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4bbf439b09
Unlike ->read(), ->read_iter() instances *must* return the amount
of data they'd left in iterator. For ->read() returning less than
it has actually copied is a QoI issue; read(fd, unmapped_page - 5, 8)
is allowed to fill all 5 bytes of destination and return 4; it's
not nice to caller, but POSIX allows pretty much anything in such
situation, up to and including a SIGSEGV.
generic_file_splice_read() uses pipe-backed iterator as destination;
there a short copy comes from pipe being full, not from running into
an un{mapped,writable} page in the middle of destination as we
have for iovec-backed iterators read(2) uses. And there we rely
upon the ->read_iter() reporting the actual amount it has left
in destination.
Conversion of a ->read() instance into ->read_iter() has to watch
out for that. If you really need an "all or nothing" kind of
behaviour somewhere, you need to do iov_iter_revert() to prune
the partial copy.
In case of seq_read_iter() we can handle short copy just fine;
the data is in m->buf and next call will fetch it from there.
Fixes: d4d50710a8 (seq_file: add seq_read_iter)
Tested-by: Nathan Chancellor <natechancellor@gmail.com>
Signed-off-by: Al Viro <viro@zeniv.linux.org.uk>
Linux kernel
============
There are several guides for kernel developers and users. These guides can
be rendered in a number of formats, like HTML and PDF. Please read
Documentation/admin-guide/README.rst first.
In order to build the documentation, use ``make htmldocs`` or
``make pdfdocs``. The formatted documentation can also be read online at:
https://www.kernel.org/doc/html/latest/
There are various text files in the Documentation/ subdirectory,
several of them using the Restructured Text markup notation.
Please read the Documentation/process/changes.rst file, as it contains the
requirements for building and running the kernel, and information about
the problems which may result by upgrading your kernel.