linux-kernel - Re: [RFC 0/7] introduce memory hinting API for external process

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Message-ID: <20190520164605.GA11665@cmpxchg.org>
Date:   Mon, 20 May 2019 12:46:05 -0400
From:   Johannes Weiner <hannes@...xchg.org>
To:     Minchan Kim <minchan@...nel.org>
Cc:     Andrew Morton <akpm@...ux-foundation.org>,
        LKML <linux-kernel@...r.kernel.org>,
        linux-mm <linux-mm@...ck.org>, Michal Hocko <mhocko@...e.com>,
        Tim Murray <timmurray@...gle.com>,
        Joel Fernandes <joel@...lfernandes.org>,
        Suren Baghdasaryan <surenb@...gle.com>,
        Daniel Colascione <dancol@...gle.com>,
        Shakeel Butt <shakeelb@...gle.com>,
        Sonny Rao <sonnyrao@...gle.com>,
        Brian Geffon <bgeffon@...gle.com>
Subject: Re: [RFC 0/7] introduce memory hinting API for external process

On Mon, May 20, 2019 at 12:52:47PM +0900, Minchan Kim wrote:
> - Approach
> 
> The approach we chose was to use a new interface to allow userspace to
> proactively reclaim entire processes by leveraging platform information.
> This allowed us to bypass the inaccuracy of the kernel’s LRUs for pages
> that are known to be cold from userspace and to avoid races with lmkd
> by reclaiming apps as soon as they entered the cached state. Additionally,
> it could provide many chances for platform to use much information to
> optimize memory efficiency.
> 
> IMHO we should spell it out that this patchset complements MADV_WONTNEED
> and MADV_FREE by adding non-destructive ways to gain some free memory
> space. MADV_COLD is similar to MADV_WONTNEED in a way that it hints the
> kernel that memory region is not currently needed and should be reclaimed
> immediately; MADV_COOL is similar to MADV_FREE in a way that it hints the
> kernel that memory region is not currently needed and should be reclaimed
> when memory pressure rises.

I agree with this approach and the semantics. But these names are very
vague and extremely easy to confuse since they're so similar.

MADV_COLD could be a good name, but for deactivating pages, not
reclaiming them - marking memory "cold" on the LRU for later reclaim.

For the immediate reclaim one, I think there is a better option too:
In virtual memory speak, putting a page into secondary storage (or
ensuring it's already there), and then freeing its in-memory copy, is
called "paging out". And that's what this flag is supposed to do. So
how about MADV_PAGEOUT?

With that, we'd have:

MADV_FREE: Mark data invalid, free memory when needed
MADV_DONTNEED: Mark data invalid, free memory immediately

MADV_COLD: Data is not used for a while, free memory when needed
MADV_PAGEOUT: Data is not used for a while, free memory immediately

What do you think?