linux-kernel - Re: Unexpected splice "always copy" behavior observed

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Date:	Tue, 18 May 2010 11:53:13 -0400
From:	Steven Rostedt <rostedt@...dmis.org>
To:	Mathieu Desnoyers <mathieu.desnoyers@...icios.com>
Cc:	Peter Zijlstra <peterz@...radead.org>,
	Frederic Weisbecker <fweisbec@...il.com>,
	Pierre Tardy <tardyp@...il.com>, Ingo Molnar <mingo@...e.hu>,
	Arnaldo Carvalho de Melo <acme@...hat.com>,
	Tom Zanussi <tzanussi@...il.com>,
	Paul Mackerras <paulus@...ba.org>,
	linux-kernel@...r.kernel.org, arjan@...radead.org,
	ziga.mahkovec@...il.com, davem <davem@...emloft.net>,
	linux-mm@...ck.org, Andrew Morton <akpm@...ux-foundation.org>,
	KOSAKI Motohiro <kosaki.motohiro@...fujitsu.com>,
	Christoph Lameter <cl@...ux-foundation.org>,
	Tejun Heo <tj@...nel.org>, Jens Axboe <jens.axboe@...cle.com>,
	Linus Torvalds <torvalds@...ux-foundation.org>,
	Nick Piggin <npiggin@...e.de>
Subject: Re: Unexpected splice "always copy" behavior observed

Hehe, I just notice this this morning too, while investigating.


On Tue, 2010-05-18 at 11:34 -0400, Mathieu Desnoyers wrote:
> Hi,
> 
> I'm currently digging into the splice code to figure out why it's always in copy
> mode even though I specified the SPLICE_F_MOVE flag and released the page
> references from the LTTng ring buffer. I'm splicing to a pipe and then from the
> pipe to an ext3 filesystem (2.6.33.4 kernel). I've got the feeling I'm missing
> something and I don't like that.
> 
> My simple test case is to add a printk around the splice copy:
> 
> fs/splice.c: pipe_to_file()
>        if (buf->page != page) {
>                 /*
>                  * Careful, ->map() uses KM_USER0!
>                  */
>                 char *src = buf->ops->map(pipe, buf, 1);
>                 char *dst = kmap_atomic(page, KM_USER1);
> 
>                 printk(KERN_WARNING "SPLICE COPY!!!\n");
>                 memcpy(dst + offset, src + buf->offset, this_len);
>                 flush_dcache_page(page);
>                 kunmap_atomic(dst, KM_USER1);
>                 buf->ops->unmap(pipe, buf, src);

I used trace_printk() since it is not as invasive.

>         }
> 
> I'll start with a disclaimer that I only recently improved my splice
> understanding, so AFAIU:

Same here ;-)

> 
> * pipe_to_file() allocates a struct page *page on its stack.
> 
> * It is passed, uninitialized, to
> 
>         ret = pagecache_write_begin(file, mapping, sd->pos, this_len,
>                                 AOP_FLAG_UNINTERRUPTIBLE, &page, &fsdata);
> 
>     that looks already odd to me, as I would expect pipe_to_file to populate
>     this page pointer with buf->page initially if the proper conditions are met.
> 
> * Looking at the ext2 and ext3 write_begin code, neither are using the pagep
>   parameter:
> 
>   ext2:
> 
> static int
> ext2_write_begin(struct file *file, struct address_space *mapping,
>                 loff_t pos, unsigned len, unsigned flags,
>                 struct page **pagep, void **fsdata)
> {
>         *pagep = NULL;
>         return __ext2_write_begin(file, mapping, pos, len, flags, pagep,fsdata);
> }
> 
> 
>   ext3:
> 
> static int ext3_write_begin(struct file *file, struct address_space *mapping,
>                                 loff_t pos, unsigned len, unsigned flags,
>                                 struct page **pagep, void **fsdata)
> {
>         struct page *page;
>         ....
> 
> retry:
>         page = grab_cache_page_write_begin(mapping, index, flags);
>         if (!page)
>                 return -ENOMEM;
>         *pagep = page;
> 
> * So, considering the test to check if the page content must be copied:
> 
>        if (buf->page != page) {
> 
>   how is it ever possible that buf->page == page ?

I'm currently looking at the network code to see if it is better.

-- Steve


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