Re: [RFCv5 4/5] iomap: Allocate iop in ->write_begin() early

[Christoph Hellwig <hch@infradead.org>]

On Mon, May 08, 2023 at 12:57:59AM +0530, Ritesh Harjani (IBM) wrote:
> Earlier when the folio is uptodate, we only allocate iop at writeback

s/Earlier/Currently/ ?

> time (in iomap_writepage_map()). This is ok until now, but when we are
> going to add support for per-block dirty state bitmap in iop, this
> could cause some performance degradation. The reason is that if we don't
> allocate iop during ->write_begin(), then we will never mark the
> necessary dirty bits in ->write_end() call. And we will have to mark all
> the bits as dirty at the writeback time, that could cause the same write
> amplification and performance problems as it is now.
> 
> However, for all the writes with (pos, len) which completely overlaps
> the given folio, there is no need to allocate an iop during
> ->write_begin(). So skip those cases.

This reads a bit backwards, I'd suggest to mention early
allocation only happens for sub-page writes before going into the
details.

The changes themselves looks good to me.