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

[Matthew Wilcox <willy@infradead.org>]

On Fri, May 19, 2023 at 08:48:37PM +0530, Ritesh Harjani wrote:
> Christoph Hellwig <hch@infradead.org> writes:
> 
> > 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.
> >
> 
> sub-page is a bit confusing here. Because we can have a large folio too
> with blocks within that folio. So we decided to go with per-block
> terminology [1].
> 
> [1]: https://lore.kernel.org/linux-xfs/ZFR%2FGuVca5nFlLYF@casper.infradead.org/
> 
> I am guessing you would like to me to re-write the above para. Is this better?
> 
> "We dont need to allocate an iop in ->write_begin() for writes where the
> position and length completely overlap with the given folio.
> Therefore, such cases are skipped."

... and reorder that paragraph to be first.