linux-kernel - Re: [PATCH v2 1/2] mm/migrate_device.c: Copy pte dirty bit to page

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Date:   Tue, 16 Aug 2022 16:35:04 -0400
From:   Peter Xu <peterx@...hat.com>
To:     huang ying <huang.ying.caritas@...il.com>
Cc:     Alistair Popple <apopple@...dia.com>, linux-mm@...ck.org,
        akpm@...ux-foundation.org, linux-kernel@...r.kernel.org,
        "Sierra Guiza, Alejandro (Alex)" <alex.sierra@....com>,
        Felix Kuehling <Felix.Kuehling@....com>,
        Jason Gunthorpe <jgg@...dia.com>,
        John Hubbard <jhubbard@...dia.com>,
        David Hildenbrand <david@...hat.com>,
        Ralph Campbell <rcampbell@...dia.com>,
        Matthew Wilcox <willy@...radead.org>,
        Karol Herbst <kherbst@...hat.com>,
        Lyude Paul <lyude@...hat.com>, Ben Skeggs <bskeggs@...hat.com>,
        Logan Gunthorpe <logang@...tatee.com>, paulus@...abs.org,
        linuxppc-dev@...ts.ozlabs.org, Huang Ying <ying.huang@...el.com>,
        stable@...r.kernel.org
Subject: Re: [PATCH v2 1/2] mm/migrate_device.c: Copy pte dirty bit to page

On Tue, Aug 16, 2022 at 04:10:29PM +0800, huang ying wrote:
> > @@ -193,11 +194,10 @@ static int migrate_vma_collect_pmd(pmd_t *pmdp,
> >                         bool anon_exclusive;
> >                         pte_t swp_pte;
> >
> > +                       flush_cache_page(vma, addr, pte_pfn(*ptep));
> > +                       pte = ptep_clear_flush(vma, addr, ptep);
> 
> Although I think it's possible to batch the TLB flushing just before
> unlocking PTL.  The current code looks correct.

If we're with unconditionally ptep_clear_flush(), does it mean we should
probably drop the "unmapped" and the last flush_tlb_range() already since
they'll be redundant?

If that'll need to be dropped, it looks indeed better to still keep the
batch to me but just move it earlier (before unlock iiuc then it'll be
safe), then we can keep using ptep_get_and_clear() afaiu but keep "pte"
updated.

Thanks,

-- 
Peter Xu