在 2024/11/11 16:22, Christoph Hellwig 写道:
On Mon, Nov 11, 2024 at 02:57:06PM +1030, Qu Wenruo wrote:
Hi,
Recently I'm working on migrating btrfs_buffered_write() to utilize
write_begin() and write_end() callbacks.
Why? They aren't exactly efficient, and it's just going to create
more Churn for Goldwyn's iomap work.
So it is not recommended to go the write_begin() and write_end()
callbacks at all?
Or just not recommended for btrfs?
I know there are limits like those call backs do not support
IOCB_NOWAIT, and the memory allocation inefficient problem (it should
only affect ocfs2), but shouldn't we encourage to use the more common
paths where all other fses go?
Currently only the following filesystems really utilizing that pointer:
- bcachefs
Which is a structure of 24 bytes without any extra pointer.
And as pointed out last time willy and I did go through the users of
write_begin/end this is just dead code that is never called.
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.
And that scheme was one of my suggestions back then, together with
removing write_begin/end from address_space_operations because they
aren't operations called by MM/pagecache code, but just callbacks
provided by the file system to perform_generic_write.
Mind to point me to the old discussion thread? I'd like to know why we
didn't go that path.
Thanks,
Qu
