linux-kernel - Re: [PATCH v2 1/2] iov_iter: optimise iov_iter

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Date:   Fri, 20 Nov 2020 01:49:04 +0000
From:   Matthew Wilcox <willy@...radead.org>
To:     Pavel Begunkov <asml.silence@...il.com>
Cc:     linux-fsdevel@...r.kernel.org,
        Alexander Viro <viro@...iv.linux.org.uk>,
        Jens Axboe <axboe@...nel.dk>, linux-block@...r.kernel.org,
        linux-kernel@...r.kernel.org
Subject: Re: [PATCH v2 1/2] iov_iter: optimise iov_iter_npages for bvec

On Fri, Nov 20, 2020 at 01:39:05AM +0000, Pavel Begunkov wrote:
> On 20/11/2020 01:20, Matthew Wilcox wrote:
> > On Thu, Nov 19, 2020 at 11:24:38PM +0000, Pavel Begunkov wrote:
> >> The block layer spends quite a while in iov_iter_npages(), but for the
> >> bvec case the number of pages is already known and stored in
> >> iter->nr_segs, so it can be returned immediately as an optimisation
> > 
> > Er ... no, it doesn't.  nr_segs is the number of bvecs.  Each bvec can
> > store up to 4GB of contiguous physical memory.
> 
> Ah, really, missed min() with PAGE_SIZE in bvec_iter_len(), then it's a
> stupid statement. Thanks!
> 
> Are there many users of that? All these iterators are a huge burden,
> just to count one 4KB page in bvec it takes 2% of CPU time for me.

__bio_try_merge_page() will create multipage BIOs, and that's
called from a number of places including
bio_try_merge_hw_seg(), bio_add_page(), and __bio_iov_iter_get_pages()

so ... yeah, it's used a lot.