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

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

在 2024/11/12 16:36, Christoph Hellwig 写道:
> On Tue, Nov 12, 2024 at 04:33:56PM +1030, Qu Wenruo wrote:
>> IIRC it's related to the get_user_page() shenanigans but not 100% sure.
>
> While there might be a few stragglers left, everyone should be using
> pin_user_pages when manipulating page contents.  The long tolerated
> just pin and mark dirty is officially a bug now.

Great to know that.

>
>> I'm talking about the iomap_set_range_dirty() call inside
>> iomap_writepage_map(), for the "if (i_blocks_per_folio() > 1)" branch.
>
> That does not set the page dirty.  It propagatates the per-folio
> dirty state into the per-block dirty bitmap allocated in the line
> above.

I know, I just want to say it's marking the whole folio to be written back.

And in that out-of-band case, shouldn't we error out now instead of
marking the range to be written back?
Especially such out-of-band folio is unstable and can lead to other
problems, like content change halfway and causing csum mismatch.

And if that's the case, I think it will be much easier for btrfs to get
rid of its cow fixup workaround.

Thanks,
Qu