Re: [RFCv2 1/6] mm: introduce MADV_COLD

[Michal Hocko <mhocko@kernel.org>]

On Fri 31-05-19 22:39:04, Minchan Kim wrote:
> On Fri, May 31, 2019 at 10:47:52AM +0200, Michal Hocko wrote:
> > On Fri 31-05-19 15:43:08, Minchan Kim wrote:
> > > When a process expects no accesses to a certain memory range, it could
> > > give a hint to 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_COLD hint to madvise(2) syscall.
> > > MADV_COLD 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.
> > > 
> > > Internally, it works via deactivating pages from active list to inactive's
> > > head if the page is private because inactive list could be full of
> > > used-once pages which are first candidate for the reclaiming and that's a
> > > reason why MADV_FREE move pages to head of inactive LRU list. Therefore,
> > > if the memory pressure happens, they will be reclaimed earlier than other
> > > active pages unless there is no access until the time.
> > 
> > [I am intentionally not looking at the implementation because below
> > points should be clear from the changelog - sorry about nagging ;)]
> > 
> > What kind of pages can be deactivated? Anonymous/File backed.
> > Private/shared? If shared, are there any restrictions?
> 
> Both file and private pages could be deactived from each active LRU
> to each inactive LRU if the page has one map_count. In other words,
> 
>     if (page_mapcount(page) <= 1)
>         deactivate_page(page);

Why do we restrict to pages that are single mapped?

> > Are there any restrictions on mappings? E.g. what would be an effect of
> > this operation on hugetlbfs mapping?
> 
> VM_LOCKED|VM_HUGETLB|VM_PFNMAP vma will be skipped like MADV_FREE|DONTNEED

OK documenting that this is restricted to the same vmas as MADV_FREE|DONTNEED
is really useful to mention.

> 
> > 
> > Also you are talking about inactive LRU but what kind of LRU is that? Is
> > it the anonymous LRU? If yes, don't we have the same problem as with the
> 
> active file page -> inactive file LRU
> active anon page -> inacdtive anon LRU
> 
> > early MADV_FREE implementation when enough page cache causes that
> > deactivated anonymous memory doesn't get reclaimed anytime soon. Or
> > worse never when there is no swap available?
> 
> I think MADV_COLD is a little bit different symantic with MAVD_FREE.
> MADV_FREE means it's okay to discard when the memory pressure because
> the content of the page is *garbage*. Furthemore, freeing such pages is
> almost zero overhead since we don't need to swap out and access
> afterward causes minor fault. Thus, it would make sense to put those
> freeable pages in inactive file LRU to compete other used-once pages.
> 
> However, MADV_COLD doesn't means it's a garbage and freeing requires
> swap out/swap in afterward. So, it would be better to move inactive
> anon's LRU list, not file LRU. Furthermore, it would avoid unnecessary
> scanning of those cold anonymous if system doesn't have a swap device.

Please document this, if this is really a desirable semantic because
then you have the same set of problems as we've had with the early
MADV_FREE implementation mentioned above.

-- 
Michal Hocko
SUSE Labs