linux-kernel - Re: [PATCH v2] mm: do not update memcg stats for NR_{FILE/SHMEM}

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Date: Mon, 6 May 2024 22:31:36 +0200
From: David Hildenbrand <david@...hat.com>
To: Yosry Ahmed <yosryahmed@...gle.com>
Cc: Andrew Morton <akpm@...ux-foundation.org>,
 Shakeel Butt <shakeel.butt@...ux.dev>, Johannes Weiner <hannes@...xchg.org>,
 Michal Hocko <mhocko@...nel.org>, Roman Gushchin <roman.gushchin@...ux.dev>,
 Muchun Song <muchun.song@...ux.dev>, linux-mm@...ck.org,
 cgroups@...r.kernel.org, linux-kernel@...r.kernel.org,
 syzbot+9319a4268a640e26b72b@...kaller.appspotmail.com
Subject: Re: [PATCH v2] mm: do not update memcg stats for
 NR_{FILE/SHMEM}_PMDMAPPED

On 06.05.24 22:18, Yosry Ahmed wrote:
> On Mon, May 06, 2024 at 09:50:10PM +0200, David Hildenbrand wrote:
>> On 06.05.24 21:29, Yosry Ahmed wrote:
>>> Previously, all NR_VM_EVENT_ITEMS stats were maintained per-memcg,
>>> although some of those fields are not exposed anywhere. Commit
>>> 14e0f6c957e39 ("memcg: reduce memory for the lruvec and memcg stats")
>>> changed this such that we only maintain the stats we actually expose
>>> per-memcg via a translation table.
>>>
>>> Additionally, commit 514462bbe927b ("memcg: warn for unexpected events
>>> and stats") added a warning if a per-memcg stat update is attempted for
>>> a stat that is not in the translation table. The warning started firing
>>> for the NR_{FILE/SHMEM}_PMDMAPPED stat updates in the rmap code. These
>>> stats are not maintained per-memcg, and hence are not in the translation
>>> table.
>>>
>>> Do not use __lruvec_stat_mod_folio() when updating NR_FILE_PMDMAPPED and
>>> NR_SHMEM_PMDMAPPED. Use __mod_node_page_state() instead, which updates
>>> the global per-node stats only.
>>>
>>> Reported-by: syzbot+9319a4268a640e26b72b@...kaller.appspotmail.com
>>> Closes: https://lore.kernel.org/lkml/0000000000001b9d500617c8b23c@google.com
>>> Fixes: 514462bbe927 ("memcg: warn for unexpected events and stats")
>>> Acked-by: Shakeel Butt <shakeel.butt@...ux.dev>
>>> Signed-off-by: Yosry Ahmed <yosryahmed@...gle.com>
>>> ---
>>>    mm/rmap.c | 15 +++++++++------
>>>    1 file changed, 9 insertions(+), 6 deletions(-)
>>>
>>> diff --git a/mm/rmap.c b/mm/rmap.c
>>> index 12be4241474ab..ed7f820369864 100644
>>> --- a/mm/rmap.c
>>> +++ b/mm/rmap.c
>>> @@ -1435,13 +1435,14 @@ static __always_inline void __folio_add_file_rmap(struct folio *folio,
>>>    		struct page *page, int nr_pages, struct vm_area_struct *vma,
>>>    		enum rmap_level level)
>>>    {
>>> +	pg_data_t *pgdat = folio_pgdat(folio);
>>>    	int nr, nr_pmdmapped = 0;
>>>    	VM_WARN_ON_FOLIO(folio_test_anon(folio), folio);
>>>    	nr = __folio_add_rmap(folio, page, nr_pages, level, &nr_pmdmapped);
>>>    	if (nr_pmdmapped)
>>> -		__lruvec_stat_mod_folio(folio, folio_test_swapbacked(folio) ?
>>> +		__mod_node_page_state(pgdat, folio_test_swapbacked(folio) ?
>>>    			NR_SHMEM_PMDMAPPED : NR_FILE_PMDMAPPED, nr_pmdmapped);
>>>    	if (nr)
>>>    		__lruvec_stat_mod_folio(folio, NR_FILE_MAPPED, nr);
>>> @@ -1493,6 +1494,7 @@ static __always_inline void __folio_remove_rmap(struct folio *folio,
>>>    		enum rmap_level level)
>>>    {
>>>    	atomic_t *mapped = &folio->_nr_pages_mapped;
>>> +	pg_data_t *pgdat = folio_pgdat(folio);
>>>    	int last, nr = 0, nr_pmdmapped = 0;
>>>    	bool partially_mapped = false;
>>>    	enum node_stat_item idx;
>>> @@ -1540,13 +1542,14 @@ static __always_inline void __folio_remove_rmap(struct folio *folio,
>>>    	}
>>>    	if (nr_pmdmapped) {
>>> +		/* NR_{FILE/SHMEM}_PMDMAPPED are not maintained per-memcg */
>>>    		if (folio_test_anon(folio))
>>> -			idx = NR_ANON_THPS;
>>> -		else if (folio_test_swapbacked(folio))
>>> -			idx = NR_SHMEM_PMDMAPPED;
>>> +			__lruvec_stat_mod_folio(folio, NR_ANON_THPS, -nr_pmdmapped);
>>>    		else
>>> -			idx = NR_FILE_PMDMAPPED;
>>> -		__lruvec_stat_mod_folio(folio, idx, -nr_pmdmapped);
>>> +			__mod_node_page_state(pgdat,
>>
>> folio_pgdat(folio) should fit here easily. :)
>>
>> But I would actually suggest something like the following in mm/rmap.c
> 
> I am not a big fan of this. Not because I don't like the abstraction,
> but because I think it doesn't go all the way. It abstracts a very
> certain case: updating nr_pmdmapped for file folios.
> 

Right. It only removes some of the ugliness ;)

> I think if we opt for abstracting the stats updates in mm/rmap.c, we
> should go all the way with something like the following (probably split
> as two patches: refactoring then bug fix). WDYT about the below?
> 
> diff --git a/mm/rmap.c b/mm/rmap.c
> index 12be4241474ab..70d6f6309da01 100644
> --- a/mm/rmap.c
> +++ b/mm/rmap.c
> @@ -1269,6 +1269,28 @@ static void __page_check_anon_rmap(struct folio *folio, struct page *page,
>   		       page);
>   }
>   
> +static void __foio_mod_stat(struct folio *folio, int nr, int nr_pmdmapped)
> +{
> +	int idx;
> +
> +	if (nr) {
> +		idx = folio_test_anon(folio) ? NR_ANON_MAPPED : NR_FILE_MAPPED;
> +		__lruvec_stat_mod_folio(folio, idx, nr);
> +	}
> +	if (nr_pmdmapped) {
> +		if (folio_test_anon(folio)) {
> +			idx = NR_ANON_THPS;
> +			__lruvec_stat_mod_folio(folio, idx, nr_pmdmapped);
> +		} else {
> +			/* NR_*_PMDMAPPED are not maintained per-memcg */
> +			idx = folio_test_swapbacked(folio) ?
> +				NR_SHMEM_PMDMAPPED : NR_FILE_PMDMAPPED;
> +			__mod_node_page_state(folio_pgdat(folio), idx,
> +					      nr_pmdmapped);
> +		}
> +	}
> +}
> +

I didn't suggest that, because in the _anon and _file functions we'll 
end up introducing unnecessary folio_test_anon() checks that the 
compiler cannot optimize out.

But at least in the removal path it's a clear win.

-- 
Cheers,

David / dhildenb