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

[Johannes Weiner]

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 kernelâs 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?