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Date:   Wed, 30 Mar 2022 12:17:09 -0400
From:   Johannes Weiner <hannes@cmpxchg.org>
To:     Christoph Hellwig <hch@infradead.org>
Cc:     CGEL <cgel.zte@gmail.com>, axboe@kernel.dk,
        viro@zeniv.linux.org.uk, linux-block@vger.kernel.org,
        linux-kernel@vger.kernel.org, linux-fsdevel@vger.kernel.org,
        akpm@linux-foundation.org, Yang Yang <yang.yang29@zte.com.cn>,
        Ran Xiaokai <ran.xiaokai@zte.com.cn>
Subject: Re: [PATCH] block/psi: make PSI annotations of submit_bio only work
 for file pages
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On Wed, Mar 30, 2022 at 08:54:09AM -0700, Christoph Hellwig wrote:
> On Wed, Mar 30, 2022 at 11:45:56AM -0400, Johannes Weiner wrote:
> > > FYI, I started redoing that version and I think with all the cleanups
> > > to filemap.c and the readahead code this can be done fairly nicely now:
> > > 
> > > http://git.infradead.org/users/hch/block.git/commitdiff/666abb29c6db870d3941acc5ac19e83fbc72cfd4
> > 
> > Yes, it's definitely much nicer now with the MM instantiating the
> > pages for ->readpage(s).
> > 
> > But AFAICS this breaks compressed btrfs (and erofs?) because those
> > still do additional add_to_page_cache_lru() and bio submissions.
> 
> In btrfs, add_ra_bio_pages only passed freshly allocated pages to
> add_to_page_cache_lru.  These can't really have PageWorkingSet set,
> can they?  In erofs they can also come from a local page pool, but
> I think otherwise the same applies.

It's add_to_page_cache_lru() that sets the flag.

Basically, when a PageWorkingset (hot) page gets reclaimed, the bit is
stored in the vacated tree slot. When the entry is brought back in,
add_to_page_cache_lru() transfers it to the newly allocated page.

add_to_page_cache_lru()
  filemap_add_folio()
    __filemap_add_folio(mapping, folio, index, gfp, &shadow)
      *shadow = *slot
      *slot = folio
  if (shadow)
    workingset_refault(folio, shadow)
      folio_set_workingset()