bvec/iter: disallow zero-length segment bvecs

zero-length bvec segments are allowed in general, but not handled by bio and down the block layer so filtered out. This inconsistency may be confusing and prevent from optimisations. As zero-length segments are useless and places that were generating them are patched, declare them not allowed. Reviewed-by: Christoph Hellwig <[email protected]> Signed-off-by: Pavel Begunkov <[email protected]> Reviewed-by: Ming Lei <[email protected]> Signed-off-by: Jens Axboe <[email protected]>
kawasaki · Jan 25, 2021 · 9b2e001 · 9b2e001
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diff --git a/Documentation/block/biovecs.rst b/Documentation/block/biovecs.rst
@@ -40,6 +40,8 @@ normal code doesn't have to deal with bi_bvec_done.
    There is a lower level advance function - bvec_iter_advance() - which takes
    a pointer to a biovec, not a bio; this is used by the bio integrity code.
 
+As of 5.12 bvec segments with zero bv_len are not supported.
+
 What's all this get us?
 =======================
 

diff --git a/Documentation/filesystems/porting.rst b/Documentation/filesystems/porting.rst
@@ -865,3 +865,10 @@ no matter what.  Everything is handled by the caller.
 
 clone_private_mount() returns a longterm mount now, so the proper destructor of
 its result is kern_unmount() or kern_unmount_array().
+
+---
+
+**mandatory**
+
+zero-length bvec segments are disallowed, they must be filtered out before
+passed on to an iterator.
diff --git a/lib/iov_iter.c b/lib/iov_iter.c
@@ -72,8 +72,6 @@
 	__start.bi_bvec_done = skip;			\
 	__start.bi_idx = 0;				\
 	for_each_bvec(__v, i->bvec, __bi, __start) {	\
-		if (!__v.bv_len)			\
-			continue;			\
 		(void)(STEP);				\
 	}						\
 }