On Tue, Jan 17, 2023 at 6:44 PM Matthew Wilcox <willy@xxxxxxxxxxxxx> wrote: > > On Tue, Jan 17, 2023 at 05:06:57PM -0800, Suren Baghdasaryan wrote: > > On Tue, Jan 17, 2023 at 7:47 AM Michal Hocko <mhocko@xxxxxxxx> wrote: > > > > > > On Mon 09-01-23 12:53:23, Suren Baghdasaryan wrote: > > > > Introduce lock_vma_under_rcu function to lookup and lock a VMA during > > > > page fault handling. When VMA is not found, can't be locked or changes > > > > after being locked, the function returns NULL. The lookup is performed > > > > under RCU protection to prevent the found VMA from being destroyed before > > > > the VMA lock is acquired. VMA lock statistics are updated according to > > > > the results. > > > > For now only anonymous VMAs can be searched this way. In other cases the > > > > function returns NULL. > > > > > > Could you describe why only anonymous vmas are handled at this stage and > > > what (roughly) has to be done to support other vmas? lock_vma_under_rcu > > > doesn't seem to have any anonymous vma specific requirements AFAICS. > > > > TBH I haven't spent too much time looking into file-backed page faults > > yet but a couple of tasks I can think of are: > > - Ensure that all vma->vm_ops->fault() handlers do not rely on > > mmap_lock being read-locked; > > I think this way lies madness. There are just too many device drivers > that implement ->fault. My plan is to call the ->map_pages() method > under RCU without even read-locking the VMA. If that doesn't satisfy > the fault, then drop all the way back to taking the mmap_sem for read > before calling into ->fault. Sounds reasonable to me but I guess the devil is in the details... >
