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

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Message-ID: <20201120012017.GJ29991@casper.infradead.org>
Date:   Fri, 20 Nov 2020 01:20:17 +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 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.