linux-kernel - Re: [PATCH 4/4] pipe_fs_i.h: add pipe_buf

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Message-ID: <Yi1bakVfs/l6CNE0@casper.infradead.org>
Date:   Sun, 13 Mar 2022 02:48:10 +0000
From:   Matthew Wilcox <willy@...radead.org>
To:     Al Viro <viro@...iv.linux.org.uk>
Cc:     Max Kellermann <max.kellermann@...il.com>,
        linux-fsdevel@...r.kernel.org, linux-kernel@...r.kernel.org
Subject: Re: [PATCH 4/4] pipe_fs_i.h: add pipe_buf_init()

On Sun, Mar 13, 2022 at 02:37:43AM +0000, Al Viro wrote:
> On Fri, Feb 25, 2022 at 07:54:31PM +0100, Max Kellermann wrote:
> 
> >  			/* Insert it into the buffer array */
> >  			buf = &pipe->bufs[head & mask];
> > -			buf->page = page;
> > -			buf->ops = &anon_pipe_buf_ops;
> > -			buf->offset = 0;
> > -			buf->len = 0;
> > -			if (is_packetized(filp))
> > -				buf->flags = PIPE_BUF_FLAG_PACKET;
> > -			else
> > -				buf->flags = PIPE_BUF_FLAG_CAN_MERGE;
> > +			pipe_buf_init(buf, page, 0, 0,
> > +				      &anon_pipe_buf_ops,
> > +				      is_packetized(filp) ? PIPE_BUF_FLAG_PACKET : PIPE_BUF_FLAG_CAN_MERGE);
> 
> *cringe*
> FWIW, packetized case is very rare, so how about turning that into
> 			pipe_buf_init(buf, page, 0, 0,
> 				      &anon_pipe_buf_ops,
> 				      PIPE_BUF_FLAG_CAN_MERGE);
> 			if (unlikely(is_packetized(filp)))
> 				buf->flags = PIPE_BUF_FLAG_PACKET;
> Your pipe_buf_init() is inlined, so it shouldn't be worse from the optimizer
> POV - it should be able to start with calculating that value and then storing
> that, etc.

That's not equivalent.  I think the better option here is to always
initialise flags to 0 (and not have a parameter for it):

			pipe_buf_init(buf, page, 0, 0, &anon_pipe_buf_ops);
			if (is_packetized(filp))
				buf->flags = PIPE_BUF_FLAG_PACKET;
			else
				buf->flags = PIPE_BUF_FLAG_CAN_MERGE;