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SubjectRe: [PATCH] fix unbalanced page refcounting in bio_map_user_iov
On Sun, Sep 24, 2017 at 03:27:39PM +0100, Al Viro wrote:

> At the very least, we need bmd->iter = *iter; bmd->iter.iov = bmd->iov;
> instead of that iov_iter_init() in there. I'm not sure how far back does
> it go; looks like "block: support large requests in blk_rq_map_user_iov"
> is the earliest possible point, but it might need more digging to make
> sure. v4.5+, if that's when the problems began...
>
> Anyway, I'd added the obvious fix to #work.iov_iter, reordered it and
> force-pushed the result.

While we are at it, calculation of nr_pages in bio_copy_user_iov() is bloody
odd - why, in the name of everything unholy, does it care about the iovec
boundaries in there? We are copying data anyway; why does allocation of bio
care about the fragmentation of the other end of copying? Shouldn't it be
simply max(DIV_ROUND_UP(offset + len, PAGE_SIZE), BIO_MAX_PAGES)?

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