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

[Christoph Hellwig <hch@infradead.org>]

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.

> 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.