Re: [PATCH 2/2] block: no-copy bvec for direct IO

[Johannes Weiner]

On Wed, Dec 09, 2020 at 08:40:05AM +0000, Christoph Hellwig wrote:
> > +	/*
> > +	 * In practice groups of pages tend to be accessed/reclaimed/refaulted
> > +	 * together. To not go over bvec for those who didn't set BIO_WORKINGSET
> > +	 * approximate it by looking at the first page and inducing it to the
> > +	 * whole bio
> > +	 */
> > +	if (unlikely(PageWorkingset(iter->bvec->bv_page)))
> > +		bio_set_flag(bio, BIO_WORKINGSET);
> 
> IIRC the feedback was that we do not need to deal with BIO_WORKINGSET
> at all for direct I/O.

Yes, this hunk is incorrect. We must not use this flag for direct IO.
It's only for paging IO, when you bring in the data at page->mapping +
page->index. Otherwise you tell the pressure accounting code that you
are paging in a thrashing page, when really you're just reading new
data into a page frame that happens to be hot.

(As per the other thread, bio_add_page() currently makes that same
mistake for direct IO. I'm fixing that.)