Re: [PATCH] block: add bio_iov_iter_nvecs for figuring out nr_vecs

[Johannes Weiner <hannes@cmpxchg.org>]

Sorry, I'm only now getting back to this.

On Fri, Dec 04, 2020 at 12:48:49PM +0000, Christoph Hellwig wrote:
> On Thu, Dec 03, 2020 at 05:36:07PM -0500, Johannes Weiner wrote:
> > Correct, it's only interesting for pages under LRU management - page
> > cache and swap pages. It should not matter for direct IO.
> > 
> > The VM uses the page flag to tell the difference between cold faults
> > (empty cache startup e.g.), and thrashing pages which are being read
> > back not long after they have been reclaimed. This influences reclaim
> > behavior, but can also indicate a general lack of memory.
> 
> I really wonder if we should move setting the flag out of bio_add_page
> and into the writeback code, as it will do the wrong things for
> non-writeback I/O, that is direct I/O or its in-kernel equivalents.

Good point. When somebody does direct IO reads into a user page that
happens to have the flag set, we misattribute submission delays.

There is some background discussion from when I first submitted the
patch, which did the annotations on the writeback/page cache side:

https://lore.kernel.org/lkml/20190722201337.19180-1-hannes@cmpxchg.org/

Fragility is a concern, as this is part of the writeback code that is
spread out over several fs-specific implementations, and it's somewhat
easy to get the annotation wrong.

Some possible options I can think of:

1 open-coding the submit_bio() annotations in writeback code, like the original patch
  pros: no bio layer involvement at all - no BIO_WORKINGSET flag
  cons: lots of copy-paste code & comments

2 open-coding if (PageWorkingset()) bio_set_flag(BIO_WORKINGSET) in writeback code
  pros: slightly less complex callsite code, eliminates read check in submit_bio()
  cons: still somewhat copy-pasty (but the surrounding code is as well)

3 adding a bio_add_page_memstall() as per Dave in the original patch thread
  pros: minimal churn and self-documenting (may need a better name)
  cons: easy to incorrectly use raw bio_add_page() in writeback code

4 writeback & direct-io versions for bio_add_page()
  pros: hard to misuse
  cons: awkward interface/layering

5 flag bio itself as writeback or direct-io (BIO_BUFFERED?)
  pros: single version of bio_add_page()
  cons: easy to miss setting the flag, similar to 3

Personally, I'm torn between 2 and 5. What do you think?