On Tuesday April 24, npiggin@xxxxxxx wrote:
> + write_begin: This is intended as a replacement for prepare_write. Called
> + by the generic buffered write code to ask the filesystem to prepare
> + to write len bytes at the given offset in the file. flags is a field
> + for AOP_FLAG_xxx flags, described in include/linux/mm.h.
Putting "This is intended as a replacement.." there sees a bit
dangerous. It could well accidentally remain when the documentation
for prepare_write gets removed. I would make it a separate paragraph
and flesh it out. And include text from prepare_write before that
gets removed.
write_begin:
This is intended as a replacement for prepare_write. The key
differences being that:
- it returns a locked page (in *pagep) rather than
being given a pre-locked page:
- it can pass arbitrary state to write_end rather than
having to hide stuff in some filesystem-internal
data structure
- The (largely undocumented) flags option.
Called by the generic bufferred write code to ask an
address_space to prepare to write len bytes at the given
offset in the file.
The address_space should check that the write will be able to
complete, by allocating space if necessary and doing any other
internal housekeeping. If the write will update parts of any
basic-blocks on storage, then those blocks should be pre-read
(if they haven't been read already) so that the updated blocks
can be written out properly.
The possible flags are listed in include/linux/fs.h (not
mm.h) and include
AOP_FLAG_UNINTERRUPTIBLE:
It is unclear how this should be used. No
current code handles it.
(together with the rest...)
> +
> + The filesystem must return the locked pagecache page for the caller
> + to write into.
> +
> + A void * may be returned in fsdata, which then gets passed into
> + write_end.
> +
> + Returns < 0 on failure, in which case all cleanup must be done and
> + write_end not called. 0 on success, in which case write_end must
> + be called.
As you are not including perform_write in the current patchset, maybe
it is best not to include the documentation yet either?
> + perform_write: This is a single-call, bulk version of write_begin/write_end
> + operations. It is only used in the buffered write path (write_begin
> + must still be implemented), and not for in-kernel writes to pagecache.
> + It takes an iov_iter structure, which provides a descriptor for the
> + source data (and has associated iov_iter_xxx helpers to operate on
> + that data). There are also file, mapping, and pos arguments, which
> + specify the destination of the data.
> +
> + Returns < 0 on failure if nothing was written out, otherwise returns
> + the number of bytes copied into pagecache.
> +
> + fs/libfs.c provides a reasonable template to start with, demonstrating
> + iov_iter routines, and iteration over the destination pagecache.
> +
NeilBrown
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