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 06:06:57PM +1030, Qu Wenruo wrote:
> > 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?

They aren't a very efficient model, so I would not recommend to add
new users.

> 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?

I'd recommend to use iomap.

> > 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.

Probably because no one did the work yet.  I don't have a pointer at
hand, but it was a discussion willy kicked up about converting
write_begin/end to folios.