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SubjectRe: [PATCH] fix unbalanced page refcounting in bio_map_user_iov
On Sun, 24 Sep 2017 10:27:39 -0400,
Al Viro wrote:

> BTW, there's something fishy in bio_copy_user_iov(). If the area we'd asked for
> had been too large for a single bio, we are going to create a bio and have
> bio_add_pc_page() eventually fill it up to limit. Then we return into
> __blk_rq_map_user_iov(), advance iter and call bio_copy_user_iov() again.
> Fine, but... now we might have non-zero iter->iov_offset. And this
> bmd->is_our_pages = map_data ? 0 : 1;
> memcpy(bmd->iov, iter->iov, sizeof(struct iovec) * iter->nr_segs);
> iov_iter_init(&bmd->iter, iter->type, bmd->iov,
> iter->nr_segs, iter->count);
> does not even look at iter->iov_offset. As the result, when it gets to
> bio_uncopy_user(), we copy the data from each bio into the *beginning* of
> the user area, overwriting that from the other bio.

Yeah, something is wrong with bio_copy_user_iov. Our datapath hangs when IO flows through unmodified SG (it forces bio_copy if iov_count is set). I did not look at details, but same IO pattern and memory layout work well with bio_map (module refcount problem).

> Anyway, I'd added the obvious fix to #work.iov_iter, reordered it and
> force-pushed the result.

I'll give it a try, thanks!
--
wbr, Vitaly

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