[PATCH mm-unstable] mm/khugepaged: fix collapse_pte_mapped_thp() versus uffd

[Hugh Dickins]

On Tue, 22 Aug 2023, Matthew Wilcox wrote:
> On Tue, Aug 22, 2023 at 11:34:19AM -0700, Hugh Dickins wrote:
> > (Yes, the locking is a bit confusing: but mainly for the unrelated reason,
> > that with the split locking configs, we never quite know whether this lock
> > is the same as that lock or not, and so have to be rather careful.)
> 
> Is it time to remove the PTE split locking config option?  I believe all
> supported architectures have at least two levels of page tables, so if we
> have split ptlocks, ptl and pml are always different from each other (it's
> just that on two level machines, pmd == pud == p4d == pgd).  With huge
> thread counts now being the norm, it's hard to see why anybody would want
> to support SMP and !SPLIT_PTE_PTLOCKS.  To quote the documentation ...
> 
>   Split page table lock for PTE tables is enabled compile-time if
>   CONFIG_SPLIT_PTLOCK_CPUS (usually 4) is less or equal to NR_CPUS.
>   If split lock is disabled, all tables are guarded by mm->page_table_lock.
> 
> You can barely buy a wrist-watch without eight CPUs these days.

Whilst I'm still happy with my 0-CPU wrist-watch, I do think you're right:
that SPLIT_PTLOCK_CPUS business was really just a safety-valve for when
introducing split ptlock in the first place, 4 pulled out of a hat, and
the unsplit ptlock path quite under-tested.

But I'll leave it to someone else do the job of removing it whenever.

Hugh