mm: Allow the [page|pfn]_mkwrite callbacks to drop the mmap_sem

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>
Reviewed-by: Ralph Campbell <rcampbell@nvidia.com>

mm/memory.c

@@ -2238,7 +2238,7 @@ 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_NOPAGE | VM_FAULT_RETRY)))
 		return ret;
 	if (unlikely(!(ret & VM_FAULT_LOCKED))) {
 		lock_page(page);
@@ -2515,7 +2515,7 @@ 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_NOPAGE | VM_FAULT_RETRY))
 			return ret;
 		return finish_mkwrite_fault(vmf);
 	}
@@ -2536,7 +2536,8 @@ 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_NOPAGE |
+				       VM_FAULT_RETRY)))) {
 			put_page(vmf->page);
 			return tmp;
 		}
@@ -3601,7 +3602,8 @@ 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_NOPAGE |
+					VM_FAULT_RETRY)))) {
 			put_page(vmf->page);
 			return tmp;
 		}