On Tue, Jan 05, 2021 at 01:03:48PM -0500, Andrea Arcangeli wrote:

On Tue, Jan 05, 2021 at 04:37:27PM +0100, Peter Zijlstra wrote:

quoted

(your other email clarified this point; the COW needs to copy while
holding the PTL and we need TLBI under PTL if we're to change this)

The COW doesn't need to hold the PT lock, the TLBI broadcast doesn't
need to be delivered under PT lock either.

Simply there need to be a TLBI broadcast before the copy. The patch I
sent here https://lkml.kernel.org/r/X+QLr1WmGXMs33Ld@redhat.com that
needs to be cleaned up with some abstraction and better commentary
also misses a smp_mb() in the case flush_tlb_page is not called, but
that's a small detail.

That's horrific crap. All of that tlb-pending stuff is batshit, and this
makes it worse.

quoted

And I'm thinking the speculative page fault series steps right into all
this, it fundamentally avoids mmap_sem and entirely relies on the PTL.

I thought about that but that only applies to some kind of "anon" page
fault.

That must be something new; it used to handle all faults. I specifically
spend quite a bit of time getting the file crud right (which Linus
initially fingered for being horrible broken).

SPF fundamentally elides the mmap_sem, which Linus said must serialize
faults.

Here the problem isn't just the page fault, the problem is not to
regress clear_refs to block on page fault I/O, and all

IIRC we do the actual reads without any locks held, just like
VM_FAULT_RETRY does today. You take the fault, find you need IO, drop
locks, do IO, retake fault.

MAP_PRIVATE/MAP_SHARED filebacked faults bitting the disk to read
/usr/ will still prevent clear_refs from running (and the other way
around) if it has to take the mmap_sem for writing.

I don't look at the speculative page fault for a while but last I
checked there was nothing there that can tame the above major
regression from CPU speed to disk I/O speed that would be inflicted on
both clear_refs on huge mm and on uffd-wp.

All of the clear_refs nonsense is immaterial to SPF. Also, who again
cares about clear_refs? Why is it important?