.dev>
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On Mon, 22 May 2023, Qi Zheng wrote:
> On 2023/5/22 12:58, Hugh Dickins wrote:
> > map_pte() use pte_offset_map_nolock(), to make sure of the ptl belonging
> > to pte, even if pmd entry is then changed racily: page_vma_mapped_walk()
> > use that instead of getting pte_lockptr() later, or restart if map_pte()
> > found no page table.
> > 
> > Signed-off-by: Hugh Dickins <hughd@google.com>
> > ---
> >   mm/page_vma_mapped.c | 28 ++++++++++++++++++++++------
> >   1 file changed, 22 insertions(+), 6 deletions(-)
> > 
> > diff --git a/mm/page_vma_mapped.c b/mm/page_vma_mapped.c
> > index 947dc7491815..2af734274073 100644
> > --- a/mm/page_vma_mapped.c
> > +++ b/mm/page_vma_mapped.c
> > @@ -156,6 +168,7 @@ bool page_vma_mapped_walk(struct page_vma_mapped_walk
> > *pvmw)
> >    struct vm_area_struct *vma = pvmw->vma;
> >    struct mm_struct *mm = vma->vm_mm;
> >    unsigned long end;
> > +	spinlock_t *ptl;
> >    pgd_t *pgd;
> >    p4d_t *p4d;
> >    pud_t *pud;
> > @@ -257,8 +270,11 @@ bool page_vma_mapped_walk(struct page_vma_mapped_walk
> > *pvmw)
> >      step_forward(pvmw, PMD_SIZE);
> >      continue;
> >   		}
> > -		if (!map_pte(pvmw))
> > +		if (!map_pte(pvmw, &ptl)) {
> > +			if (!pvmw->pte)
> > +				goto restart;
> 
> Could pvmw->pmd be changed? Otherwise, how about just jumping to the
> retry label below?
> 
> @@ -205,6 +205,8 @@ bool page_vma_mapped_walk(struct page_vma_mapped_walk
> *pvmw)
>                 }
> 
>                 pvmw->pmd = pmd_offset(pud, pvmw->address);
> +
> +retry:
>                 /*
>                  * Make sure the pmd value isn't cached in a register by the
>                  * compiler and used as a stale value after we've observed a

You're right, that could be done, and that's where I'd have inserted
the label if there were none already.  I just thought the fewer goto
labels the better, so reused the restart already there.  If you feel
strongly that it's actively misleading, I can certainly make that
change; but it's too rare an occurrence to be worth optimizing for.

Hugh