Re: [PATCH][RFC] splice support

[Andrew Morton <akpm@osdl.org>]

Jens Axboe <axboe@suse.de> wrote:
>
> 
> ...
>
> > - I think the `size_t left' in do_splice_to() can overflow if f_pos is
> >   sufficiently different from i_size.
> 
> They're both loff_t.

Nope:

+static long do_splice_to(struct file *in, struct inode *pipe, size_t len,
+			 unsigned long flags)
+{
+	if (in->f_op && in->f_op->splice_read) {
+		loff_t isize = i_size_read(in->f_mapping->host);
+		size_t left;
+
+		if (unlikely(in->f_pos >= isize))
+			return 0;
+	
+		left = isize - in->f_pos;

It's doing

		32bit = 64bit - 64bit;

> 
> > - In generic_file_splice_read():
> > 
> >   - nonatomic modification of f_pos.  Is i_mutex held?  (see
> >     generic_file_llseek())
> 
> Fixed.

OK.  In some ways I agree with Nick that a pwrite/pread-like interface is
nicer, so things are more stateless and threads don't have to fight over
f_pos.  Dunno..

> >   - These pages can get truncated at any time they're unlocked.  Does
> >     the code cope with all that?
> 
> I guess page_cache_pipe_buf_map() needs the same ->mapping check?

That would seem appropriate.

btw, that function might have a problem I think - it returns NULL with
the page locked, but pipe_to_sendpage() and other callers don't appear to
unlock it.

> > - hm.  What happens if the pages which find_get_pages() returned are
> > not contiguous in pagecache?  I think your `pages' array gets all
> > jumbled up.
>
> Hmm please expand. 

find_get_pages() does "find me the next N pages above `index' which are
presently in pagecache'.  So it can return an array of page*'s which do not
represent contiguous pages in the file - there can be holes in there.

IOW: pages[n]->index !necessarily= pages[n+1]->index-1

Maybe the code handles that by making sure that all the pages in the range
are already in pagecache - I didn't check.  But that would take some heroic
locking.

> >   - release_pages() might be faster than one-at-a-time page_cache_release()
> 
> We should not hit that case very often. Not sure how to handle the
> 'cold' right now, so I'll just leave it.

OK.  ("cold" is a wild-ass guess as to whether you think those pages'
contents are likely to be be in CPU cache.  I'd guess "yes", so cold=0).