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

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Message-ID: <874jy9aqts.fsf@yhuang6-desk2.ccr.corp.intel.com>
Date:   Fri, 19 Aug 2022 10:51:27 +0800
From:   "Huang, Ying" <ying.huang@...el.com>
To:     Peter Xu <peterx@...hat.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

Peter Xu <peterx@...hat.com> writes:

> 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?

As you have said below, we don't use non-present PTEs and flush present
PTEs before we access the pages.

>> 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.

Yes.  I think so too.

> 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.

I don't think it's a good idea to be redundant.  That may hide the real
issue.

Best Regards,
Huang, Ying