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>

diff --git a/Documentation/filesystems/locking.rst b/Documentation/filesystems/locking.rst
index 7de7a72..aa1a233 100644 (file)
--- a/Documentation/filesystems/locking.rst
+++ b/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

diff --git a/mm/memory.c b/mm/memory.c
index 7716f2c..ec7e89c 100644 (file)
--- a/mm/memory.c
+++ b/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 */