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

[Michal Hocko <mhocko@kernel.org>]

On Tue 04-06-19 08:02:05, Minchan Kim wrote:
> Hi Johannes,
> 
> On Mon, Jun 03, 2019 at 05:50:59PM -0400, Johannes Weiner wrote:
> > On Mon, Jun 03, 2019 at 10:32:30PM +0200, Michal Hocko wrote:
> > > On Mon 03-06-19 13:27:17, Johannes Weiner wrote:
> > > > On Mon, Jun 03, 2019 at 09:16:07AM +0200, Michal Hocko wrote:
> > > > > On Fri 31-05-19 23:34:07, Minchan Kim wrote:
> > > > > > 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.
> > > > > 
> > > > > So what? In other words why should it be restricted solely based on the
> > > > > map count. I can see a reason to restrict based on the access
> > > > > permissions because we do not want to simplify all sorts of side channel
> > > > > attacks but memory reclaim is capable of reclaiming shared pages and so
> > > > > far I haven't heard any sound argument why madvise should skip those.
> > > > > Again if there are any reasons, then document them in the changelog.
> > > > 
> > > > I think it makes sense. It could be explained, but it also follows
> > > > established madvise semantics, and I'm not sure it's necessarily
> > > > Minchan's job to re-iterate those.
> > > > 
> > > > Sharing isn't exactly transparent to userspace. The kernel does COW,
> > > > ksm etc. When you madvise, you can really only speak for your own
> > > > reference to that memory - "*I* am not using this."
> > > > 
> > > > This is in line with other madvise calls: MADV_DONTNEED clears the
> > > > local page table entries and drops the corresponding references, so
> > > > shared pages won't get freed. MADV_FREE clears the pte dirty bit and
> > > > also has explicit mapcount checks before clearing PG_dirty, so again
> > > > shared pages don't get freed.
> > > 
> > > Right, being consistent with other madvise syscalls is certainly a way
> > > to go. And I am not pushing one way or another, I just want this to be
> > > documented with a reasoning behind. Consistency is certainly an argument
> > > to use.
> > > 
> > > On the other hand these non-destructive madvise operations are quite
> > > different and the shared policy might differ as a result as well. We are
> > > aging objects rather than destroying them after all. Being able to age
> > > a pagecache with a sufficient privileges sounds like a useful usecase to
> > > me. In other words you are able to cause the same effect indirectly
> > > without the madvise operation so it kinda makes sense to allow it in a
> > > more sophisticated way.
> > 
> > Right, I don't think it's about permission - as you say, you can do
> > this indirectly. Page reclaim is all about relative page order, so if
> > we thwarted you from demoting some pages, you could instead promote
> > other pages to cause a similar end result.
> > 
> > I think it's about intent. You're advising the kernel that *you're*
> > not using this memory and would like to have it cleared out based on
> > that knowledge. You could do the same by simply allocating the new
> > pages and have the kernel sort it out. However, if the kernel sorts it
> > out, it *will* look at other users of the page, and it might decide
> > that other pages are actually colder when considering all users.
> > 
> > When you ignore shared state, on the other hand, the pages you advise
> > out could refault right after. And then, not only did you not free up
> > the memory, but you also caused IO that may interfere with bringing in
> > the new data for which you tried to create room in the first place.
> > 
> > So I don't think it ever makes sense to override it.
> > 
> > But it might be better to drop the explicit mapcount check and instead
> > make the local pte young and call shrink_page_list() without the
>                      ^
>                      old?
> 
> > TTU_IGNORE_ACCESS, ignore_references flags - leave it to reclaim code
> > to handle references and shared pages exactly the same way it would if
> > those pages came fresh off the LRU tail, excluding only the reference
> > from the mapping that we're madvising.
> 
> You are confused from the name change. Here, MADV_COLD is deactivating
> , not pageing out. Therefore, shrink_page_list doesn't matter.
> And madvise_cold_pte_range already makes the local pte *old*(I guess
> your saying was typo).
> I guess that's exactly what Michal wanted: just removing page_mapcount
> check and defers to decision on normal page reclaim policy:
> If I didn't miss your intention, it seems you and Michal are on same page.
> (Please correct me if you want to say something other)

Indeed.

> I could drop the page_mapcount check at next revision.

Yes please.
-- 
Michal Hocko
SUSE Labs