linux-kernel - Re: [PATCH 2/2] iov_iter: optimise iter type checking

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Date:   Fri, 11 Dec 2020 02:01:00 +0000
From:   Al Viro <viro@...iv.linux.org.uk>
To:     Pavel Begunkov <asml.silence@...il.com>
Cc:     Christoph Hellwig <hch@...radead.org>,
        Jens Axboe <axboe@...nel.dk>, linux-block@...r.kernel.org,
        linux-kernel@...r.kernel.org
Subject: Re: [PATCH 2/2] iov_iter: optimise iter type checking

On Thu, Nov 19, 2020 at 05:12:44PM +0000, Pavel Begunkov wrote:
> On 19/11/2020 17:03, Christoph Hellwig wrote:
> > On Thu, Nov 19, 2020 at 03:29:43PM +0000, Pavel Begunkov wrote:
> >> The problem here is that iov_iter_is_*() helpers check types for
> >> equality, but all iterate_* helpers do bitwise ands. This confuses
> >> a compiler, so even if some cases were handled separately with
> >> iov_iter_is_*(), it can't eliminate and skip unreachable branches in
> >> following iterate*().
> > 
> > I think we need to kill the iov_iter_is_* helpers, renumber to not do
> > the pointless bitmask and just check for equality (might turn into a
> > bunch of nice switch statements actually).
> 
> There are uses like below though, and that would also add some overhead
> on iov_iter_type(), so it's not apparent to me which version would be
> cleaner/faster in the end. But yeah, we can experiment after landing
> this patch.
> 
> if (type & (ITER_BVEC|ITER_KVEC))

There are exactly 3 such places, and all of them would've been just as well
with case ITER_BVEC: case ITER_KVEC: ... in a switch.

Hmm...  I wonder which would work better:

enum iter_type {
        ITER_IOVEC = 0,
        ITER_KVEC = 2,
        ITER_BVEC = 4,
        ITER_PIPE = 6,
        ITER_DISCARD = 8,
};
iov_iter_type(iter)	(((iter)->type) & ~1)
iov_iter_rw(iter)	(((iter)->type) & 1)

or

enum iter_type {
        ITER_IOVEC,
        ITER_KVEC,
        ITER_BVEC,
        ITER_PIPE,
        ITER_DISCARD,
};
iov_iter_type(iter)	(((iter)->type) & (~0U>>1))
// callers of iov_iter_rw() are almost all comparing with explicit READ or WRITE
iov_iter_rw(iter)	(((iter)->type) & ~(~0U>>1) ? WRITE : READ)
with places like iov_iter_kvec() doing
	i->type = ITER_KVEC | ((direction == WRITE) ? BIT(31) : 0);

Preferences?