netfs: Fix netfs_page_mkwrite() to check folio->mapping is valid

Fix netfs_page_mkwrite() to check that folio->mapping is valid once it has
taken the folio lock (as filemap_page_mkwrite() does).  Without this,
generic/247 occasionally oopses with something like the following:

    BUG: kernel NULL pointer dereference, address: 0000000000000000
    #PF: supervisor read access in kernel mode
    #PF: error_code(0x0000) - not-present page

    RIP: 0010:trace_event_raw_event_netfs_folio+0x61/0xc0
    ...
    Call Trace:
     <TASK>
     ? __die_body+0x1a/0x60
     ? page_fault_oops+0x6e/0xa0
     ? exc_page_fault+0xc2/0xe0
     ? asm_exc_page_fault+0x22/0x30
     ? trace_event_raw_event_netfs_folio+0x61/0xc0
     trace_netfs_folio+0x39/0x40
     netfs_page_mkwrite+0x14c/0x1d0
     do_page_mkwrite+0x50/0x90
     do_pte_missing+0x184/0x200
     __handle_mm_fault+0x42d/0x500
     handle_mm_fault+0x121/0x1f0
     do_user_addr_fault+0x23e/0x3c0
     exc_page_fault+0xc2/0xe0
     asm_exc_page_fault+0x22/0x30

This is due to the invalidate_inode_pages2_range() issued at the end of the
DIO write interfering with the mmap'd writes.

Fixes: 102a7e2 ("netfs: Allow buffered shared-writeable mmap through netfs_page_mkwrite()")
Signed-off-by: David Howells <dhowells@redhat.com>
Link: https://lore.kernel.org/r/780211.1719318546@warthog.procyon.org.uk
Reviewed-by: Jeff Layton <jlayton@kernel.org>
cc: Matthew Wilcox <willy@infradead.org>
cc: Jeff Layton <jlayton@kernel.org>
cc: netfs@lists.linux.dev
cc: v9fs@lists.linux.dev
cc: linux-afs@lists.infradead.org
cc: linux-cifs@vger.kernel.org
cc: linux-mm@kvack.org
cc: linux-fsdevel@vger.kernel.org
Signed-off-by: Christian Brauner <brauner@kernel.org>

Author: dhowells

fs/netfs/buffered_write.c

@@ -523,6 +523,7 @@ vm_fault_t netfs_page_mkwrite(struct vm_fault *vmf, struct netfs_group *netfs_gr
 	struct netfs_group *group;
 	struct folio *folio = page_folio(vmf->page);
 	struct file *file = vmf->vma->vm_file;
+	struct address_space *mapping = file->f_mapping;
 	struct inode *inode = file_inode(file);
 	struct netfs_inode *ictx = netfs_inode(inode);
 	vm_fault_t ret = VM_FAULT_RETRY;
@@ -534,6 +535,11 @@ vm_fault_t netfs_page_mkwrite(struct vm_fault *vmf, struct netfs_group *netfs_gr
 
 	if (folio_lock_killable(folio) < 0)
 		goto out;
+	if (folio->mapping != mapping) {
+		folio_unlock(folio);
+		ret = VM_FAULT_NOPAGE;
+		goto out;
+	}
 
 	if (folio_wait_writeback_killable(folio)) {
 		ret = VM_FAULT_LOCKED;
@@ -549,7 +555,7 @@ vm_fault_t netfs_page_mkwrite(struct vm_fault *vmf, struct netfs_group *netfs_gr
 	group = netfs_folio_group(folio);
 	if (group != netfs_group && group != NETFS_FOLIO_COPY_TO_CACHE) {
 		folio_unlock(folio);
-		err = filemap_fdatawait_range(inode->i_mapping,
+		err = filemap_fdatawait_range(mapping,
 					      folio_pos(folio),
 					      folio_pos(folio) + folio_size(folio));
 		switch (err) {