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

[Ralph Campbell <rcampbell@xxxxxxxxxx>]

On 4/12/19 9:04 AM, Thomas Hellstrom 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: "Jérôme Glisse" <jglisse@xxxxxxxxxx>
> Cc: linux-mm@xxxxxxxxx
> Cc: linux-kernel@xxxxxxxxxxxxxxx
>
> Signed-off-by: Thomas Hellstrom <thellstrom@xxxxxxxxxx>

Reviewed-by: Ralph Campbell <rcampbell@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)))

A very minor nit, for consistency elsewhere in mm/memory.c,
could you make this be:
	(VM_FAULT_ERROR | VM_FAULT_NOPAGE | VM_FAULT_RETRY)

>   		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;
>   		}