Re: [patch|rfc] block: don't mark buffers beyond end of disk as mapped

[Jeff Moyer]

Jeff Moyer <jmoyer@xxxxxxxxxx> writes:

> Nick Piggin <npiggin@xxxxxxxxx> writes:
>
>> On 2 May 2012 00:08, Jeff Moyer <jmoyer@xxxxxxxxxx> wrote:
>>> Nick Piggin <npiggin@xxxxxxxxx> writes:
>>>
>>>> Not a bad fix. But it's kind of sad to have i_size checking logic also in
>>>> block_read_full_page, that does not cope with this.
>>>>
>>>> I have found there are parts of the kernel (readahead) that try to read
>>>> beyond EOF and seem to get angry if we return an error (by not
>>>> marking uptodate in readpage) in that case though :(
>>>>
>>>> But, either way, I think it's very reasonable to not mark buffers beyond
>>>> end of device as mapped. So I think your patch is fine.
>>>>
>>>> I guess for ext[234], it does not read metadata close to the end of the
>>>> device or you were using 4K sized blocks?
>>>
>>> Well, the test case just reads directly from the loop device, bypassing
>>> the file system, and I did use 1KB blocks when making the file system, so
>>> it is quite puzzling.
>>
>> It's because buffer_head creation does not go through the same paths
>> for bdev file access versus getblk APIs.
>>
>> blkdev_get_block does the right thing there
>>
>> In fact, it's probably good to unify the checks here, i.e., use max_blocks()
>
> You really think it's worth it?  I mean, it's just an i_size_read and a
> shift, and there is precedent for it inside fs/buffer.c.  I'd prefer to
> keep the patch as-is, but will change it if you feel that strongly about
> it.

Anyway, here is the other version of the patch, using max_block as you
suggested.

Cheers,
Jeff

Signed-off-by: Jeff Moyer <jmoyer@xxxxxxxxxx>

diff --git a/fs/block_dev.c b/fs/block_dev.c
index e08f6a2..ba11c30 100644
--- a/fs/block_dev.c
+++ b/fs/block_dev.c
@@ -70,7 +70,7 @@ static void bdev_inode_switch_bdi(struct inode *inode,
 	spin_unlock(&dst->wb.list_lock);
 }
 
-static sector_t max_block(struct block_device *bdev)
+sector_t blkdev_max_block(struct block_device *bdev)
 {
 	sector_t retval = ~((sector_t)0);
 	loff_t sz = i_size_read(bdev->bd_inode);
@@ -163,7 +163,7 @@ static int
 blkdev_get_block(struct inode *inode, sector_t iblock,
 		struct buffer_head *bh, int create)
 {
-	if (iblock >= max_block(I_BDEV(inode))) {
+	if (iblock >= blkdev_max_block(I_BDEV(inode))) {
 		if (create)
 			return -EIO;
 
@@ -185,7 +185,7 @@ static int
 blkdev_get_blocks(struct inode *inode, sector_t iblock,
 		struct buffer_head *bh, int create)
 {
-	sector_t end_block = max_block(I_BDEV(inode));
+	sector_t end_block = blkdev_max_block(I_BDEV(inode));
 	unsigned long max_blocks = bh->b_size >> inode->i_blkbits;
 
 	if ((iblock + max_blocks) > end_block) {
diff --git a/fs/buffer.c b/fs/buffer.c
index 351e18e..ad5938c 100644
--- a/fs/buffer.c
+++ b/fs/buffer.c
@@ -921,6 +921,7 @@ init_page_buffers(struct page *page, struct block_device *bdev,
 	struct buffer_head *head = page_buffers(page);
 	struct buffer_head *bh = head;
 	int uptodate = PageUptodate(page);
+	sector_t end_block = blkdev_max_block(I_BDEV(bdev->bd_inode));
 
 	do {
 		if (!buffer_mapped(bh)) {
@@ -929,7 +930,8 @@ init_page_buffers(struct page *page, struct block_device *bdev,
 			bh->b_blocknr = block;
 			if (uptodate)
 				set_buffer_uptodate(bh);
-			set_buffer_mapped(bh);
+			if (block < end_block)
+				set_buffer_mapped(bh);
 		}
 		block++;
 		bh = bh->b_this_page;
diff --git a/include/linux/fs.h b/include/linux/fs.h
index 8de6755..25c40b9 100644
--- a/include/linux/fs.h
+++ b/include/linux/fs.h
@@ -2051,6 +2051,7 @@ extern void unregister_blkdev(unsigned int, const char *);
 extern struct block_device *bdget(dev_t);
 extern struct block_device *bdgrab(struct block_device *bdev);
 extern void bd_set_size(struct block_device *, loff_t size);
+extern sector_t blkdev_max_block(struct block_device *bdev);
 extern void bd_forget(struct inode *inode);
 extern void bdput(struct block_device *);
 extern void invalidate_bdev(struct block_device *);
--
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