Re: [PATCH] mm/mglru: Fix young counter undercount for large folios
From: Baolin Wang
Date: Wed Aug 12 2026 - 21:32:08 EST
On 8/13/26 9:20 AM, Barry Song wrote:
On Thu, Aug 13, 2026 at 9:09 AM Baolin Wang
<baolin.wang@xxxxxxxxxxxxxxxxx> wrote:
On 8/13/26 8:53 AM, Barry Song wrote:
On Wed, Aug 12, 2026 at 6:17 PM Baolin Wang
<baolin.wang@xxxxxxxxxxxxxxxxx> wrote:
On 8/12/26 2:59 PM, Hui Zhu wrote:
From: Hui Zhu <zhuhui@xxxxxxxxxx>
In lru_gen_look_around(), the young counter tracks the number of young
PTEs. The original folio's contribution is represented by the initial
value of young: test_and_clear_young_ptes_notify() is called on it at
function entry, and the function returns early if it is not young. In
the subsequent loop, the original folio is skipped (its accessed bits
were already cleared), so it is not double-counted.
However, young is initialized to 1 regardless of the folio size. When
the original folio is a large folio with nr PTEs, its young count is
underestimated by nr - 1. This inconsistency can cause
suitable_to_scan() to return false, preventing the PMD from being added
to the bloom filter and reducing aging accuracy for mTHP workloads.
Initialize young to nr so the original folio is accounted the same way
as other young folios in the loop (young += nr).
Signed-off-by: Hui Zhu <zhuhui@xxxxxxxxxx>
---
Good catch. Please also add the Fixes tag:
Fixes: 56e5b60b2114 ("mm: support batched checking of the young flag for
MGLRU")
With that,
Reviewed-by: Baolin Wang <baolin.wang@xxxxxxxxxxxxxxxxx>
Hi Baolin, Hui,
I am not convinced this is the correct patch. test_and_clear_young_ptes_notify()
only indicates that there is at least one young PTE among the nr PTEs;
it does not mean that all of the PTEs are young.
Am I missing something?
You are right. But I explained why this is done in my original commit
56e5b60b2114:
"
Note that we also update the 'young' counter and
'mm_stats[MM_LEAF_YOUNG]' counter with the batched count in the
lru_gen_look_around() and walk_pte_range(). However, the batched
operations may inflate these two counters, because in a large folio not
all PTEs may have been accessed. (Additionally, tracking how many PTEs
have been accessed within a large folio is not very meaningful, since
the mm core actually tracks access/dirty on a per-folio basis, not per
page). The impact analysis is as follows:
1. The 'mm_stats[MM_LEAF_YOUNG]' counter has no functional impact and is
mainly for debugging.
2. The 'young' counter is used to decide whether to place the current
PMD entry into the bloom filters by suitable_to_scan() (so that next
time we can check whether it has been accessed again), which may set the
hash bit in the bloom filters for a PMD entry that hasn't seen much
access. However, bloom filters inherently allow some error, so this
effect appears negligible.
"
Based on this, I think changing it to 'nr' is reasonable. For an
accessed large folio, it's better to have the bloom filter rescan the
PMD and keep it in memory instead of reclaiming it incorrectly.
I am not sure if this is the best policy, but we don't seem to have
a practical way to get the exact number of accessed PTEs, so this may
be acceptable.
As I mentioned earlier, it seems unnecessary to implement this, since core-mm tracks access flag at per-folio granularity. Moreover, bloom filter itself allows for some error.
However, could we at least update the changelog to
clarify that this is intentional?
" However, young is initialized to 1 regardless of the folio size. When
the original folio is a large folio with nr PTEs, its young count is
underestimated by nr - 1. This inconsistency can cause
suitable_to_scan() to return false, preventing the PMD from being added"
Agree. Looks better.
Its young count is not underestimated; we are intentionally
overestimating it. Also, nr does not necessarily equal
folio_nr_pages(), does it?
Right.