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

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Date:   Sat, 23 Sep 2017 21:33:23 +0100
From:   Al Viro <viro@...IV.linux.org.uk>
To:     Vitaly Mayatskikh <v.mayatskih@...il.com>
Cc:     linux-block@...r.kernel.org, linux-kernel@...r.kernel.org,
        Jens Axboe <axboe@...nel.dk>
Subject: Re: [PATCH] fix unbalanced page refcounting in bio_map_user_iov

On Sat, Sep 23, 2017 at 06:19:26PM +0100, Al Viro wrote:
> On Sat, Sep 23, 2017 at 05:55:37PM +0100, Al Viro wrote:
> 
> > IOW, the loop on failure exit should go through the bio, like __bio_unmap_user()
> > does.  We *also* need to put everything left unused in pages[], but only from the
> > last iteration through iov_for_each().
> > 
> > Frankly, I would prefer to reuse the pages[], rather than append to it on each
> > iteration.  Used iov_iter_get_pages_alloc(), actually.
> 
> Something like completely untested diff below, perhaps...

> +				unsigned n = PAGE_SIZE - offs;
> +				unsigned prev_bi_vcnt = bio->bi_vcnt;

Sorry, that should've been followed by
				if (n > bytes)
					n = bytes;

Anyway, a carved-up variant is in vfs.git#work.iov_iter.  It still needs
review and testing; the patch Vitaly has posted in this thread plus 6
followups, hopefully more readable than aggregate diff.

Comments?