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    SubjectRe: [PATCH][RFC] splice support
    Jens Axboe <> 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.


    +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

    > > - 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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