Re: [PATCH 1/9] mm: Allow the [page|pfn]_mkwrite callbacks to drop the mmap_sem

[Souptick Joarder]

On Wed, Apr 17, 2019 at 4:28 PM Thomas Hellstrom <thellstrom@xxxxxxxxxx> wrote:
>
> Hi, Souptick,
>
> On Sat, 2019-04-13 at 20:41 +0530, Souptick Joarder wrote:
> > On Fri, Apr 12, 2019 at 9:34 PM Thomas Hellstrom <
> > thellstrom@xxxxxxxxxx> wrote:
> > > Driver fault callbacks are allowed to drop the mmap_sem when
> > > expecting
> > > long hardware waits to avoid blocking other mm users. Allow the
> > > mkwrite
> > > callbacks to do the same by returning early on VM_FAULT_RETRY.
> > >
> > > In particular we want to be able to drop the mmap_sem when waiting
> > > for
> > > a reservation object lock on a GPU buffer object. These locks may
> > > be
> > > held while waiting for the GPU.
> > >
> > > Cc: Andrew Morton <akpm@xxxxxxxxxxxxxxxxxxxx>
> > > Cc: Matthew Wilcox <willy@xxxxxxxxxxxxx>
> > > Cc: Will Deacon <will.deacon@xxxxxxx>
> > > Cc: Peter Zijlstra <peterz@xxxxxxxxxxxxx>
> > > Cc: Rik van Riel <riel@xxxxxxxxxxx>
> > > Cc: Minchan Kim <minchan@xxxxxxxxxx>
> > > Cc: Michal Hocko <mhocko@xxxxxxxx>
> > > Cc: Huang Ying <ying.huang@xxxxxxxxx>
> > > Cc: Souptick Joarder <jrdr.linux@xxxxxxxxx>
> > > Cc: "JÃrÃme Glisse" <jglisse@xxxxxxxxxx>
> > > Cc: linux-mm@xxxxxxxxx
> > > Cc: linux-kernel@xxxxxxxxxxxxxxx
> > >
> > > Signed-off-by: Thomas Hellstrom <thellstrom@xxxxxxxxxx>
> > > ---
> > >  mm/memory.c | 10 ++++++----
> > >  1 file changed, 6 insertions(+), 4 deletions(-)
> > >
> > > diff --git a/mm/memory.c b/mm/memory.c
> > > index e11ca9dd823f..a95b4a3b1ae2 100644
> > > --- a/mm/memory.c
> > > +++ b/mm/memory.c
> > > @@ -2144,7 +2144,7 @@ static vm_fault_t do_page_mkwrite(struct
> > > vm_fault *vmf)
> > >         ret = vmf->vma->vm_ops->page_mkwrite(vmf);
> > >         /* Restore original flags so that caller is not surprised
> > > */
> > >         vmf->flags = old_flags;
> > > -       if (unlikely(ret & (VM_FAULT_ERROR | VM_FAULT_NOPAGE)))
> > > +       if (unlikely(ret & (VM_FAULT_ERROR | VM_FAULT_RETRY |
> > > VM_FAULT_NOPAGE)))
> >
> > With this patch there will multiple instances of (VM_FAULT_ERROR |
> > VM_FAULT_RETRY | VM_FAULT_NOPAGE)
> > in mm/memory.c. Does it make sense to wrap it in a macro and use it ?
>
> Even though the code will look neater, it might be trickier to follow a
> particular error path. Could we perhaps postpone to a follow-up patch?

Sure. follow-up-patch is fine.

>
> Thomas
>
>
>
> >
> > >                 return ret;
> > >         if (unlikely(!(ret & VM_FAULT_LOCKED))) {
> > >                 lock_page(page);
> > > @@ -2419,7 +2419,7 @@ static vm_fault_t wp_pfn_shared(struct
> > > vm_fault *vmf)
> > >                 pte_unmap_unlock(vmf->pte, vmf->ptl);
> > >                 vmf->flags |= FAULT_FLAG_MKWRITE;
> > >                 ret = vma->vm_ops->pfn_mkwrite(vmf);
> > > -               if (ret & (VM_FAULT_ERROR | VM_FAULT_NOPAGE))
> > > +               if (ret & (VM_FAULT_ERROR | VM_FAULT_RETRY |
> > > VM_FAULT_NOPAGE))
> > >                         return ret;
> > >                 return finish_mkwrite_fault(vmf);
> > >         }
> > > @@ -2440,7 +2440,8 @@ static vm_fault_t wp_page_shared(struct
> > > vm_fault *vmf)
> > >                 pte_unmap_unlock(vmf->pte, vmf->ptl);
> > >                 tmp = do_page_mkwrite(vmf);
> > >                 if (unlikely(!tmp || (tmp &
> > > -                                     (VM_FAULT_ERROR |
> > > VM_FAULT_NOPAGE)))) {
> > > +                                     (VM_FAULT_ERROR |
> > > VM_FAULT_RETRY |
> > > +                                      VM_FAULT_NOPAGE)))) {
> > >                         put_page(vmf->page);
> > >                         return tmp;
> > >                 }
> > > @@ -3494,7 +3495,8 @@ static vm_fault_t do_shared_fault(struct
> > > vm_fault *vmf)
> > >                 unlock_page(vmf->page);
> > >                 tmp = do_page_mkwrite(vmf);
> > >                 if (unlikely(!tmp ||
> > > -                               (tmp & (VM_FAULT_ERROR |
> > > VM_FAULT_NOPAGE)))) {
> > > +                               (tmp & (VM_FAULT_ERROR |
> > > VM_FAULT_RETRY |
> > > +                                       VM_FAULT_NOPAGE)))) {
> > >                         put_page(vmf->page);
> > >                         return tmp;
> > >                 }
> > > --
> > > 2.20.1
> > >