[RFC][PATCH] vfs: check inode size on no_cached_page

[Wu Fengguang <fengguang.wu@intel.com>]

[This patch may not necessarily be merged, but at least we should
 be aware of the problem.]

When user space requests past-EOF data, do_generic_file_read() will
issue a bonus readpage call, which may be unfavorable.

do_generic_file_read:
        -> find_page:
        -> find_get_page()             = NULL
        -> page_cache_sync_readahead()
        -> find_get_page()             = NULL
        -> no_cached_page:
        -> readpage:
                -> nfs_readpage()      = error
        -> readpage_error:

Reported-by: Xu Chenfeng <xcf@ustc.edu.cn>
Signed-off-by: Wu Fengguang <fengguang.wu@intel.com>
---
 mm/filemap.c |    5 +++++
 1 file changed, 5 insertions(+)

--- mm.orig/mm/filemap.c
+++ mm/mm/filemap.c
@@ -1269,6 +1269,11 @@ readpage_error:
 		goto out;
 
 no_cached_page:
+		isize = i_size_read(inode);
+		end_index = (isize - 1) >> PAGE_CACHE_SHIFT;
+		if (unlikely(!isize || index > end_index))
+			goto out;
+
 		/*
 		 * Ok, it wasn't cached, so we need to create a new
 		 * page..