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

[Pavel Begunkov <asml.silence@gmail.com>]

On 10/12/2020 13:18, Johannes Weiner wrote:
> 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?

I was thinking that easier would be inverted 3, i.e. letting add_page
with the annotation be and use a special version of it for direct IO.
IIRC we only to change bio_iov_iter_get_pages() + its helpers for that.

-- 
Pavel Begunkov