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

[Usama Arif <usamaarif642@gmail.com>]

On 05/09/2025 15:47, David Hildenbrand wrote:
> 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.
> 

Yes that makes sense! and we start with the highest order list.