Re: [PATCH v1] mm/huge_memory: fix shrinking of all-zero THPs with max_ptes_none default

[David Hildenbrand <david@redhat.com>]

On 05.09.25 16:43, Usama Arif wrote:
> 
> 
> On 05/09/2025 15:37, Zi Yan wrote:
>> On 5 Sep 2025, at 10:11, David Hildenbrand wrote:
>>
>>> We added an early exit in thp_underused(), probably to avoid scanning
>>> pages when there is no chance for success.
>>>
>>> However, assume we have max_ptes_none = 511 (default).
>>>
>>> Nothing should stop us from freeing all pages part of a THP that
>>> is completely zero (512) and khugepaged will for sure not try to
>>> instantiate a THP in that case (512 shared zeropages).
>>>
>>> This can just trivially happen if someone writes a single 0 byte into a
>>> PMD area, or of course, when data ends up being zero later.
>>>
>>> So let's remove that early exit.
>>>
>>> Do we want to CC stable? Hm, not sure. Probably not urgent.
>>>
>>> Note that, as default, the THP shrinker is active
>>> (/sys/kernel/mm/transparent_hugepage/shrink_underused = 1), and all
>>> THPs are added to the deferred split lists. However, with the
>>> max_ptes_none default we would never scan them. We would not do that. If
>>> that's not desirable, we should just disable the shrinker as default,
>>> also not adding all THPs to the deferred split lists.
>>>
>>> Easy to reproduce:
>>>
>>> 1) Allocate some THPs filled with 0s
>>>
>>> <prog.c>
>>>   #include <string.h>
>>>   #include <stdio.h>
>>>   #include <stdlib.h>
>>>   #include <unistd.h>
>>>   #include <sys/mman.h>
>>>
>>>   const size_t size = 1024*1024*1024;
>>>
>>>   int main(void)
>>>   {
>>>           size_t offs;
>>>           char *area;
>>>
>>>           area = mmap(0, size, PROT_READ | PROT_WRITE,
>>>                       MAP_ANON | MAP_PRIVATE, -1, 0);
>>>           if (area == MAP_FAILED) {
>>>                   printf("mmap failed\n");
>>>                   exit(-1);
>>>           }
>>>           madvise(area, size, MADV_HUGEPAGE);
>>>
>>>           for (offs = 0; offs < size; offs += getpagesize())
>>>                   area[offs] = 0;
>>>           pause();
>>>   }
>>> <\prog.c>
>>>
>>> 2) Trigger the shrinker
>>>
>>> E.g., memory pressure through memhog
>>>
>>> 3) Observe that THPs are not getting reclaimed
>>>
>>> $ cat /proc/`pgrep prog`/smaps_rollup
>>>
>>> Would list ~1GiB of AnonHugePages. With this fix, they would get
>>> reclaimed as expected.
>>>
>>> Fixes: dafff3f4c850 ("mm: split underused THPs")
>>> Cc: Andrew Morton <akpm@linux-foundation.org>
>>> Cc: Lorenzo Stoakes <lorenzo.stoakes@oracle.com>
>>> Cc: Zi Yan <ziy@nvidia.com>
>>> Cc: Baolin Wang <baolin.wang@linux.alibaba.com>
>>> Cc: "Liam R. Howlett" <Liam.Howlett@oracle.com>
>>> Cc: Nico Pache <npache@redhat.com>
>>> Cc: Ryan Roberts <ryan.roberts@arm.com>
>>> Cc: Dev Jain <dev.jain@arm.com>
>>> Cc: Barry Song <baohua@kernel.org>
>>> Cc: Usama Arif <usamaarif642@gmail.com>
>>> Signed-off-by: David Hildenbrand <david@redhat.com>
>>> ---
>>>   mm/huge_memory.c | 3 ---
>>>   1 file changed, 3 deletions(-)
>>>
>> LGTM. Acked-by: Zi Yan <ziy@nvidia.com>
>>
>> I also notice that thp_underused() checks num_zero_pages directly
>> against khugepaged_max_ptes_none. This means mTHPs will never be regarded
>> as underused. A similar issue you are discussing in Nico’s khugepaged
>> mTHP support. Maybe checks against these khugepaged_max* variables
>> should be calculated based on nr_pages of a large folio, like
>> making these variables a ratio in other discussion.
> 
> I unfortunately didnt follow the series in the latest revisions.
> 
> In the earlier revisions, I think it was decided to not add mTHPs to shrinker
> as a start, as there are diminshing returns for smaller THPs and having a lot
> of smaller mTHPs in the deferred list might mean that we get to PMD mapped THPs
> a lot slower?

Probably we would want lists per order etc.

-- 
Cheers

David / dhildenb