Re: [RFC 0/6]mm: add new LRU list for MADV_FREE pages

[Michal Hocko]

On Tue 31-01-17 16:38:10, Johannes Weiner wrote:
> On Tue, Jan 31, 2017 at 11:45:47AM -0800, Shaohua Li wrote:
> > On Tue, Jan 31, 2017 at 01:59:49PM -0500, Johannes Weiner wrote:
> > > Hi Shaohua,
> > > 
> > > On Sun, Jan 29, 2017 at 09:51:17PM -0800, Shaohua Li wrote:
> > > > We are trying to use MADV_FREE in jemalloc. Several issues are found. Without
> > > > solving the issues, jemalloc can't use the MADV_FREE feature.
> > > > - Doesn't support system without swap enabled. Because if swap is off, we can't
> > > >   or can't efficiently age anonymous pages. And since MADV_FREE pages are mixed
> > > >   with other anonymous pages, we can't reclaim MADV_FREE pages. In current
> > > >   implementation, MADV_FREE will fallback to MADV_DONTNEED without swap enabled.
> > > >   But in our environment, a lot of machines don't enable swap. This will prevent
> > > >   our setup using MADV_FREE.
> > > > - Increases memory pressure. page reclaim bias file pages reclaim against
> > > >   anonymous pages. This doesn't make sense for MADV_FREE pages, because those
> > > >   pages could be freed easily and refilled with very slight penality. Even page
> > > >   reclaim doesn't bias file pages, there is still an issue, because MADV_FREE
> > > >   pages and other anonymous pages are mixed together. To reclaim a MADV_FREE
> > > >   page, we probably must scan a lot of other anonymous pages, which is
> > > >   inefficient. In our test, we usually see oom with MADV_FREE enabled and nothing
> > > >   without it.
> > > 
> > > Fully agreed, the anon LRU is a bad place for these pages.
> > > 
> > > > For the first two issues, introducing a new LRU list for MADV_FREE pages could
> > > > solve the issues. We can directly reclaim MADV_FREE pages without writting them
> > > > out to swap, so the first issue could be fixed. If only MADV_FREE pages are in
> > > > the new list, page reclaim can easily reclaim such pages without interference
> > > > of file or anonymous pages. The memory pressure issue will disappear.
> > > 
> > > Do we actually need a new page flag and a special LRU for them? These
> > > pages are basically like clean cache pages at that point. What do you
> > > think about clearing their PG_swapbacked flag on MADV_FREE and moving
> > > them to the inactive file list? The way isolate+putback works should
> > > not even need much modification, something like clear_page_mlock().
> > > 
> > > When the reclaim scanner finds anon && dirty && !swapbacked, it can
> > > again set PG_swapbacked and goto keep_locked to move the page back
> > > into the anon LRU to get reclaimed according to swapping rules.
> > 
> > Interesting idea! Not sure though, the MADV_FREE pages are actually anonymous
> > pages, this will introduce confusion. On the other hand, if the MADV_FREE pages
> > are mixed with inactive file pages, page reclaim need to reclaim a lot of file
> > pages first before reclaim the MADV_FREE pages. This doesn't look good. The
> > point of a separate LRU is to avoid scan other anon/file pages.
> 
> The LRU code and the rest of VM already use independent page type
> distinctions. That's because shmem pages are !PageAnon - they have a
> page->mapping that points to a real address space, not an anon_vma -
> but they are swapbacked and thus go through the anon LRU. This would
> just do the reverse: put PageAnon pages on the file LRU when they
> don't contain valid data and are thus not swapbacked.
> 
> As far as mixing with inactive file pages goes, it'd be possible to
> link the MADV_FREE pages to the tail of the inactive list, rather than
> the head. That said, I'm not sure reclaiming use-once filesystem cache
> before MADV_FREE is such a bad policy. MADV_FREE retains the vmas for
> the sole purpose of reusing them in the (near) future. That is
> actually a stronger reuse signal than we have for use-once file pages.
> If somebody does continuous writes to a logfile or a one-off search
> through one or more files, we should actually reclaim that cache
> before we go after MADV_FREE pages that are temporarily invalidated.

I completely agree here. LRU_*_FILE will be a bit misnomer (LRU_*CACHE
would sound more appropriate). I expect there would be few places which
account based on the LRU list but those shouldn't be that hard to fix.
-- 
Michal Hocko
SUSE Labs