On Fri, May 19, 2023 at 08:48:37PM +0530, Ritesh Harjani wrote: > Christoph Hellwig <hch@xxxxxxxxxxxxx> 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@xxxxxxxxxxxxxxxxxxxx/ > > 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.
