linux-kernel - Re: [PATCH] block/psi: make PSI annotations of submit

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Message-ID: <YkU49BQ4rPheOG0f@infradead.org>
Date:   Wed, 30 Mar 2022 22:15:32 -0700
From:   Christoph Hellwig <hch@...radead.org>
To:     Johannes Weiner <hannes@...xchg.org>
Cc:     Christoph Hellwig <hch@...radead.org>, CGEL <cgel.zte@...il.com>,
        axboe@...nel.dk, viro@...iv.linux.org.uk,
        linux-block@...r.kernel.org, linux-kernel@...r.kernel.org,
        linux-fsdevel@...r.kernel.org, akpm@...ux-foundation.org,
        Yang Yang <yang.yang29@....com.cn>,
        Ran Xiaokai <ran.xiaokai@....com.cn>
Subject: Re: [PATCH] block/psi: make PSI annotations of submit_bio only work
 for file pages

On Wed, Mar 30, 2022 at 12:17:09PM -0400, Johannes Weiner wrote:
> 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.

Ok.  In this case my patch didn't quite do the right thing for readahead
either.  But that does leave a question for the btrfs compressed
case, which only adds extra pages to a read to readahad a bigger
cluster size - that is these pages are not read at the request of the
VM.  Does it really make sense to do PSI accounting for them in that
case?