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

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Date:   Thu, 18 Aug 2022 10:44:46 -0400
From:   Peter Xu <peterx@...hat.com>
To:     "Huang, Ying" <ying.huang@...el.com>
Cc:     Nadav Amit <nadav.amit@...il.com>,
        Alistair Popple <apopple@...dia.com>,
        huang ying <huang.ying.caritas@...il.com>,
        Linux MM <linux-mm@...ck.org>,
        Andrew Morton <akpm@...ux-foundation.org>,
        LKML <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, stable@...r.kernel.org
Subject: Re: [PATCH v2 1/2] mm/migrate_device.c: Copy pte dirty bit to page

On Thu, Aug 18, 2022 at 02:34:45PM +0800, Huang, Ying wrote:
> > In this specific case, the only way to do safe tlb batching in my mind is:
> >
> > 	pte_offset_map_lock();
> > 	arch_enter_lazy_mmu_mode();
> >         // If any pending tlb, do it now
> >         if (mm_tlb_flush_pending())
> > 		flush_tlb_range(vma, start, end);
> >         else
> >                 flush_tlb_batched_pending();
> 
> I don't think we need the above 4 lines.  Because we will flush TLB
> before we access the pages.

Could you elaborate?

> Can you find any issue if we don't use the above 4 lines?

It seems okay to me to leave stall tlb at least within the scope of this
function. It only collects present ptes and flush propoerly for them.  I
don't quickly see any other implications to other not touched ptes - unlike
e.g. mprotect(), there's a strong barrier of not allowing further write
after mprotect() returns.

Still I don't know whether there'll be any side effect of having stall tlbs
in !present ptes because I'm not familiar enough with the private dev swap
migration code.  But I think having them will be safe, even if redundant.

Thanks,

-- 
Peter Xu