About using on-stack fsdata pointer for write_begin() and write_end() callbacks

[Qu Wenruo <quwenruo.btrfs@gmx.com>]

Hi,

Recently I'm working on migrating btrfs_buffered_write() to utilize
write_begin() and write_end() callbacks.

Currently only the following filesystems really utilizing that pointer:

- bcachefs
   Which is a structure of 24 bytes without any extra pointer.

- f2fs (for compression)
   Which is holding a pointer to an array of pages.

- ext4
   Only utilize that pointer as a flag for ext4_da_write_begin()

- ocfs2
   This a large structure holding a lot of things

- (Future) btrfs
   Only holds a pointer and a bool.
   (Also needs a way to pass ki_flags to support IOCB_NOWAIT though)

Thus I'm wondering should we make perform_generic_write() to accept a
*fsdata pointer, other than making write_begin() to allocate one.
So that we only need to allocate the memory (or use the on-stack one)
once per write, other than once per folio.

This will cause no change to f2fs/ext4, but should benefit
ocfs2/bcachefs and of-course btrfs.

Or is there some special corner case that relies on the current behavior?

Thanks,
Qu