mm: hold the RCU read lock over calls to ->map_pages

Prevent filesystems from doing things which sleep in their map_pages
method.  This is in preparation for a pagefault path protected only by
RCU.

Link: https://lkml.kernel.org/r/20230327174515.1811532-4-willy@infradead.org
Signed-off-by: Matthew Wilcox (Oracle) <willy@infradead.org>
Cc: Darrick J. Wong <djwong@kernel.org>
Cc: Dave Chinner <david@fromorbit.com>
Cc: David Howells <dhowells@redhat.com>
Signed-off-by: Andrew Morton <akpm@linux-foundation.org>

Documentation/filesystems/locking.rst

@@ -645,7 +645,7 @@ ops		mmap_lock	PageLocked(page)
 open:		yes
 close:		yes
 fault:		yes		can return with page locked
-map_pages:	yes
+map_pages:	read
 page_mkwrite:	yes		can return with page locked
 pfn_mkwrite:	yes
 access:		yes
@@ -661,7 +661,7 @@ locked. The VM will unlock the page.
 
 ->map_pages() is called when VM asks to map easy accessible pages.
 Filesystem should find and map pages associated with offsets from "start_pgoff"
-till "end_pgoff". ->map_pages() is called with page table locked and must
+till "end_pgoff". ->map_pages() is called with the RCU lock held and must
 not block.  If it's not possible to reach a page without blocking,
 filesystem should skip it. Filesystem should use do_set_pte() to setup
 page table entry. Pointer to entry associated with the page is passed in

mm/memory.c

@@ -4450,6 +4450,7 @@ static vm_fault_t do_fault_around(struct vm_fault *vmf)
 	/* The page offset of vmf->address within the VMA. */
 	pgoff_t vma_off = vmf->pgoff - vmf->vma->vm_pgoff;
 	pgoff_t from_pte, to_pte;
+	vm_fault_t ret;
 
 	/* The PTE offset of the start address, clamped to the VMA. */
 	from_pte = max(ALIGN_DOWN(pte_off, nr_pages),
@@ -4465,9 +4466,13 @@ static vm_fault_t do_fault_around(struct vm_fault *vmf)
 			return VM_FAULT_OOM;
 	}
 
-	return vmf->vma->vm_ops->map_pages(vmf,
-		vmf->pgoff + from_pte - pte_off,
-		vmf->pgoff + to_pte - pte_off);
+	rcu_read_lock();
+	ret = vmf->vma->vm_ops->map_pages(vmf,
+			vmf->pgoff + from_pte - pte_off,
+			vmf->pgoff + to_pte - pte_off);
+	rcu_read_unlock();
+
+	return ret;
 }
 
 /* Return true if we should do read fault-around, false otherwise */