Re: [PATCH v3 19/20] PCI/P2PDMA: introduce pci_mmap_p2pmem()

[Jason Gunthorpe]

On Fri, Oct 01, 2021 at 02:13:14PM -0600, Logan Gunthorpe wrote:
> 
> 
> On 2021-10-01 11:45 a.m., Jason Gunthorpe wrote:
> >> Before the invalidation, an active flag is cleared to ensure no new
> >> mappings can be created while the unmap is proceeding.
> >> unmap_mapping_range() should sequence itself with the TLB flush and
> > 
> > AFIAK unmap_mapping_range() kicks off the TLB flush and then
> > returns. It doesn't always wait for the flush to fully finish. Ie some
> > cases use RCU to lock the page table against GUP fast and so the
> > put_page() doesn't happen until the call_rcu completes - after a grace
> > period. The unmap_mapping_range() does not wait for grace periods.
> 
> Admittedly, the tlb flush code isn't the easiest code to understand.
> But, yes it seems at least on some arches the pages are freed by
> call_rcu(). But can't this be fixed easily by adding a synchronize_rcu()
> call after calling unmap_mapping_range()? Certainly after a
> synchronize_rcu(), the TLB has been flushed and it is safe to free those
> pages.

It would close this issue, however synchronize_rcu() is very slow
(think > 1second) in some cases and thus cannot be inserted here.

I'm also not completely sure that rcu is the only case, I don't know
how every arch handles its gather structure.. I have a feeling the
general intention was for this to be asynchronous

My preferences are to either remove devmap from gup_fast, or fix it to
not use special pages - the latter being obviously better.

Jason