Re: [PATCH RFC v3 3/3] iomap: bound ioend size to 4096 pages

[Christoph Hellwig <hch@infradead.org>]

On Mon, May 17, 2021 at 01:17:22PM -0400, Brian Foster wrote:
> The iomap writeback infrastructure is currently able to construct
> extremely large bio chains (tens of GBs) associated with a single
> ioend. This consolidation provides no significant value as bio
> chains increase beyond a reasonable minimum size. On the other hand,
> this does hold significant numbers of pages in the writeback
> state across an unnecessarily large number of bios because the ioend
> is not processed for completion until the final bio in the chain
> completes. Cap an individual ioend to a reasonable size of 4096
> pages (16MB with 4k pages) to avoid this condition.

Note that once we get huge page/folio support in the page cache this
sucks as we can trivially handle much larger sizes with very little
iteration.

I wonder if both this limit and the previous one should be based on the
number of pages added instead.  And in fact maybe if we only want the
limit at add to ioend time and skip the defer to workqueue part entirely.