Peter Xu <peterx@xxxxxxxxxx> writes:
> 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.
Thanks, I would have been completely lost about what you were talking
about without this :-)
> 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.
In the above scenario migrate_vma_collect_pmd() will observe pte_none.
This will mark the src_pfn[] array as needing a new zero page which will
be installed by migrate_vma_pages()->migrate_vma_insert_page().
So there is no data to be copied hence there can't be any data
corruption. Remember these are private anonymous pages, so any
zap_pte_range() indicates the data is no longer needed (eg.
MADV_DONTNEED).
>>
>> 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?
That comment is out dated. We used to try locking the page again but
that was removed by ab09243aa95a ("mm/migrate.c: remove
MIGRATE_PFN_LOCKED"). See
https://lkml.kernel.org/r/20211025041608.289017-1-apopple@xxxxxxxxxx
Will post a clean-up for it.
>> 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
