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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