VFS mmap wrong behavior when I/O failure occurs

Related FS:

    EXT2, EXT3, XFS

Related files:
fs/buffer.c mm/filemap.c

Bug description:
Make a partition in USB storage HDD, create a 64M file. Write a program, do: open - mmap - read memory at offset 16M
(from disk) - munmap - close. After each operation, pause for a while, such as 3 seconds. Between mmap and read memory, unplug a USB wire to force a I/O error. The read memory will report no error and get 0x00 back, while the right result is SIGBUS.

Bug analysis:
When accessing a page mmaped, kernel calls
do_no_page->..->filemap_nopage->mapping->a_ops->readpage to read a page from disk. In filemap_nopage(), if !Uptodate(page) (i.e.:
readpage() fails) , it will ClearPageError(page) and try readpage() again. Most FS will call mpage_readpage(). For EXT2/EXT3/XFS, it calls block_read_full_page().

1st readpage()->block_read_full_page():
get_block fails(because of I/O failure),

			if (iblock < lblock) {
				if (get_block(inode, iblock, bh, 0))
					SetPageError(page);
			}
			if (!buffer_mapped(bh)) {
				void *kaddr = kmap_atomic(page, KM_USER0);
				memset(kaddr + i * blocksize, 0, blocksize);
				flush_dcache_page(page);
				kunmap_atomic(kaddr, KM_USER0);
				set_buffer_uptodate(bh); //NOTICE HERE
				continue;
			}
			...

Then ClearPageError(page);

2nd readpage()->block_read_full_page():
for every buffer:

		if (buffer_uptodate(bh))
			continue;

So at the end, the page/buffer is uptodate, no Error set. filemap_nopage will happily return a page memset with 0 without any error!

Way around:
A. do not set buffer uptodate
Or
B. do not call readpage() twice.

Patch:
Since the buffer is memset to 0, no need to set_buffer_uptodate. diff -uNp linux-2.6.11.8-orig/fs/buffer.c linux-2.6.11.8/fs/buffer.c
--- linux-2.6.11.8-orig/fs/buffer.c 2005-05-11 14:41:03.000000000
-0400
+++ linux-2.6.11.8/fs/buffer.c 2005-05-11 14:38:55.000000000 -0400 @@ -2105,7 +2105,6 @@ int block_read_full_page(struct page *pa
memset(kaddr + i * blocksize, 0,

blocksize);
                                flush_dcache_page(page);
                                kunmap_atomic(kaddr, KM_USER0);
-                               set_buffer_uptodate(bh);
                                continue;
                        }
                        /*

----
Best Regards,
Qu Fuping

Your email client wordwrapped the patch. Please fix it.

This patch will break the kernel's regular handling of file holes - !buffer_mapped() means that there was no disk mapping for this buffer: it sits over a hole in the file. Zeroing out the buffer and marking it uptodate is correct behaviour.

You probably want something like this:

@@ -2094,9 +2094,12 @@ int block_read_full_page(struct page *pa
 			continue;
 
 		if (!buffer_mapped(bh)) {
+			int err = 0;
+
 			fully_mapped = 0;
 			if (iblock < lblock) {
-				if (get_block(inode, iblock, bh, 0))
+				err = get_block(inode, iblock, bh, 0);
+				if (err)
 					SetPageError(page);
 			}
 			if (!buffer_mapped(bh)) {

@@ -2104,7 +2107,8 @@ int block_read_full_page(struct page *pa
 				memset(kaddr + i * blocksize, 0, blocksize);
 				flush_dcache_page(page);
 				kunmap_atomic(kaddr, KM_USER0);
-				set_buffer_uptodate(bh);
+				if (!err)
+					set_buffer_uptodate(bh);
 				continue;
 			}
 			/*

_

Yes, you make the point, that is what I really want.

So the final patch will be like above:
P.S. my mail client is a little buggy, i can't handle it correctly :(

diff -uNp linux-2.6.11.8-orig/fs/buffer.c linux-2.6.11.8/fs/buffer.c
@@ -2095,17 +2095,21 @@ int block_read_full_page(struct page *pa
 			continue;
 
 		if (!buffer_mapped(bh)) {
+			int err = 0;
+			
 			fully_mapped = 0;
 			if (iblock < lblock) {
-				if (get_block(inode, iblock, bh, 0))
-					SetPageError(page);
+				err = get_block(inode, iblock, bh, 0)
+					if(err)
+						SetPageError(page);
 			}
 			if (!buffer_mapped(bh)) {
 				void *kaddr = kmap_atomic(page, KM_USER0);
 				memset(kaddr + i * blocksize, 0, blocksize);
 				flush_dcache_page(page);
 				kunmap_atomic(kaddr, KM_USER0);
-				set_buffer_uptodate(bh);
+				if(!err)
+					set_buffer_uptodate(bh);
 				continue;
 			}
 			/*