Messages in this thread | | | Subject | Re: Unexpected splice "always copy" behavior observed | From | Steven Rostedt <> | Date | Tue, 18 May 2010 11:53:13 -0400 |
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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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