[PATCH] block/bio.c: update bi_iter.bi_size before recounting segments

[Ming Lei]

The patch of "bio: modify __bio_add_page() to accept pages that
don't start a new segment" changes the way for adding one page
to bio:

	- previously by adding page after checking successfully
	- now by trying to add page and recover if it fails

Unfortunately the patch forgets to update bio->bi_iter.bi_size
before trying to add page, then the last vector for holding
the added page may not be covered if recouning segments is needed,
so bio->bi_phys_segments may become not consistent with the
actual bio page buffers after the page is added successfully
to the bio(after bi_iter.bi_size is added by 'len')

Suppose the page in the last vector can't be merged to bio, tragedy
will happen when __bio_add_page() is called to add another page:

	- blk_recount_segments() is called and the actual segments get
	figured out correctly

	- the actual segments may become queue_max_segments(q) plus one
	in failure path

	- driver will find the segment count is too big to handle.

The patch fixes the virtio-blk oops bug reported from Jet Chen in
below link:

	http://marc.info/?l=linux-kernel&m=140113053817095&w=2

Cc: Jens Axboe <axboe@xxxxxxxxx>
Cc: Maurizio Lombardi <mlombard@xxxxxxxxxx>
Cc: Dongsu Park <dongsu.park@xxxxxxxxxxxxxxxx>
Cc: Christoph Hellwig <hch@xxxxxx>
Cc: Kent Overstreet <kmo@xxxxxxxxxxxxx>
Cc: Andrew Morton <akpm@xxxxxxxxxxxxxxxxxxxx>
Reported-by: Jet Chen <jet.chen@xxxxxxxxx>
Tested-by: Jet Chen <jet.chen@xxxxxxxxx>
Signed-off-by: Ming Lei <ming.lei@xxxxxxxxxxxxx>
---
Andrew, could you put the patch in your -mm tree
because the previous two patches were routed from
your tree?

 block/bio.c |    4 +++-
 1 file changed, 3 insertions(+), 1 deletion(-)

diff --git a/block/bio.c b/block/bio.c
index 0443694..f9bae56 100644
--- a/block/bio.c
+++ b/block/bio.c
@@ -744,6 +744,7 @@ static int __bio_add_page(struct request_queue *q, struct bio *bio, struct page
 				}
 			}
 
+			bio->bi_iter.bi_size += len;
 			goto done;
 		}
 	}
@@ -761,6 +762,7 @@ static int __bio_add_page(struct request_queue *q, struct bio *bio, struct page
 	bvec->bv_offset = offset;
 	bio->bi_vcnt++;
 	bio->bi_phys_segments++;
+	bio->bi_iter.bi_size += len;
 
 	/*
 	 * Perform a recount if the number of segments is greater
@@ -802,7 +804,6 @@ static int __bio_add_page(struct request_queue *q, struct bio *bio, struct page
 		bio->bi_flags &= ~(1 << BIO_SEG_VALID);
 
  done:
-	bio->bi_iter.bi_size += len;
 	return len;
 
  failed:
@@ -810,6 +811,7 @@ static int __bio_add_page(struct request_queue *q, struct bio *bio, struct page
 	bvec->bv_len = 0;
 	bvec->bv_offset = 0;
 	bio->bi_vcnt--;
+	bio->bi_iter.bi_size -= len;
 	blk_recount_segments(q, bio);
 	return 0;
 }
-- 
1.7.9.5

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