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

[Peter Xu <peterx@redhat.com>]

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