Re: [PATCH v2 0/9] fix max discard sectors limit

[James Bottomley <James.Bottomley@HansenPartnership.com>]

On Sat, 2013-04-20 at 01:40 +0900, Namjae Jeon wrote:
> From: Namjae Jeon <namjae.jeon@samsung.com>
> 
> linux-v3.8-rc1 and later support for plug for blkdev_issue_discard with
> commit 0cfbcafcae8b7364b5fa96c2b26ccde7a3a296a9 
> (block: add plug for blkdev_issue_discard )
> 
> For example,
> 1) DISCARD rq-1 with size size 4GB
> 2) DISCARD rq-2 with size size 1GB
> 
> If these 2 discard requests get merged, final request size will be 5GB.
> 
> In this case, request's __data_len field may overflow as it can store
> max 4GB(unsigned int).
> 
> This issue was observed while doing mkfs.f2fs on 5GB SD card:
> https://lkml.org/lkml/2013/4/1/292
> 
> # mkfs.f2fs /dev/mmcblk0p3
> Info: sector size = 512
> Info: total sectors = 11370496 (in 512bytes)
> Info: zone aligned segment0 blkaddr: 512
> [  257.789764] blk_update_request: bio idx 0 >= vcnt 0
> 
> mkfs process gets stuck in D state and I see the following in the dmesg:
> 
> [  257.789733] __end_that: dev mmcblk0: type=1, flags=122c8081
> [  257.789764]   sector 4194304, nr/cnr 2981888/4294959104
> [  257.789764]   bio df3840c0, biotail df3848c0, buffer   (null), len 1526726656
> [  257.789764] blk_update_request: bio idx 0 >= vcnt 0
> [  257.794921] request botched: dev mmcblk0: type=1, flags=122c8081
> [  257.794921]   sector 4194304, nr/cnr 2981888/4294959104
> [  257.794921]   bio df3840c0, biotail df3848c0, buffer   (null), len 1526726656
> 
> Few drivers(e.g. mmc, mtd..) set q->limits.max_discard_sectors
> more than UINT_MAX >> 9 sectors which is incorrect and it may lead to overflow
> of request's __data_len field if merged discard request's size exceeds 4GB.
> 
> This patchset fixes this issue by updating helper function
> blk_queue_max_discard_sectors which is used to set max_discard_sectors limit.
> 
> This patchset also replaces "q->limits.max_discard_sector = max_discard_sectors"
> with blk_queue_max_discard_sectors call in other drivers like mmc, mtd etc.

I really don't understand this explanation.  How can you be affected by
the incorrect setting of q->limits.max_discard sectors when  n the
blkdev_issue_discard() code you see:

	max_discard_sectors = min(q->limits.max_discard_sectors, UINT_MAX >>
9);

?

The problem is not that we issue discards bigger than __data_len can
allow, the problem is that we merge them larger than __data_len will
allow.  That means the merge code needs fixing to pay attention to
max_discard_sectors, so isn't this the correct fix:

James

---

diff --git a/include/linux/blkdev.h b/include/linux/blkdev.h
index 78feda9..33f358f 100644
--- a/include/linux/blkdev.h
+++ b/include/linux/blkdev.h
@@ -838,7 +838,7 @@ static inline unsigned int blk_queue_get_max_sectors(struct request_queue *q,
 						     unsigned int cmd_flags)
 {
 	if (unlikely(cmd_flags & REQ_DISCARD))
-		return q->limits.max_discard_sectors;
+		return min(q->limits.max_discard_sectors, UINT_MAX >> 9);
 
 	if (unlikely(cmd_flags & REQ_WRITE_SAME))
 		return q->limits.max_write_same_sectors;