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

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Date:   Wed, 24 Aug 2022 16:48:38 -0400
From:   Peter Xu <peterx@...hat.com>
To:     Alistair Popple <apopple@...dia.com>
Cc:     "Huang, Ying" <ying.huang@...el.com>,
        Nadav Amit <nadav.amit@...il.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 Wed, Aug 24, 2022 at 04:25:44PM -0400, Peter Xu wrote:
> On Wed, Aug 24, 2022 at 11:56:25AM +1000, Alistair Popple wrote:
> > >> 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.
> > 
> > What side-effect were you thinking of? I don't see any issue with not
> > TLB flushing stale device-private TLBs prior to the migration because
> > they're not accessible anyway and shouldn't be in any TLB.
> 
> Sorry to be misleading, I never meant we must add them.  As I said it's
> just that I don't know the code well so I don't know whether it's safe to
> not have it.
> 
> IIUC it's about whether having stall system-ram stall tlb in other
> processor would matter or not here.  E.g. some none pte that this code
> collected (boosted both "cpages" and "npages" for a none pte) could have
> stall tlb in other cores that makes the page writable there.

For this one, let me give a more detailed example.

It's about whether below could happen:

       thread 1                thread 2                 thread 3
       --------                --------                 --------
                          write to page P (data=P1)
                            (cached TLB writable)
  zap_pte_range()
    pgtable lock
    clear pte for page P
    pgtable unlock
    ...
                                                     migrate_vma_collect
                                                       pte none, npages++, cpages++
                                                       allocate device page
                                                       copy data (with P1)
                                                       map pte as device swap 
                          write to page P again
                          (data updated from P1->P2)
  flush tlb

Then at last from processor side P should have data P2 but actually from
device memory it's P1. Data corrupt.

> 
> When I said I'm not familiar with the code, it's majorly about one thing I
> never figured out myself, in that migrate_vma_collect_pmd() has this
> optimization to trylock on the page, collect if it succeeded:
> 
>   /*
>    * Optimize for the common case where page is only mapped once
>    * in one process. If we can lock the page, then we can safely
>    * set up a special migration page table entry now.
>    */
>    if (trylock_page(page)) {
>           ...
>    } else {
>           put_page(page);
>           mpfn = 0;
>    }
> 
> But it's kind of against a pure "optimization" in that if trylock failed,
> we'll clear the mpfn so the src[i] will be zero at last.  Then will we
> directly give up on this page, or will we try to lock_page() again
> somewhere?
> 
> The future unmap op is also based on this "cpages", not "npages":
> 
> 	if (args->cpages)
> 		migrate_vma_unmap(args);
> 
> So I never figured out how this code really works.  It'll be great if you
> could shed some light to it.
> 
> Thanks,
> 
> -- 
> Peter Xu

-- 
Peter Xu