Re: [RFC PATCH] mm: don't promote exclusive file folios of dying processes

[Johannes Weiner <hannes@cmpxchg.org>]

On Thu, Apr 17, 2025 at 05:54:57AM +0800, Barry Song wrote:
> On Thu, Apr 17, 2025 at 2:18 AM Johannes Weiner <hannes@cmpxchg.org> wrote:
> > Right, I'm more broadly objecting to the patch and its premise, but
> > thought the exclusive filtering would at least mitigate its downsides
> > somewhat. You raise good points that it's not as clear cut.
> >
> > IMO this is too subtle and unpredictable for everybody else. The
> > kernel can't see the future, but access locality and recent use is a
> > proven predictor. We generally don't discard access information,
> > unless the user asks us to, and that's what the madvise calls are for.
> 
> David pointed out some exceptions - the recency of dying processes might
> still be useful to new processes, particularly in cases like:
> 
>   while true; do app; done
> 
> Here, 'app' is repeatedly restarted but always maintains a single running
> instance. I agree this seems correct.
> 
> However, we can also find many cases where a dying process means its folios
> instantly become cold. For example:

Of course, there are many of them. Just like any access could be the
last one to that page for the next hour. But you don't know which ones
they are. Just like you don't know if I'm shutting down firefox
because that's enough internet for one day, or if I'm just restarting
it to clear out the 107 tabs I've lost track off.

> I agree that "access locality and recent use" is generally a good heuristic,
> but it must have some correlation (strong or weak) with the process lifecycle.

I don't agree. It's a cache shared between past, present and future
processes. The lifecycle of an individual processes is not saying much.

Unless you know something about userspace, and the exact data at hand,
that the kernel doesn't, which is why the Android usecase of MADV_COLD
or PAGEOUT for background apps makes sense to me, but generally tying
it to a process death does not.