On Tue, Aug 01, 2023 at 02:26:03PM +0200, David Hildenbrand wrote: > On 01.08.23 14:23, Jason Gunthorpe wrote: > > On Tue, Aug 01, 2023 at 02:22:12PM +0200, David Hildenbrand wrote: > > > On 01.08.23 14:19, Jason Gunthorpe wrote: > > > > On Tue, Aug 01, 2023 at 05:32:38AM +0000, Kasireddy, Vivek wrote: > > > > > > > > > > You get another invalidate because the memfd removes the zero pages > > > > > > that hmm_range_fault installed in the PTEs before replacing them with > > > > > > actual writable pages. Then you do the move, and another > > > > > > hmm_range_fault, and basically the whole thing over again. Except this > > > > > > time instead of returning zero pages it returns actual writable > > > > > > page. > > > > > > > > > Ok, when I tested earlier (by registering an invalidate callback) but without > > > > > hmm_range_fault(), I did not find this additional invalidate getting triggered. > > > > > Let me try with hmm_range_fault() and see if everything works as expected. > > > > > Thank you for your help. > > > > > > > > If you do not get an invalidate then there is a pretty serious bug in > > > > the mm that needs fixing. > > > > > > > > Anything hmm_range_fault() returns must be invalidated if the > > > > underying CPU mapping changes for any reasons. Since hmm_range_fault() > > > > will populate zero pages when reading from a hole in a memfd, it must > > > > also get an invalidation when the zero pages are changed into writable > > > > pages. > > > > > > Can you point me at the code that returns that (shared) zero page? > > > > It calls handle_mm_fault() - shouldn't that do it? Same as if the CPU > > read faulted the page? > > To the best of my knowledge, the shared zeropage is only used in > MAP_PRIVATE|MAP_AON mappings and in weird DAX mappings. > > If that changed, we have to fix FOLL_PIN|FOLL_LONGTERM for MAP_SHARED VMAs. > > If you read-fault on a memfd hole, you should get a proper "zeroed" > pagecache page that effectively "filled that hole" -- so there is no file > hole anymore. Sounds fine then :) Jason
