Re: [PATCH 7.2 v16 07/13] mm/khugepaged: add per-order mTHP collapse failure statistics

Nico Pache
2026-04-29T18:21:45Z

On 4/27/26 2:21 PM, David Hildenbrand (Arm) wrote:
> On 4/19/26 20:57, Nico Pache wrote:
>> Add three new mTHP statistics to track collapse failures for different
>> orders when encountering swap PTEs, excessive none PTEs, and shared PTEs:
>>
>> - collapse_exceed_swap_pte: Increment when mTHP collapse fails due to swap
>> PTEs
>>
>> - collapse_exceed_none_pte: Counts when mTHP collapse fails due to
>> exceeding the none PTE threshold for the given order
>>
>> - collapse_exceed_shared_pte: Counts when mTHP collapse fails due to shared
>> PTEs
>>
>> These statistics complement the existing THP_SCAN_EXCEED_* events by
>> providing per-order granularity for mTHP collapse attempts. The stats are
>> exposed via sysfs under
>> `/sys/kernel/mm/transparent_hugepage/hugepages-*/stats/` for each
>> supported hugepage size.
>>
>> As we currently dont support collapsing mTHPs that contain a swap or
>
> s/dont/do not/
>
>> shared entry, those statistics keep track of how often we are
>> encountering failed mTHP collapses due to these restrictions.
>>
>> Now that we plan to support mTHP collapse for anon pages, lets also track
>
> "We will add support for mTHP collapse for anonymous pages next; let's also ..."
>
>> when this happens at the PMD level within the per-mTHP stats.
>
> What about file collapse? For example, we do adjust
> count_vm_event(THP_SCAN_EXCEED_SWAP_PTE) and
> count_vm_event(THP_SCAN_EXCEED_NONE_PTE) there.
>
> Wouldn't we want to update the HPAGE_PMD_ORDER side of things there already? or
> would we want to use a different counter for that?

Maybe? My thought process was that because we dont support mTHP in Shmem
that those stats shouldnt be updated? not sure the right answer tbh--
hence why this comment says "now that.. for anon pages, lets track..".

>
>>
>> Signed-off-by: Nico Pache <npache@redhat.com>
>> ---
>> Documentation/admin-guide/mm/transhuge.rst | 24 ++++++++++++++++++++++
>> include/linux/huge_mm.h | 3 +++
>> mm/huge_memory.c | 7 +++++++
>> mm/khugepaged.c | 21 +++++++++++++++++--
>> 4 files changed, 53 insertions(+), 2 deletions(-)
>>
>> diff --git a/Documentation/admin-guide/mm/transhuge.rst b/Documentation/admin-guide/mm/transhuge.rst
>> index c51932e6275d..eebb1f6bbc6c 100644
>> --- a/Documentation/admin-guide/mm/transhuge.rst
>> +++ b/Documentation/admin-guide/mm/transhuge.rst
>> @@ -714,6 +714,30 @@ nr_anon_partially_mapped
>> an anonymous THP as "partially mapped" and count it here, even though it
>> is not actually partially mapped anymore.
>>
>> +collapse_exceed_none_pte
>> + The number of collapse attempts that failed due to exceeding the
>> + max_ptes_none threshold. For mTHP collapse, Currently only max_ptes_none
>> + values of 0 and (HPAGE_PMD_NR - 1) are supported. Any other value will
>> + emit a warning and no mTHP collapse will be attempted. khugepaged will
>> + try to collapse to the largest enabled (m)THP size; if it fails, it will
>> + try the next lower enabled mTHP size. This counter records the number of
>> + times a collapse attempt was skipped for exceeding the max_ptes_none
>> + threshold, and khugepaged will move on to the next available mTHP size.
>
> Why is everything after the first sentence worth documenting here? This doesn't
> read like it belongs to a failure counter?
>
>> +
>> +collapse_exceed_swap_pte
>> + The number of anonymous mTHP PTE ranges which were unable to collapse due
>> + to containing at least one swap PTE. Currently khugepaged does not
>> + support collapsing mTHP regions that contain a swap PTE. This counter can
>> + be used to monitor the number of khugepaged mTHP collapses that failed
>> + due to the presence of a swap PTE.
>
> Can we similarly simplify that (and make it consistent with the one above) to
>
> "The number of collapse attempts that failed due to exceeding the max_ptes_swap
> threshold."
>
>> +
>> +collapse_exceed_shared_pte
>> + The number of anonymous mTHP PTE ranges which were unable to collapse due
>> + to containing at least one shared PTE. Currently khugepaged does not
>> + support collapsing mTHP PTE ranges that contain a shared PTE. This
>> + counter can be used to monitor the number of khugepaged mTHP collapses
>> + that failed due to the presence of a shared PTE.
>
> Same here
>
> "The number of collapse attempts that failed due to exceeding the
> max_ptes_shared threshold."

These all used to be very similar to what you have here. But Lorenzo
asked me to expand all this (IIRC) several times. So here we are! I
believe I even mentioned in the V15 that these are probably the "best"
defined counters in the whole doc lol.

Cheers,

-- Nico

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