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

[Souptick Joarder <jrdr.linux@gmail.com>]

On Fri, Apr 12, 2019 at 9:34 PM Thomas Hellstrom <thellstrom@vmware.com> 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@linux-foundation.org>
> Cc: Matthew Wilcox <willy@infradead.org>
> Cc: Will Deacon <will.deacon@arm.com>
> Cc: Peter Zijlstra <peterz@infradead.org>
> Cc: Rik van Riel <riel@surriel.com>
> Cc: Minchan Kim <minchan@kernel.org>
> Cc: Michal Hocko <mhocko@suse.com>
> Cc: Huang Ying <ying.huang@intel.com>
> Cc: Souptick Joarder <jrdr.linux@gmail.com>
> Cc: "Jérôme Glisse" <jglisse@redhat.com>
> Cc: linux-mm@kvack.org
> Cc: linux-kernel@vger.kernel.org
>
> Signed-off-by: Thomas Hellstrom <thellstrom@vmware.com>
> ---
>  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 ?

>                 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
>