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 16:37, 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.

Yeah, factoring that out and cleaning it up is my next step.

But note that mTHPs are not a candidate for the shrinker right now. (see 
my explanation in reply to Nicos patch)

-- 
Cheers

David / dhildenb