Re: [resend][PATCH] mm, vmscan: fix do_try_to_free_pages() livelock

[Kamezawa Hiroyuki]

(2012/06/18 9:43), Minchan Kim wrote:
> On 06/17/2012 02:48 AM, Aaditya Kumar wrote:
>
> > On Fri, Jun 15, 2012 at 12:57 PM, Minchan Kim<minchan@xxxxxxxxxx>  wrote:
> >
> > > > pgdat_balanced() doesn't recognized zone. Therefore kswapd may sleep
> > > > if node has multiple zones. Hm ok, I realized my descriptions was
> > > > slightly misleading. priority 0 is not needed. bakance_pddat() calls
> > > > pgdat_balanced()
> > > > every priority. Most easy case is, movable zone has a lot of free pages and
> > > > normal zone has no reclaimable page.
> > > >
> > > > btw, current pgdat_balanced() logic seems not correct. kswapd should
> > > > sleep only if every zones have much free pages than high water mark
> > > > _and_ 25% of present pages in node are free.
> > >
> > >
> > > Sorry. I can't understand your point.
> > > Current kswapd doesn't sleep if relevant zones don't have free pages above high watermark.
> > > It seems I am missing your point.
> > > Please anybody correct me.
> >
> > Since currently direct reclaim is given up based on
> > zone->all_unreclaimable flag,
> > so for e.g in one of the scenarios:
> >
> > Lets say system has one node with two zones (NORMAL and MOVABLE) and we
> > hot-remove the all the pages of the MOVABLE zone.
> >
> > While migrating pages during memory hot-unplugging, the allocation function
> > (for new page to which the page in MOVABLE zone would be moved)  can end up
> > looping in direct reclaim path for ever.
> >
> > This is so because when most of the pages in the MOVABLE zone have
> > been migrated,
> > the zone now contains lots of free memory (basically above low watermark)
> > BUT all are in MIGRATE_ISOLATE list of the buddy list.
> >
> > So kswapd() would not balance this zone as free pages are above low watermark
> > (but all are in isolate list). So zone->all_unreclaimable flag would
> > never be set for this zone
> > and allocation function would end up looping forever. (assuming the
> > zone NORMAL is
> > left with no reclaimable memory)
>
>
> Thanks a lot, Aaditya! Scenario you mentioned makes perfect.
> But I don't see it's a problem of kswapd.
>
> a5d76b54 made new migration type 'MIGRATE_ISOLATE' which is very irony type because there are many free pages in free list
> but we can't allocate it. :(
> It doesn't reflect right NR_FREE_PAGES while many places in the kernel use NR_FREE_PAGES to trigger some operation.
> Kswapd is just one of them confused.
> As right fix of this problem, we should fix hot plug code, IMHO which can fix CMA, too.
>
> This patch could make inconsistency between NR_FREE_PAGES and SumOf[free_area[order].nr_free]
> and it could make __zone_watermark_ok confuse so we might need to fix move_freepages_block itself to reflect
> free_area[order].nr_free exactly.
>
> Any thought?
>
> Side Note: I still need KOSAKI's patch with fixed description regardless of this problem because set zone->all_unreclaimable of only kswapd is very fragile.
>
> diff --git a/mm/page_alloc.c b/mm/page_alloc.c
> index 4403009..19de56c 100644
> --- a/mm/page_alloc.c
> +++ b/mm/page_alloc.c
> @@ -5593,8 +5593,10 @@ int set_migratetype_isolate(struct page *page)
>
>   out:
>          if (!ret) {
> +               int pages_moved;
>                  set_pageblock_migratetype(page, MIGRATE_ISOLATE);
> -               move_freepages_block(zone, page, MIGRATE_ISOLATE);
> +               pages_moved = move_freepages_block(zone, page, MIGRATE_ISOLATE);
> +               __mod_zone_page_state(zone, NR_FREE_PAGES, -pages_moved);
>          }
>
>          spin_unlock_irqrestore(&zone->lock, flags);
> @@ -5607,12 +5609,14 @@ void unset_migratetype_isolate(struct page *page, unsigned migratetype)
>   {
>          struct zone *zone;
>          unsigned long flags;
> +       int pages_moved;
>          zone = page_zone(page);
>          spin_lock_irqsave(&zone->lock, flags);
>          if (get_pageblock_migratetype(page) != MIGRATE_ISOLATE)
>                  goto out;
>          set_pageblock_migratetype(page, migratetype);
> -       move_freepages_block(zone, page, migratetype);
> +       pages_moved = move_freepages_block(zone, page, migratetype);
> +       __mod_zone_page_state(zone, NR_FREE_PAGES, pages_moved);
>   out:
>          spin_unlock_irqrestore(&zone->lock, flags);
>   }


I think this patch is very good.

Thanks,
-Kame


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