linux-kernel - Re: [PATCH] fix unbalanced page refcounting in bio_map_user

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Date:   Sun, 24 Sep 2017 21:48:29 -0400
From:   Vitaly Mayatskikh <v.mayatskih@...il.com>
To:     Al Viro <viro@...IV.linux.org.uk>
Cc:     linux-block@...r.kernel.org, linux-kernel@...r.kernel.org,
        Jens Axboe <axboe@...nel.dk>, Christoph Hellwig <hch@....de>
Subject: Re: [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