linux-kernel - Re: [PATCH 08/31] mm/page_vma_mapped: pte_offset_map_nolock() not pte

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Message-ID: <83ec98e0-d17-6d59-e276-557c13ac58ea@google.com>
Date:   Tue, 23 May 2023 19:44:40 -0700 (PDT)
From:   Hugh Dickins <hughd@...gle.com>
To:     Qi Zheng <qi.zheng@...ux.dev>
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>,
        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 08/31] mm/page_vma_mapped: pte_offset_map_nolock() not
 pte_lockptr()

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@...gle.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