Re: [PATCH v10 07/10] mm: Device exclusive memory access

On Thu, Jun 10, 2021 at 10:18:25AM +1000, Alistair Popple wrote:

quoted

quoted

The main problem is split_huge_pmd_address() unconditionally calls a mmu
notifier so I would need to plumb in passing an owner everywhere which could
get messy.

Could I ask why?  split_huge_pmd_address() will notify with CLEAR, so I'm a bit
confused why we need to pass over the owner.

Sure, it is the same reason we need to pass it for the exclusive notifier.
Any invalidation during the make exclusive operation will break the mmu read
side critical section forcing a retry of the operation. The owner field is what
is used to filter out invalidations (such as the exclusive invalidation) that
don't need to be retried.

Do you mean the mmu_interval_read_begin|retry() calls?

Hmm, the thing is.. to me FOLL_SPLIT_PMD should have similar effect to explicit
call split_huge_pmd_address(), afaict.  Since both of them use __split_huge_pmd()
internally which will generate that unwanted CLEAR notify.

If that's the case, I think it fails because split_huge_pmd_address() will
trigger that CLEAR notify unconditionally (even if it's not a thp; not sure
whether it should be optimized to not notify at all... definitely another
story), while FOLL_SPLIT_PMD will skip the notify as it calls split_huge_pmd()
instead, who checks the pmd before calling __split_huge_pmd().

Does it also mean that if there's a real THP it won't really work?  As then
FOLL_SPLIT_PMD will start to trigger that CLEAR notify too, I think..

-- 
Peter Xu