On Thu, Mar 30 2006, Andrew Morton wrote:
> Jens Axboe <[email protected]> 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;
My mistake, I looked at a different spot. Fixed.
> >
> > > - 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..
I can go either way, I tend to agree that passing in offset may be the
best solution.
> > > - 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.
Will fix that up.
> > > - 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.
It doesn't, I'm assuming that find_get_pages() returns consequtive pages
atm. Would seem like the sane interface :-)
We continue doing find_or_create_page() on the remaining, but using 'i'
as the 'index' addition. So if we had non-conseq pages, we'd be screwed.
Needs a little thinking, input welcome..
--
Jens Axboe
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