On 2021-10-01 4:14 p.m., Jason Gunthorpe wrote: > 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. It shouldn't be *that* slow, at least not the vast majority of the time... it seems a bit unreasonable that a CPU wouldn't schedule for more than a second. But these aren't fast paths and synchronize_rcu() already gets called in the unbind path for p2pdma a of couple times. I'm sure it would also be fine to slow down the vma_close() path as well. > 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 Yeah, this is not clear to me either. > My preferences are to either remove devmap from gup_fast, or fix it to > not use special pages - the latter being obviously better. Yeah, I rather expect DAX users want the optimization provided by gup_fast. I don't think P2PDMA users would be happy about being stuck with slow gup either. Loga
