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    SubjectRe: Unexpected splice "always copy" behavior observed
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    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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