From: Chris Mason <mason@suse.com>
When a block device is accessed via read/write, it is possible for some of
the buffers on a page to be mapped and others not. __getblk and friends
assume this can't happen, and can end up looping forever when pages have
some unmapped buffers. Picture:
lseek(/dev/xxx, 2048, SEEK_SET)
write(/dev/xxx, 2048 bytes)
Assuming the block size is 1k, page 0 has 4 buffers, two are mapped by
__block_prepare_write and two are not. Next, another process triggers
getblk(/dev/xxx, blocknr = 0);
__getblk_slow will loop forever. __find_get_block fails because the buffer
isn't mapped. grow_dev_page does nothing because there are buffers on the
page with the correct size. madhav@veritas.com and others at Veritas
tracked this down.
The fix below has two parts. First, it changes __find_get_block to avoid
the buffer_error warnings when it finds unmapped buffers on the page.
Second, it changes grow_dev_page to map the buffers on the page by calling
init_page_buffers. init_page_buffers is changed so we don't stomp on
uptodate bits for the buffers.
Signed-off-by: Andrew Morton <akpm@osdl.org>
---
25-akpm/fs/buffer.c | 36 +++++++++++++++++++++++-------------
1 files changed, 23 insertions(+), 13 deletions(-)
diff -puN fs/buffer.c~__getblk_slow-can-loop-forever-when-pages-are-partially fs/buffer.c
--- 25/fs/buffer.c~__getblk_slow-can-loop-forever-when-pages-are-partially Fri Dec 10 15:19:13 2004
+++ 25-akpm/fs/buffer.c Fri Dec 10 15:19:13 2004
@@ -426,6 +426,7 @@ __find_get_block_slow(struct block_devic
struct buffer_head *bh;
struct buffer_head *head;
struct page *page;
+ int all_mapped = 1;
index = block >> (PAGE_CACHE_SHIFT - bd_inode->i_blkbits);
page = find_get_page(bd_mapping, index);
@@ -443,14 +444,23 @@ __find_get_block_slow(struct block_devic
get_bh(bh);
goto out_unlock;
}
+ if (!buffer_mapped(bh))
+ all_mapped = 0;
bh = bh->b_this_page;
} while (bh != head);
- printk("__find_get_block_slow() failed. "
- "block=%llu, b_blocknr=%llu\n",
- (unsigned long long)block, (unsigned long long)bh->b_blocknr);
- printk("b_state=0x%08lx, b_size=%u\n", bh->b_state, bh->b_size);
- printk("device blocksize: %d\n", 1 << bd_inode->i_blkbits);
+ /* we might be here because some of the buffers on this page are
+ * not mapped. This is due to various races between
+ * file io on the block device and getblk. It gets dealt with
+ * elsewhere, don't buffer_error if we had some unmapped buffers
+ */
+ if (all_mapped) {
+ printk("__find_get_block_slow() failed. "
+ "block=%llu, b_blocknr=%llu\n",
+ (unsigned long long)block, (unsigned long long)bh->b_blocknr);
+ printk("b_state=0x%08lx, b_size=%u\n", bh->b_state, bh->b_size);
+ printk("device blocksize: %d\n", 1 << bd_inode->i_blkbits);
+ }
out_unlock:
spin_unlock(&bd_mapping->private_lock);
page_cache_release(page);
@@ -1098,18 +1108,16 @@ init_page_buffers(struct page *page, str
{
struct buffer_head *head = page_buffers(page);
struct buffer_head *bh = head;
- unsigned int b_state;
-
- b_state = 1 << BH_Mapped;
- if (PageUptodate(page))
- b_state |= 1 << BH_Uptodate;
+ int uptodate = PageUptodate(page);
do {
- if (!(bh->b_state & (1 << BH_Mapped))) {
+ if (!buffer_mapped(bh)) {
init_buffer(bh, NULL, NULL);
bh->b_bdev = bdev;
bh->b_blocknr = block;
- bh->b_state = b_state;
+ if (uptodate)
+ set_buffer_uptodate(bh);
+ set_buffer_mapped(bh);
}
block++;
bh = bh->b_this_page;
@@ -1138,8 +1146,10 @@ grow_dev_page(struct block_device *bdev,
if (page_has_buffers(page)) {
bh = page_buffers(page);
- if (bh->b_size == size)
+ if (bh->b_size == size) {
+ init_page_buffers(page, bdev, block, size);
return page;
+ }
if (!try_to_free_buffers(page))
goto failed;
}
_