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

[Minchan Kim <minchan@kernel.org>]

On Fri, May 31, 2019 at 04:03:32PM +0200, Michal Hocko wrote:
> 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?

Because page table in one of process shared the page would have access bit
so finally we couldn't reclaim the page. The more process it is shared,
the more fail to reclaim.

> 
> > > 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.

Sure.

> 
> > 
> > > 
> > > 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.

IIRC, the problem of MADV_FREE was that we couldn't discard freeable
pages because VM never scan anonymous LRU with swapless system.
However, it's not the our case because we should reclaim them, not
discarding.

I will include it in the description.

Thanks.