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.
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,