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SubjectRe: [PATCH][RFC] splice support
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).

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