Re: [PATCH v4 6/8] mm/swap: implement workingset detection for anonymous LRU

[Joonsoo Kim]

2020ë 3ì 24ì (í) ìì 2:17, Johannes Weiner <hannes@xxxxxxxxxxx>ëì ìì:
>
> On Mon, Mar 23, 2020 at 02:52:10PM +0900, js1304@xxxxxxxxx wrote:
> > From: Joonsoo Kim <iamjoonsoo.kim@xxxxxxx>
> >
> > This patch implements workingset detection for anonymous LRU.
> > All the infrastructure is implemented by the previous patches so this patch
> > just activates the workingset detection by installing/retrieving
> > the shadow entry.
> >
> > Signed-off-by: Joonsoo Kim <iamjoonsoo.kim@xxxxxxx>
> > ---
> >  include/linux/swap.h |  6 ++++++
> >  mm/memory.c          |  7 ++++++-
> >  mm/swap_state.c      | 20 ++++++++++++++++++--
> >  mm/vmscan.c          |  7 +++++--
> >  4 files changed, 35 insertions(+), 5 deletions(-)
> >
> > diff --git a/include/linux/swap.h b/include/linux/swap.h
> > index 273de48..fb4772e 100644
> > --- a/include/linux/swap.h
> > +++ b/include/linux/swap.h
> > @@ -408,6 +408,7 @@ extern struct address_space *swapper_spaces[];
> >  extern unsigned long total_swapcache_pages(void);
> >  extern void show_swap_cache_info(void);
> >  extern int add_to_swap(struct page *page);
> > +extern void *get_shadow_from_swap_cache(swp_entry_t entry);
> >  extern int add_to_swap_cache(struct page *page, swp_entry_t entry,
> >                       gfp_t gfp, void **shadowp);
> >  extern int __add_to_swap_cache(struct page *page, swp_entry_t entry);
> > @@ -566,6 +567,11 @@ static inline int add_to_swap(struct page *page)
> >       return 0;
> >  }
> >
> > +static inline void *get_shadow_from_swap_cache(swp_entry_t entry)
> > +{
> > +     return NULL;
> > +}
> > +
> >  static inline int add_to_swap_cache(struct page *page, swp_entry_t entry,
> >                                       gfp_t gfp_mask, void **shadowp)
> >  {
> > diff --git a/mm/memory.c b/mm/memory.c
> > index 5f7813a..91a2097 100644
> > --- a/mm/memory.c
> > +++ b/mm/memory.c
> > @@ -2925,10 +2925,15 @@ vm_fault_t do_swap_page(struct vm_fault *vmf)
> >                       page = alloc_page_vma(GFP_HIGHUSER_MOVABLE, vma,
> >                                                       vmf->address);
> >                       if (page) {
> > +                             void *shadow;
> > +
> >                               __SetPageLocked(page);
> >                               __SetPageSwapBacked(page);
> >                               set_page_private(page, entry.val);
> > -                             lru_cache_add_anon(page);
> > +                             shadow = get_shadow_from_swap_cache(entry);
> > +                             if (shadow)
> > +                                     workingset_refault(page, shadow);
>
> Hm, this is calling workingset_refault() on a page that isn't charged
> to a cgroup yet. That means the refault stats and inactive age counter
> will be bumped incorrectly in the root cgroup instead of the real one.

Okay.

> > +                             lru_cache_add(page);
> >                               swap_readpage(page, true);
> >                       }
> >               } else {
>
> You need to look up and remember the shadow entry at the top and call
> workingset_refault() after mem_cgroup_commit_charge() has run.

Okay. I will call workingset_refault() after charging.

I completely missed that workingset_refault() should be called after charging.
workingset_refault() in __read_swap_cache_async() also has the same problem.

> It'd be nice if we could do the shadow lookup for everybody in
> lookup_swap_cache(), but that's subject to race conditions if multiple
> faults on the same swap page happen in multiple vmas concurrently. The
> swapcache bypass scenario is only safe because it checks that there is
> a single pte under the mmap sem to prevent forking. So it looks like
> you have to bubble up the shadow entry through swapin_readahead().

The problem looks not that easy. Hmm...

In current code, there is a large time gap between the shadow entries
are poped up and the page is charged to the memcg, especially,
for readahead-ed pages. We cannot maintain the shadow entries of
the readahead-ed pages until the pages are charged.

My plan to solve this problem is propagating the charged mm to
__read_swap_cache_async(), like as file cache, charging when
the page is added on to the swap cache and calling workingset_refault()
there. Charging will only occur if:

1. faulted page
2. readahead-ed page with the shadow entry for the same memcg

Also, readahead only happens when shadow entry's memcg is the same
with the charged memcg. If not the same, it's mostly not ours so
readahead isn't needed.

Please let me know how you think of the feasibility of this idea.

Thanks.