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

[Qu Wenruo <wqu@suse.com>]

在 2024/11/12 15:35, Christoph Hellwig 写道:
> 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.

No wonder no one is adding support for IOCB_NOWAIT.

> 
>> 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.

Definitely the way we will go in the long run.

Although I'm still struggling on the out-of-band dirty folio (someone 
marked a folio dirty without notifying the fs) handling.

The iomap writepages implementation will just mark all the folio range 
dirty and start mapping.

So I guess there must be some fs specific handling inside the mapping 
function, let me dig it deeper and check Goldwyn's work for details.

Anyway thanks a lot pointing out that the write_begin() model is already 
kinda deprecated.

Thanks,
Qu