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Subject: Re: [PATCH v1 0/4] block: fix bio_will_gap()
To: Ming Lei <ming.lei@canonical.com>, Jens Axboe <axboe@kernel.dk>,
        linux-kernel@vger.kernel.org
References: <1455852022-14188-1-git-send-email-ming.lei@canonical.com>
Cc: linux-block@vger.kernel.org, Christoph Hellwig <hch@infradead.org>,
        Kent Overstreet <kent.overstreet@gmail.com>,
        Keith Busch <keith.busch@intel.com>, Elliott Robert <elliott@hpe.com>
From: Sagi Grimberg <sagig@dev.mellanox.co.il>
Message-ID: <56C9862C.70908@dev.mellanox.co.il>
Date: Sun, 21 Feb 2016 11:41:00 +0200
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> Hi Guys,
>
> The bio passed to bio_will_gap() may be fast cloned from upper
> layer(dm, md, bcache, fs, ...), or from bio splitting in block
> core. Unfortunately bio_will_gap() just figures out the last
> bvec via 'bi_io_vec[prev->bi_vcnt - 1]' directly, and this way
> is obviously wrong in case of fast-cloned bio.
>
> It is observed that lots of BIOs are still merged even if
> the virt boundary limit is violated by the merge, and the issue
> was reported from Sagi Grimberg.
>
> This patch introduces two helpers for getting the first and last
> bvec of one bio and applys them to fix the issue. Sagi tested
> the last patchset and confirmed the fix.
>
> V1:
> 	- get bvec directly for non-cloned bio
> 	- implement bio_get_last_bvec() with single bio_advance_iter(),
> 	and avoid to use bio_for_each_segment() which looks a bit inefficient
> 	- avoid to double check queue_virt_boundary() in bio_will_gap()

Thanks Ming,

Jens, can this make the next 4.5-rc since this regression was detected
in 4.5?

Thanks,
Sagi.