Unexpected splice "always copy" behavior observed

Mathieu Desnoyers
2010-05-18T15:34:51Z

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

* 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 ?

Thanks,

Mathieu

--
Mathieu Desnoyers
Operating System Efficiency R&D Consultant
EfficiOS Inc.