Re: [RFC 1/7] mm: introduce MADV_COOL

[Minchan Kim <minchan@kernel.org>]

On Mon, May 20, 2019 at 10:16:21AM +0200, Michal Hocko wrote:
> [CC linux-api]

Thanks, Michal. I forgot to add it.

> 
> On Mon 20-05-19 12:52:48, Minchan Kim wrote:
> > When a process expects no accesses to a certain memory range
> > it could hint kernel that the pages can be reclaimed
> > when memory pressure happens but data should be preserved
> > for future use.  This could reduce workingset eviction so it
> > ends up increasing performance.
> > 
> > This patch introduces the new MADV_COOL hint to madvise(2)
> > syscall. MADV_COOL can be used by a process to mark a memory range
> > as not expected to be used in the near future. The hint can help
> > kernel in deciding which pages to evict early during memory
> > pressure.
> 
> I do not want to start naming fight but MADV_COOL sounds a bit
> misleading. Everybody thinks his pages are cool ;). Probably MADV_COLD
> or MADV_DONTNEED_PRESERVE.

Thanks for the suggestion. Since I got several suggestions, Let's discuss
them all at once in cover-letter.

> 
> > Internally, it works via deactivating memory from active list to
> > inactive's head so when the memory pressure happens, they will be
> > reclaimed earlier than other active pages unless there is no
> > access until the time.
> 
> Could you elaborate about the decision to move to the head rather than
> tail? What should happen to inactive pages? Should we move them to the
> tail? Your implementation seems to ignore those completely. Why?

Normally, inactive LRU could have used-once pages without any mapping
to user's address space. Such pages would be better candicate to
reclaim when the memory pressure happens. With deactivating only
active LRU pages of the process to the head of inactive LRU, we will
keep them in RAM longer than used-once pages and could have more chance
to be activated once the process is resumed.

> 
> What should happen for shared pages? In other words do we want to allow
> less privileged process to control evicting of shared pages with a more
> privileged one? E.g. think of all sorts of side channel attacks. Maybe
> we want to do the same thing as for mincore where write access is
> required.

It doesn't work with shared pages(ie, page_mapcount > 1). I will add it
in the description.

> -- 
> Michal Hocko
> SUSE Labs