Re: [PATCH 6/7] mm/mglru: fix potential generation folio number leak

[Baolin Wang <baolin.wang@linux.alibaba.com>]

On 8/20/26 11:45 AM, Kairui Song wrote:
> On Thu, Aug 20, 2026 at 9:52 AM Baolin Wang
> <baolin.wang@linux.alibaba.com> wrote:
>> On 8/18/26 1:38 PM, Kairui Song via B4 Relay wrote:
>>> From: Kairui Song <kasong@tencent.com>
>>>
>>> Each generation of MGLRU accounts anon and file folio numbers
>>> separately. The page table walker's update_batch_size() derives the
>>> anon / file type of a folio from its current flags, but the page table
>>> walk holds neither the lruvec lock nor the folio lock, so the type can
>>> change during that period.
>>
>> Right.
>>
>>> MADV_FREE's lazyfree path clears PG_swapbacked under the lruvec lock,
>>> so the folio is no longer considered on the anon LRU list. Lazyfreed
>>> folios can also be changed back to the anon list again. If the flip
>>> lands between folio_update_gen()'s cmpxchg and the type read in
>>> update_batch_size(), the batched delta pair is applied to the wrong
>>> type. The anon and file generation counters then carry phantom deltas
>>> that nothing reconciles, permanently skewing lrugen->nr_pages and the
>>> reclaim budgets derived from it.
>>
>> But I think the problem occurs between update_batch_size() and
>> sort_folio(). update_batch_size() only updates the anon or file folio
>> statistics, while sort_folio() moves promoted folios to the
>> corresponding type's list:
>>
>> /* promoted */
>> if (gen != lru_gen_from_seq(lrugen->min_seq[type])) {
>>          list_move(&folio->lru, &lrugen->folios[gen][type][zone]);
>>          return true;
>> }
>>
>> If the folio's anon/file type changes between these two steps (e.g., a
>> lazyfree folio), it would lead to what you described: "The anon and file
>> generation counters then carry phantom deltas that nothing reconciles,
>> permanently skewing lrugen->nr_pages and the reclaim budgets derived
>> from it."
> 
> Actually no, the counters follow eventual consistency (note the word
> "permanently"), we are fine with a drift as long as it will eventually
> be corrected. Lazy promotions creates counter drift from the physical
> location, but that is actually fixed by the sort_folio.
> 
> Now, for the type issue, use the lazyfree case as example (I think
> that's actually the only place,), lru_lazyfree will remove the folio
> form lruvec before marking it !PG_swapbacked, so during that removal
> period, the gen bits are zero (folio's gen == -1), so any lazy
> promotion CAS won't touch the counter, and only folio_update_gen will
> do it since folio_inc_gen only handles on list folios. The
> PG_swapbacked clearing in lazyfree only happens on folio wich has gen
> == -1 (off-list). And it makes sense since update PG_swapbacked need
> to update the counter and move the folio.
> 
> And if the CAS happends before the list removal, the list removal, the
> folio is on the anon list, so the CAS is moving a folio in the anon
> list, folio_update_gen will call update_batch_size asking it to update
> the anon counters, we are fine after this commit. (Before this commit,
> the CAS is moving a folio in the anon list but update_batch_size may
> occur on file coutners). The list removal will update the anon
> counter, and the subsequent list addition will account for the right
> file counter.
> 
> And if the CAS happens after the list add, we are still fine since the
> file counter is charged and sees a file type here.

Thanks for the expalnation. Now I see the problem and I think your are 
right.

(This involves the combination of the gen counter and the folio's type, 
so it would be better to have a diagram describing the race, otherwise 
it's indeed hard to follow.)