Re: [PATCH v2] mm: count zeromap read and set for swapout and swapin

[Usama Arif <usamaarif642@gmail.com>]

On 02/11/2024 12:59, Barry Song wrote:
> On Sat, Nov 2, 2024 at 8:32 PM Usama Arif <usamaarif642@gmail.com> wrote:
>>
>>
>>
>> On 02/11/2024 10:12, Barry Song wrote:
>>> From: Barry Song <v-songbaohua@oppo.com>
>>>
>>> When the proportion of folios from the zero map is small, missing their
>>> accounting may not significantly impact profiling. However, it’s easy
>>> to construct a scenario where this becomes an issue—for example,
>>> allocating 1 GB of memory, writing zeros from userspace, followed by
>>> MADV_PAGEOUT, and then swapping it back in. In this case, the swap-out
>>> and swap-in counts seem to vanish into a black hole, potentially
>>> causing semantic ambiguity.
>>>
>>> We have two ways to address this:
>>>
>>> 1. Add a separate counter specifically for the zero map.
>>> 2. Continue using the current accounting, treating the zero map like
>>> a normal backend. (This aligns with the current behavior of zRAM
>>> when supporting same-page fills at the device level.)
>>>
>>> This patch adopts option 1 as pswpin/pswpout counters are that they
>>> only apply to IO done directly to the backend device (as noted by
>>> Nhat Pham).
>>>
>>> We can find these counters from /proc/vmstat (counters for the whole
>>> system) and memcg's memory.stat (counters for the interested memcg).
>>>
>>> For example:
>>>
>>> $ grep -E 'swpin_zero|swpout_zero' /proc/vmstat
>>> swpin_zero 1648
>>> swpout_zero 33536
>>>
>>> $ grep -E 'swpin_zero|swpout_zero' /sys/fs/cgroup/system.slice/memory.stat
>>> swpin_zero 3905
>>> swpout_zero 3985
>>>
>>> Fixes: 0ca0c24e3211 ("mm: store zero pages to be swapped out in a bitmap")
>> I don't think its a hotfix (or even a fix). It was discussed in the initial
>> series to add these as a follow up and Joshua was going to do this soon.
>> Its not fixing any bug in the initial series.
> 
> I would prefer that all kernel versions with zeromap include this
> counter; otherwise,
> it could be confusing to determine where swap-in and swap-out have occurred,
> as shown by the small program below:
> 
> p =malloc(1g);
> write p to zero
> madvise_pageout
> read p;
> 
> Previously, there was 1GB of swap-in and swap-out activity reported, but
> now nothing is shown.
> 
> I don't mean to suggest that there's a bug in the zeromap code; rather,
> having this counter would help clear up any confusion.
> 
> I didn't realize Joshua was handling it. Is he still planning to? If
> so, I can leave it
> with Joshua if that was the plan :-)
> 

Please do continue with this patch, I think he was going to look at the
swapped_zero version that we discussed earlier anyways. Will let Joshua
comment on it.

>>
>>> Cc: Usama Arif <usamaarif642@gmail.com>
>>> Cc: Chengming Zhou <chengming.zhou@linux.dev>
>>> Cc: Yosry Ahmed <yosryahmed@google.com>
>>> Cc: Nhat Pham <nphamcs@gmail.com>
>>> Cc: Johannes Weiner <hannes@cmpxchg.org>
>>> Cc: David Hildenbrand <david@redhat.com>
>>> Cc: Hugh Dickins <hughd@google.com>
>>> Cc: Matthew Wilcox (Oracle) <willy@infradead.org>
>>> Cc: Shakeel Butt <shakeel.butt@linux.dev>
>>> Cc: Andi Kleen <ak@linux.intel.com>
>>> Cc: Baolin Wang <baolin.wang@linux.alibaba.com>
>>> Cc: Chris Li <chrisl@kernel.org>
>>> Cc: "Huang, Ying" <ying.huang@intel.com>
>>> Cc: Kairui Song <kasong@tencent.com>
>>> Cc: Ryan Roberts <ryan.roberts@arm.com>
>>> Signed-off-by: Barry Song <v-songbaohua@oppo.com>
>>> ---
>>>  -v2:
>>>  * add separate counters rather than using pswpin/out; thanks
>>>  for the comments from Usama, David, Yosry and Nhat;
>>>  * Usama also suggested a new counter like swapped_zero, I
>>>  prefer that one be separated as an enhancement patch not
>>>  a hotfix. will probably handle it later on.
>>>
>> I dont think either of them would be a hotfix.
> 
> As mentioned above, this isn't about fixing a bug; it's simply to ensure
> that swap-related metrics don't disappear.
> 
>>
>>>  Documentation/admin-guide/cgroup-v2.rst | 10 ++++++++++
>>>  include/linux/vm_event_item.h           |  2 ++
>>>  mm/memcontrol.c                         |  4 ++++
>>>  mm/page_io.c                            | 16 ++++++++++++++++
>>>  mm/vmstat.c                             |  2 ++
>>>  5 files changed, 34 insertions(+)
>>>
>>> diff --git a/Documentation/admin-guide/cgroup-v2.rst b/Documentation/admin-guide/cgroup-v2.rst
>>> index db3799f1483e..984eb3c9d05b 100644
>>> --- a/Documentation/admin-guide/cgroup-v2.rst
>>> +++ b/Documentation/admin-guide/cgroup-v2.rst
>>> @@ -1599,6 +1599,16 @@ The following nested keys are defined.
>>>         pglazyfreed (npn)
>>>               Amount of reclaimed lazyfree pages
>>>
>>> +       swpin_zero
>>> +             Number of pages moved into memory with zero content, meaning no
>>> +             copy exists in the backend swapfile, allowing swap-in to avoid
>>> +             I/O read overhead.
>>> +
>>> +       swpout_zero
>>> +             Number of pages moved out of memory with zero content, meaning no
>>> +             copy is needed in the backend swapfile, allowing swap-out to avoid
>>> +             I/O write overhead.
>>> +
>>
>> Maybe zero-filled pages might be a better term in both.
> 
> Do you mean dropping "with zero content" and replacing it by
> Number of zero-filled pages moved out of memory ? I'm fine
> with the change.

Yes, mainly because if you do swapout of memory that was memset 0 its still
content, just zero-filled.

Thanks,
Usama
> 
>>
>>>         zswpin
>>>               Number of pages moved in to memory from zswap.
>>>
>>> diff --git a/include/linux/vm_event_item.h b/include/linux/vm_event_item.h
>>> index aed952d04132..f70d0958095c 100644
>>> --- a/include/linux/vm_event_item.h
>>> +++ b/include/linux/vm_event_item.h
>>> @@ -134,6 +134,8 @@ enum vm_event_item { PGPGIN, PGPGOUT, PSWPIN, PSWPOUT,
>>>  #ifdef CONFIG_SWAP
>>>               SWAP_RA,
>>>               SWAP_RA_HIT,
>>> +             SWPIN_ZERO,
>>> +             SWPOUT_ZERO,
>>>  #ifdef CONFIG_KSM
>>>               KSM_SWPIN_COPY,
>>>  #endif
>>> diff --git a/mm/memcontrol.c b/mm/memcontrol.c
>>> index 5e44d6e7591e..7b3503d12aaf 100644
>>> --- a/mm/memcontrol.c
>>> +++ b/mm/memcontrol.c
>>> @@ -441,6 +441,10 @@ static const unsigned int memcg_vm_event_stat[] = {
>>>       PGDEACTIVATE,
>>>       PGLAZYFREE,
>>>       PGLAZYFREED,
>>> +#ifdef CONFIG_SWAP
>>> +     SWPIN_ZERO,
>>> +     SWPOUT_ZERO,
>>> +#endif
>>>  #ifdef CONFIG_ZSWAP
>>>       ZSWPIN,
>>>       ZSWPOUT,
>>> diff --git a/mm/page_io.c b/mm/page_io.c
>>> index 5d9b6e6cf96c..4b4ea8e49cf6 100644
>>> --- a/mm/page_io.c
>>> +++ b/mm/page_io.c
>>> @@ -204,7 +204,9 @@ static bool is_folio_zero_filled(struct folio *folio)
>>>
>>>  static void swap_zeromap_folio_set(struct folio *folio)
>>>  {
>>> +     struct obj_cgroup *objcg = get_obj_cgroup_from_folio(folio);
>>>       struct swap_info_struct *sis = swp_swap_info(folio->swap);
>>> +     int nr_pages = folio_nr_pages(folio);
>>>       swp_entry_t entry;
>>>       unsigned int i;
>>>
>>> @@ -212,6 +214,12 @@ static void swap_zeromap_folio_set(struct folio *folio)
>>>               entry = page_swap_entry(folio_page(folio, i));
>>>               set_bit(swp_offset(entry), sis->zeromap);
>>>       }
>>> +
>>> +     count_vm_events(SWPOUT_ZERO, nr_pages);
>>> +     if (objcg) {
>>> +             count_objcg_events(objcg, SWPOUT_ZERO, nr_pages);
>>> +             obj_cgroup_put(objcg);
>>> +     }
>>>  }
>>>
>>>  static void swap_zeromap_folio_clear(struct folio *folio)
>>> @@ -507,6 +515,7 @@ static void sio_read_complete(struct kiocb *iocb, long ret)
>>>  static bool swap_read_folio_zeromap(struct folio *folio)
>>>  {
>>>       int nr_pages = folio_nr_pages(folio);
>>> +     struct obj_cgroup *objcg;
>>>       bool is_zeromap;
>>>
>>>       /*
>>> @@ -521,6 +530,13 @@ static bool swap_read_folio_zeromap(struct folio *folio)
>>>       if (!is_zeromap)
>>>               return false;
>>>
>>> +     objcg = get_obj_cgroup_from_folio(folio);
>>> +     count_vm_events(SWPIN_ZERO, nr_pages);
>>> +     if (objcg) {
>>> +             count_objcg_events(objcg, SWPIN_ZERO, nr_pages);
>>> +             obj_cgroup_put(objcg);
>>> +     }
>>> +
>>>       folio_zero_range(folio, 0, folio_size(folio));
>>>       folio_mark_uptodate(folio);
>>>       return true;
>>> diff --git a/mm/vmstat.c b/mm/vmstat.c
>>> index 22a294556b58..c8ef7352f9ed 100644
>>> --- a/mm/vmstat.c
>>> +++ b/mm/vmstat.c
>>> @@ -1418,6 +1418,8 @@ const char * const vmstat_text[] = {
>>>  #ifdef CONFIG_SWAP
>>>       "swap_ra",
>>>       "swap_ra_hit",
>>> +     "swpin_zero",
>>> +     "swpout_zero",
>>>  #ifdef CONFIG_KSM
>>>       "ksm_swpin_copy",
>>>  #endif
>>
> 
> Thanks
> Barry