On Tue, May 27, 2014 at 10:43:59AM +0200, Maurizio Lombardi wrote:
>
> But now I'm suspicious of this part of commit 3979ef4dcf:
>
> failed:
> bvec->bv_page = NULL;
> bvec->bv_len = 0;
> bvec->bv_offset = 0;
> bio->bi_vcnt--; <----------------
> blk_recount_segments(q, bio);
> return 0;
>
> Is decreasing bi_vcnt sufficient to guarantee that blk_recount_segments()
> recalculates the correct number of physical segments?
> Looking at the __blk_recalc_rq_segments() it appears it may not be the case.
>
> The question is how can we restore the correct number of physical segments in
> case
> of failure without breaking anything...
>
If my hypothesis is correct, the following patch should trigger a kernel panic,
Jet Chen, can you try it and let me know whether the BUG_ON is hit or not?
diff --git a/block/bio.c b/block/bio.c
index 0443694..763868f 100644
--- a/block/bio.c
+++ b/block/bio.c
@@ -701,6 +701,7 @@ static int __bio_add_page(struct request_queue *q, struct
bio *bio, struct page
unsigned int max_sectors)
{
int retried_segments = 0;
+ unsigned int phys_segments_orig;
struct bio_vec *bvec;
/*
@@ -751,6 +752,9 @@ static int __bio_add_page(struct request_queue *q, struct
bio *bio, struct page
if (bio->bi_vcnt >= bio->bi_max_vecs)
return 0;
+ blk_recount_segments(q, bio);
+ phys_segments_orig = bio->bi_phys_segments;
+
/*
* setup the new entry, we might clear it again later if we
* cannot add the page
@@ -811,6 +815,7 @@ static int __bio_add_page(struct request_queue *q, struct
bio *bio, struct page
bvec->bv_offset = 0;
bio->bi_vcnt--;
blk_recount_segments(q, bio);
+ BUG_ON(phys_segments_orig != bio->bi_phys_segments);
return 0;
}
Regards,
Maurizio Lombardi
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