linux-kernel - Re: [PATCH] sendfile removal

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Date:	Thu, 31 May 2007 13:05:50 +0200
From:	Jens Axboe <jens.axboe@...cle.com>
To:	Christoph Hellwig <hch@...radead.org>,
	linux-kernel@...r.kernel.org, cotte@...ibm.com, hugh@...itas.com,
	neilb@...e.de, zanussi@...ibm.com,
	Linus Torvalds <torvalds@...l.org>
Subject: Re: [PATCH] sendfile removal

On Thu, May 31 2007, Christoph Hellwig wrote:
> On Thu, May 31, 2007 at 12:33:16PM +0200, Jens Axboe wrote:
> > - nfds: The ->rq_sendfile_ok optimization is gone for now. I can't
> >   determine the value of it, but I'm assuming it's there for a reason.
> >   Any chance this can be converted to splice, or use something else than
> >   ->sendfile()? CC'ed Neil.
> 
> sendfile useage in nfsd avoids a data copy and allows to use checksum
> offloading.  it's quite important for nfs server workloads.

OK, I hope Neil can provide some input on how to convert it. Of course
I'm just fishing for Neil to actually do that work :-)

> > Apart from that, it was mostly straight forward. Almost everybody uses
> > generic_file_sendfile(), which makes the conversion easy. I changed loop
> > to use do_generic_file_read() instead of sendfile, it works for me...
> 
> > diff --git a/drivers/block/loop.c b/drivers/block/loop.c
> > index 5526ead..92bac14 100644
> > --- a/drivers/block/loop.c
> > +++ b/drivers/block/loop.c
> > @@ -435,16 +435,24 @@ do_lo_receive(struct loop_device *lo,
> >  {
> >  	struct lo_read_data cookie;
> >  	struct file *file;
> > -	int retval;
> > +	read_descriptor_t desc;
> > +
> > +	desc.written = 0;
> > +	desc.count = bvec->bv_len;
> > +	desc.arg.data = &cookie;
> > +	desc.error = 0;
> >  
> >  	cookie.lo = lo;
> >  	cookie.page = bvec->bv_page;
> >  	cookie.offset = bvec->bv_offset;
> >  	cookie.bsize = bsize;
> >  	file = lo->lo_backing_file;
> > -	retval = file->f_op->sendfile(file, &pos, bvec->bv_len,
> > -			lo_read_actor, &cookie);
> > -	return (retval < 0)? retval: 0;
> > +
> > +	do_generic_file_read(file, &pos, &desc, lo_read_actor);
> 
> This change is wrong.  loop or any existing user of ->sendfile absolutely
> needs to go through a file operations vector so that file-system specific
> actions such as locking are performed.  This is required at least for the
> clustered filesystems and XFS.   The right way to implement this is
> via do_splice_direct or something similar.
> 
> do_generic_file_read is only a library function for filesystem use
> and should never be called directly.

I'll convert it to do_splice_direct(), thanks.

-- 
Jens Axboe

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