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SubjectRe: [PATCH][RFC] splice support
Jens Axboe <axboe@suse.de> wrote:
>
> Hi,
>
> Since my initial posting back in December, I've had some private queries
> about the state of splice support. The state was pretty much that it was
> a little broken, if one attempted to do file | file splicing. The
> original patch migrated pages from one file to another in this case,
> which got vm ugly really quickly. And it wasn't always the right thing
> to do, since it would mean that splicing file1 to file2 would move
> file1's page cache to file2. Sometimes this is what you want, sometimes
> it is not.
>
> So that was removed to make things work fully. It can later be
> reintroduced (and controlled with the splice flags passed in, whether to
> 'loan' or 'gift' source pages to use a McVoy term) if need be.
>
> Apart from that change, I added splice to socket support. It then
> becomes a full sendfile() replacement (unless I broke something). I'm
> attaching the current patch against 2.6.16-git, and also three test apps
> that you can use as a reference or just to play with this. The apps are:
>

- splice() take a size_t length. Should it be taking a 64-bit length?

- splice() doesn't check for (len < 0), like read() and write() do.
Should it?

- Please don't call it `len'. VFS has to deal with "lengths" which can
be in units of PAGE_CACHE_SIZE, fs blocksize, 512-bytes sectors or bytes,
and it gets confusing. Our liking for variable names like `len' and
`count' just makes it worse.

If it's in units of pages then call it `npages'. If it's in bytes then
call it `nbytes'.

What _is_ it in units of, anyway? I guess bytes, since it's size_t.

I assume all this lenning:

unsigned int this_len;

this_len = buf->len;
if (this_len > len)
this_len = len;
is dealing with bytes too. You'll be wanting a size_t in there.

- why is the `flags' arg to sys_splice() unsigned long? Can it be `int'?

- what does `flags' do, anyway? The whole thing is undocumented and
almost uncommented.

- the tmp_page trick in anon_pipe_buf_release() seems to be unrelated to
the splice() work. It should be a separate patch and any peformance
testing (needed, please) should be decoupled from that change.

- I think the `size_t left' in do_splice_to() can overflow if f_pos is
sufficiently different from i_size.

- All the operations do foo(in, out, ...). It's a bit more conventional
to do foo(out, in, ...).

- The logic in do_splice() hurts my brain. "if `in' is a pipe then
splice from `in-as-a-pipe' to `out' else if `out' is a pipe then splice
from `in' to 'out-as-a-pipe'. Make sense, I guess, but I do wonder "what
would happen if those tests were reversed?". Nothing, I guess.

- In pipe_to_file():

- Shouldn't it be using GFP_HIGHUSER()?

- local variable `index' should be unsigned long or, for clarity
value, pgoff_t.

- Incoming arg `pos' should be loff_t?

- It's racy against truncate(). After running ->readpage and
lock_page(), need to check for page->mapping == NULL.

- There's a duplicate flush_dcache_page().

- Why does it run write_one_page()??? (Don't tell me. I'll work it
out when I see the commented version ;))

- I worry a bit about the assumption in one place that a non-zero
return from commit_write() indicates an error, whereas another place
assumes that a negative return is an error. We had problems in the
past where some a_ops implementations decided to return small positive
numbers from prepare_write() or commit_write() a_ops, which broke
stuff. They shouldn't be doing that now, but it's a thing to watch out
for.

- Bug. If write_one_page() returned an error, it still unlocked the page.

- In pipe_to_sendpage():

- local variable `offset' is ulong, but elsewhere you've used uint.
The latter is better.

- Again, incoming arg `len' is confusing. I _think_ it's actually
"number of bytes to be moved from this page". A comment which explains
these things would be nice, and perhaps a better name (bytes_to_send?)

- Should incoming arg `pos' be loff_t? That would give it some meaning.

- Why does it use PAGE_SIZE and PAGE_SHIFT rather than PAGE_CACHE_*?

- In generic_file_splice_read():

- nonatomic modification of f_pos. Is i_mutex held? (see
generic_file_llseek())

- Darnit, we carried `flags' this far and ended up not using it.
(What _does_ flags do, anyway? Reads on..)

- In __generic_file_splice_read():

- local variable `index' is ulong, could be pgoff_t (for clarity)

- local variable `offset' could be uint (it is uint elsewhere, and
might generate better code).

A better name might be offset_in_page.

- local variable `pages' could be uint (but watch out for overflow!!).

A better name might be nr_pages (matches find_get_pages()). Then,
local variable `array' can be renamed to `pages', which is all much
better.

- While we're in the spirit, local var `i' would be better named
`page_nr' or something.

- Shouldn't it be using GFP_HIGHUSER?

- whoa. We move the pages into the pipe while they're still under
read I/O. Is that deliberate? (pls add nice comment).

- These pages can get truncated at any time they're unlocked. Does
the code cope with all that?

- 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.

- In move_to_pipe()

- gargh, another `offset' and `len'. No idea what they're doing, so
am unable to determine whether ulong is an appropriate type. Am keenly
looking forward to the commented version!

- Suggest you rename `pages' to `nr_pages', `array' to `pages'. And
`i' to `page_nr'.

- local var `bufs' could be renamed `nrbufs' to align with
pipe_inode_info and could be made uint.

- Do we actually need local var `bufs'? It seems to be caching info->nrbufs.

- release_pages() might be faster than one-at-a-time page_cache_release()


Anyway, that's all just low-level stuff.

What does the feature do? How would one use it in an application? Is it
intended that it be generalised to other kinds of address_spaces? If so,
which ones, and what implementation problems might we expect?

(And I still don't know what `flags' does!)
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