linux-kernel - Re: [PATCH V3 1/1] mm: add swapiness= arg to memory.reclaim

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Date:   Tue, 12 Dec 2023 16:27:09 -0500
From:   Dan Schatzberg <schatzberg.dan@...il.com>
To:     Yosry Ahmed <yosryahmed@...gle.com>
Cc:     Johannes Weiner <hannes@...xchg.org>,
        Roman Gushchin <roman.gushchin@...ux.dev>,
        Huan Yang <link@...o.com>, linux-kernel@...r.kernel.org,
        cgroups@...r.kernel.org, linux-mm@...ck.org,
        Tejun Heo <tj@...nel.org>, Zefan Li <lizefan.x@...edance.com>,
        Jonathan Corbet <corbet@....net>,
        Michal Hocko <mhocko@...nel.org>,
        Shakeel Butt <shakeelb@...gle.com>,
        Muchun Song <muchun.song@...ux.dev>,
        Andrew Morton <akpm@...ux-foundation.org>,
        David Hildenbrand <david@...hat.com>,
        Matthew Wilcox <willy@...radead.org>,
        Chris Li <chrisl@...nel.org>,
        Kefeng Wang <wangkefeng.wang@...wei.com>,
        "Vishal Moola (Oracle)" <vishal.moola@...il.com>,
        Yue Zhao <findns94@...il.com>, Hugh Dickins <hughd@...gle.com>
Subject: Re: [PATCH V3 1/1] mm: add swapiness= arg to memory.reclaim

On Mon, Dec 11, 2023 at 11:41:24AM -0800, Yosry Ahmed wrote:
> On Mon, Dec 11, 2023 at 6:04 AM Dan Schatzberg <schatzberg.dan@...il.com> wrote:
> 
> contains* the*
> 
> I think this statement was only important because no keys were
> supported, so I think we can remove it completely and rely on
> documenting the supported keys below like other interfaces, see my
> next comment.
> 
> > +       to reclaim.
> >
> >         Example::
> >
> > @@ -1304,6 +1304,17 @@ PAGE_SIZE multiple when read back.
> >         This means that the networking layer will not adapt based on
> >         reclaim induced by memory.reclaim.
> >
> > +       This file also allows the user to specify the swappiness value
> > +       to be used for the reclaim. For example:
> > +
> > +         echo "1G swappiness=60" > memory.reclaim
> > +
> > +       The above instructs the kernel to perform the reclaim with
> > +       a swappiness value of 60. Note that this has the same semantics
> > +       as the vm.swappiness sysctl - it sets the relative IO cost of
> > +       reclaiming anon vs file memory but does not allow for reclaiming
> > +       specific amounts of anon or file memory.
> > +
> 
> Can we instead follow the same format used by other nested-keyed files
> (e.g. io.max)? This usually involves a table of supported keys and
> such.

Thanks, both are good suggestions. Will address these.

> > +       while ((start = strsep(&buf, " ")) != NULL) {
> > +               if (!strlen(start))
> > +                       continue;
> > +               switch (match_token(start, if_tokens, args)) {
> > +               case MEMORY_RECLAIM_SWAPPINESS:
> > +                       if (match_int(&args[0], &swappiness))
> > +                               return -EINVAL;
> > +                       if (swappiness < 0 || swappiness > 200)
> 
> I am not a fan of extending the hardcoded 0 and 200 values, and now
> the new -1 value. Maybe it's time to create constants for the min and
> max swappiness values instead of hardcoding them everywhere? This can
> be a separate preparatory patch. Then, -1 (or any invalid value) can
> also be added as a constant with a useful name, instead of passing -1
> to all other callers.
> 
> This should make the code a little bit more readable and easier to extend.

I'm not sure I understand the concern. This check just validates that
the swappiness value inputted is between 0 and 200 (inclusive)
otherwise the interface returns -EINVAL. Are you just concerned that
these constants are not named explicitly so they can be reused
elsewhere in the code?