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SubjectRe: [patch] mm: remove sysctl to manually rescue unevictable pages
From
On Mon, Sep 26, 2011 at 5:40 PM, kautuk.c @samsung.com
<consul.kautuk@gmail.com> wrote:
> On Mon, Sep 26, 2011 at 4:59 PM, Johannes Weiner <jweiner@redhat.com> wrote:
>> On Sun, Sep 25, 2011 at 04:29:40PM +0530, Kautuk Consul wrote:
>>> write_scan_unavictable_node checks the value req returned by
>>> strict_strtoul and returns 1 if req is 0.
>>>
>>> However, when strict_strtoul returns 0, it means successful conversion
>>> of buf to unsigned long.
>>>
>>> Due to this, the function was not proceeding to scan the zones for
>>> unevictable pages even though we write a valid value to the
>>> scan_unevictable_pages sys file.
>>
>> Given that there is not a real reason for this knob (anymore) and that
>> it apparently never really worked since the day it was introduced, how
>> about we just drop all that code instead?
>>
>>        Hannes
>>
>> ---
>> From: Johannes Weiner <jweiner@redhat.com>
>> Subject: mm: remove sysctl to manually rescue unevictable pages
>>
>> At one point, anonymous pages were supposed to go on the unevictable
>> list when no swap space was configured, and the idea was to manually
>> rescue those pages after adding swap and making them evictable again.
>> But nowadays, swap-backed pages on the anon LRU list are not scanned
>> without available swap space anyway, so there is no point in moving
>> them to a separate list anymore.
>
> Is this code only for anonymous pages ?
> It seems to look at all pages in the zone both file as well as anon.
>
>>
>> The manual rescue could also be used in case pages were stranded on
>> the unevictable list due to race conditions.  But the code has been
>> around for a while now and newly discovered bugs should be properly
>> reported and dealt with instead of relying on such a manual fixup.
>
> What you say seems to be all right for anon pages, but what about file
> pages ?
> I'm not sure about how this could happen, but what if some file-system caused
> a file cache page to be set to evictable or reclaimable without
> actually removing
> that page from the unevictable list ?

What I would like to also add is that while the transition of an anon
page from and
to the unevictable lists is straight-forward, should we make the same assumption
about file cache pages ?
I am not sure about this, but could a file-system cause this kind of a problem
independent of the mlocking behaviour of a user-mode app ?

>
>>
>> Signed-off-by: Johannes Weiner <jweiner@redhat.com>
>> ---
>>  drivers/base/node.c  |    3 -
>>  include/linux/swap.h |   16 ------
>>  kernel/sysctl.c      |    7 ---
>>  mm/vmscan.c          |  130 --------------------------------------------------
>>  4 files changed, 0 insertions(+), 156 deletions(-)
>>
>> diff --git a/drivers/base/node.c b/drivers/base/node.c
>> index 9e58e71..b9d6e93 100644
>> --- a/drivers/base/node.c
>> +++ b/drivers/base/node.c
>> @@ -278,8 +278,6 @@ int register_node(struct node *node, int num, struct node *parent)
>>                sysdev_create_file(&node->sysdev, &attr_distance);
>>                sysdev_create_file_optional(&node->sysdev, &attr_vmstat);
>>
>> -               scan_unevictable_register_node(node);
>> -
>>                hugetlb_register_node(node);
>>
>>                compaction_register_node(node);
>> @@ -303,7 +301,6 @@ void unregister_node(struct node *node)
>>        sysdev_remove_file(&node->sysdev, &attr_distance);
>>        sysdev_remove_file_optional(&node->sysdev, &attr_vmstat);
>>
>> -       scan_unevictable_unregister_node(node);
>>        hugetlb_unregister_node(node);          /* no-op, if memoryless node */
>>
>>        sysdev_unregister(&node->sysdev);
>> diff --git a/include/linux/swap.h b/include/linux/swap.h
>> index b156e80..a6a9ee5 100644
>> --- a/include/linux/swap.h
>> +++ b/include/linux/swap.h
>> @@ -276,22 +276,6 @@ static inline int zone_reclaim(struct zone *z, gfp_t mask, unsigned int order)
>>  extern int page_evictable(struct page *page, struct vm_area_struct *vma);
>>  extern void scan_mapping_unevictable_pages(struct address_space *);
>>
>> -extern unsigned long scan_unevictable_pages;
>> -extern int scan_unevictable_handler(struct ctl_table *, int,
>> -                                       void __user *, size_t *, loff_t *);
>> -#ifdef CONFIG_NUMA
>> -extern int scan_unevictable_register_node(struct node *node);
>> -extern void scan_unevictable_unregister_node(struct node *node);
>> -#else
>> -static inline int scan_unevictable_register_node(struct node *node)
>> -{
>> -       return 0;
>> -}
>> -static inline void scan_unevictable_unregister_node(struct node *node)
>> -{
>> -}
>> -#endif
>> -
>>  extern int kswapd_run(int nid);
>>  extern void kswapd_stop(int nid);
>>  #ifdef CONFIG_CGROUP_MEM_RES_CTLR
>> diff --git a/kernel/sysctl.c b/kernel/sysctl.c
>> index 4f057f9..0d66092 100644
>> --- a/kernel/sysctl.c
>> +++ b/kernel/sysctl.c
>> @@ -1325,13 +1325,6 @@ static struct ctl_table vm_table[] = {
>>                .extra2         = &one,
>>        },
>>  #endif
>> -       {
>> -               .procname       = "scan_unevictable_pages",
>> -               .data           = &scan_unevictable_pages,
>> -               .maxlen         = sizeof(scan_unevictable_pages),
>> -               .mode           = 0644,
>> -               .proc_handler   = scan_unevictable_handler,
>> -       },
>>  #ifdef CONFIG_MEMORY_FAILURE
>>        {
>>                .procname       = "memory_failure_early_kill",
>> diff --git a/mm/vmscan.c b/mm/vmscan.c
>> index 7502726..c99a097 100644
>> --- a/mm/vmscan.c
>> +++ b/mm/vmscan.c
>> @@ -3396,133 +3396,3 @@ void scan_mapping_unevictable_pages(struct address_space *mapping)
>>        }
>>
>>  }
>> -
>> -/**
>> - * scan_zone_unevictable_pages - check unevictable list for evictable pages
>> - * @zone - zone of which to scan the unevictable list
>> - *
>> - * Scan @zone's unevictable LRU lists to check for pages that have become
>> - * evictable.  Move those that have to @zone's inactive list where they
>> - * become candidates for reclaim, unless shrink_inactive_zone() decides
>> - * to reactivate them.  Pages that are still unevictable are rotated
>> - * back onto @zone's unevictable list.
>> - */
>> -#define SCAN_UNEVICTABLE_BATCH_SIZE 16UL /* arbitrary lock hold batch size */
>> -static void scan_zone_unevictable_pages(struct zone *zone)
>> -{
>> -       struct list_head *l_unevictable = &zone->lru[LRU_UNEVICTABLE].list;
>> -       unsigned long scan;
>> -       unsigned long nr_to_scan = zone_page_state(zone, NR_UNEVICTABLE);
>> -
>> -       while (nr_to_scan > 0) {
>> -               unsigned long batch_size = min(nr_to_scan,
>> -                                               SCAN_UNEVICTABLE_BATCH_SIZE);
>> -
>> -               spin_lock_irq(&zone->lru_lock);
>> -               for (scan = 0;  scan < batch_size; scan++) {
>> -                       struct page *page = lru_to_page(l_unevictable);
>> -
>> -                       if (!trylock_page(page))
>> -                               continue;
>> -
>> -                       prefetchw_prev_lru_page(page, l_unevictable, flags);
>> -
>> -                       if (likely(PageLRU(page) && PageUnevictable(page)))
>> -                               check_move_unevictable_page(page, zone);
>> -
>> -                       unlock_page(page);
>> -               }
>> -               spin_unlock_irq(&zone->lru_lock);
>> -
>> -               nr_to_scan -= batch_size;
>> -       }
>> -}
>> -
>> -
>> -/**
>> - * scan_all_zones_unevictable_pages - scan all unevictable lists for evictable pages
>> - *
>> - * A really big hammer:  scan all zones' unevictable LRU lists to check for
>> - * pages that have become evictable.  Move those back to the zones'
>> - * inactive list where they become candidates for reclaim.
>> - * This occurs when, e.g., we have unswappable pages on the unevictable lists,
>> - * and we add swap to the system.  As such, it runs in the context of a task
>> - * that has possibly/probably made some previously unevictable pages
>> - * evictable.
>> - */
>> -static void scan_all_zones_unevictable_pages(void)
>> -{
>> -       struct zone *zone;
>> -
>> -       for_each_zone(zone) {
>> -               scan_zone_unevictable_pages(zone);
>> -       }
>> -}
>> -
>> -/*
>> - * scan_unevictable_pages [vm] sysctl handler.  On demand re-scan of
>> - * all nodes' unevictable lists for evictable pages
>> - */
>> -unsigned long scan_unevictable_pages;
>> -
>> -int scan_unevictable_handler(struct ctl_table *table, int write,
>> -                          void __user *buffer,
>> -                          size_t *length, loff_t *ppos)
>> -{
>> -       proc_doulongvec_minmax(table, write, buffer, length, ppos);
>> -
>> -       if (write && *(unsigned long *)table->data)
>> -               scan_all_zones_unevictable_pages();
>> -
>> -       scan_unevictable_pages = 0;
>> -       return 0;
>> -}
>> -
>> -#ifdef CONFIG_NUMA
>> -/*
>> - * per node 'scan_unevictable_pages' attribute.  On demand re-scan of
>> - * a specified node's per zone unevictable lists for evictable pages.
>> - */
>> -
>> -static ssize_t read_scan_unevictable_node(struct sys_device *dev,
>> -                                         struct sysdev_attribute *attr,
>> -                                         char *buf)
>> -{
>> -       return sprintf(buf, "0\n");     /* always zero; should fit... */
>> -}
>> -
>> -static ssize_t write_scan_unevictable_node(struct sys_device *dev,
>> -                                          struct sysdev_attribute *attr,
>> -                                       const char *buf, size_t count)
>> -{
>> -       struct zone *node_zones = NODE_DATA(dev->id)->node_zones;
>> -       struct zone *zone;
>> -       unsigned long res;
>> -       unsigned long req = strict_strtoul(buf, 10, &res);
>> -
>> -       if (!req)
>> -               return 1;       /* zero is no-op */
>> -
>> -       for (zone = node_zones; zone - node_zones < MAX_NR_ZONES; ++zone) {
>> -               if (!populated_zone(zone))
>> -                       continue;
>> -               scan_zone_unevictable_pages(zone);
>> -       }
>> -       return 1;
>> -}
>> -
>> -
>> -static SYSDEV_ATTR(scan_unevictable_pages, S_IRUGO | S_IWUSR,
>> -                       read_scan_unevictable_node,
>> -                       write_scan_unevictable_node);
>> -
>> -int scan_unevictable_register_node(struct node *node)
>> -{
>> -       return sysdev_create_file(&node->sysdev, &attr_scan_unevictable_pages);
>> -}
>> -
>> -void scan_unevictable_unregister_node(struct node *node)
>> -{
>> -       sysdev_remove_file(&node->sysdev, &attr_scan_unevictable_pages);
>> -}
>> -#endif
>> --
>> 1.7.6.2
>>
>>
>
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