Re: [PATCH 28/41] mm: introduce lock_vma_under_rcu to be used from arch-specific code

[Matthew Wilcox]

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.