On 8/31/22 12:43, Yang Shi wrote: >>> It looks powerpc does issue IPI for pmd flush. But arm64 doesn't IIRC. >>> >>> So maybe we should implement pmdp_collapse_flush() for those arches to >>> issue IPI. >> >> ... or find another way to detect and handle this in GUP-fast? >> >> Not sure if, for handling PMDs, it could be sufficient to propagate the >> pmdp pointer + value and double check that the values didn't change. > > Should work too, right before pinning the page. > > pmdp_collapse_flush() is actually just called by khugepaged, so arch > specific implementation should not be a problem and we avoid making > gup fast more complicated. > And just to pile on, about that gup fast complexity: depending upon IPIs added a lot of complexity, not just because of the IPI dependency, but more importantly because only some arches even *have* IPIs. So an entirely different set of reasoning has to be used *in addition* to working through the IPI story. And sure enough, we can see the fallout: you are uncovering lots of half-correct comments in that area. So getting rid of the dependency on IPIs in gup fast would go a long way to simplifying it, and maybe even improving overall CPU load (insert some hand-wavy notes here about IPIs being worse than things like RCU). But the real win is in the complexity reduction in gup fast. thanks, -- John Hubbard NVIDIA
