Re: copy on write for splice() from file to pipe?

On Fri, Feb 10, 2023 at 01:16:03PM +1100, Dave Chinner wrote:

On Thu, Feb 09, 2023 at 08:41:02AM -0800, Linus Torvalds wrote:

quoted

Now, I do see a couple of possible alternatives to "just create a stable copy".

For example, we very much have the notion of "confirm buffer data
before copying". It's used for things like "I started the IO on the
page, but the IO failed with an error, so even though I gave you a
splice buffer, it turns out you can't use it".

And I do wonder if we could introduce a notion of "optimistic splice",
where the splice works exactly the way it does now (you get a page
reference), but the "confirm" phase could check whether something has
changed in that mapping (using the file versioning or whatever - I'm
hand-waving) and simply fail the confirm.

That would mean that the "splice to socket" part would fail in your
chain, and you'd have to re-try it. But then the onus would be on
*you* as a splicer, not on the rest of the system to fix up your
special case.

That idea sounds fairly far out there, and complicated and maybe not
usable. So I'm just throwing it out as a "let's try to think of
alternative solutions".

Oh, that's sounds like an exact analogy to the new IOMAP_F_STALE
flag and the validity cookie we have in the iomap write path code.
The iomap contains cached, unserialised information, and the
filesystem side mapping it is derived from can change asynchronously
(e.g. by IO completion doing unwritten extent conversion). Hence the
cached iomap can become stale, and that's a data corruption vector.

The validity cookie is created when the iomap is built, and it is
passed to a filesystem callback when a folio is locked for copy-in.
This allows the IO path to detect that the filesystem side extent
map has changed during the write() operations before we modify the
contents of the folio. It is done under the locked folio so that the
validation is atomic w.r.t. the modification to the folio contents
we are about to perform.

On detection of a cookie mismatch, the write operation then sets the
IOMAP_F_STALE flag, backs out of the write to that page and ends the
write to the iomap. The iomap infrastructure then remaps the file
range from the offset of the folio at which the iomap change was
detected.  The write the proceeds with the new, up to date iomap....

We have had a similar "is the cached iomap still valid?" mechanism
on the writeback side of the page cache for years. The details are
slightly different, though I plan to move that code to use the same
IOMAP_F_STALE infrastructure in the near future because it
simplifies the writeback context wrapper shenanigans an awful lot.
And it helps make it explicit that iomaps are cached/shadowed
state, not the canonical source of reality.

Applying the same principle it to multiply referenced cached page
contents will be more complex. I suspect we might be able to
leverage inode->i_version or ctime as the "data changed" cookie as
they are both supposed to change on every explicit user data
modification made to an inode. However, I think most of the
complexity would be in requiring spliced pages to travel in some
kind of container that holds the necessary verification
information....

So while I was pondering the complexity of this and watching a great
big shiny rocket create lots of heat, light and noise, it occurred
to me that we already have a mechanism for preventing page cache
data from being changed while the folios are under IO:
SB_I_STABLE_WRITES and folio_wait_stable().

That is, md/dm RAID5/6 devices require the folio contents to be
stable during writeback to calculate parity during IO submission,
and folio_wait_stable() is the mechanism that guarantees pages don't
get changed while they are under IO.

So instead of the complex dance described above to detect data
changes at the splice destination, we simply have splice add
temporary folio state that folio_wait_stable() blocks on to prevents
new user data modification on that folio from occurring until the
splice reference and stable state flag goes away.

Whilst this would work for folios that are currently clean, I
suspect it won't work properly with folios that are already mapped
and dirty because there won't be another page fault to trap the data
being changed.  i.e. no folio_clear_dirty_for_io() call has been
made on the page to trigger new write faults. Perhaps a variant that
just propagates the pte dirty bit into the folio and clears it (i.e.
first part of folio_clear_dirty_for_io()) would be sufficient in
this case.

Cheers,

Dave.
-- 
Dave Chinner
david@fromorbit.com