Re: [PATCH v8 07/14] mm: shmem: allow THP support determination at folio allocation time

[Luiz Capitulino <luizcap@redhat.com>]

On 9/21/26 10:20 PM, Zi Yan wrote:
> On Thu Sep 17, 2026 at 9:45 PM EDT, Luiz Capitulino wrote:
>> In order to enable THP support in shmem today, besides the user
>> configuration required, the CPU must support PMD-sized pages. This
>> is the case because of the following has_transparent_hugepage()
>> usage:
>>
>> - shmem_parse_one() and shmem_parse_huge(): Check if THP is built-in and
>>    if the CPU supports PMD-sized pages
>>
>> - shmem_init(): Since the CONFIG_TRANSPARENT_HUGEPAGE guard is outside
>>    the code block calling has_transparent_hugepage(), the
>>    has_transparent_hugepage() call is exclusively checking if the CPU
>>    supports PMD-sized pages
>>
>> While it's necessary to check if CONFIG_TRANSPARENT_HUGEPAGE is enabled
>> in all cases, shmem can determine THP size support at folio allocation
>> time. Therefore, drop the has_transparent_hugepage() usage listed above
>> while keeping the CONFIG_TRANSPARENT_HUGEPAGE checks.
>>
>> Additionally, we need to check if PMD size order is supported in
>> shmem_getattr(). Use pgtable_has_pmd_leaves() for that.
>>
>> Reviewed-by: Baolin Wang <baolin.wang@linux.alibaba.com>
>> Signed-off-by: Luiz Capitulino <luizcap@redhat.com>
>> ---
>>   mm/shmem.c | 9 +++++----
>>   1 file changed, 5 insertions(+), 4 deletions(-)
>>
>> diff --git a/mm/shmem.c b/mm/shmem.c
>> index 776dff8a848e..930657d05375 100644
>> --- a/mm/shmem.c
>> +++ b/mm/shmem.c
>> @@ -690,7 +690,7 @@ static int shmem_parse_huge(const char *str)
>>   	else
>>   		return -EINVAL;
>>   
>> -	if (!has_transparent_hugepage() &&
>> +	if (!IS_ENABLED(CONFIG_TRANSPARENT_HUGEPAGE) &&
>>   	    huge != SHMEM_HUGE_NEVER && huge != SHMEM_HUGE_DENY)
>>   		return -EINVAL;
> 
> This can be deleted, since shmem_parse_huge() is guarded by
> CONFIG_TRANSPARENT_HUGEPAGE and after the change the if is always false
> under that guard.
> 
> Otherwise, LGTM. With the above code deleted, feel free to add
> 
> Reviewed-by: Zi Yan <ziy@nvidia.com>

OK, here's the diff:

diff --git a/mm/shmem.c b/mm/shmem.c
index c00bc1829fc9..d12995d41846 100644
--- a/mm/shmem.c
+++ b/mm/shmem.c
@@ -690,10 +690,6 @@ static int shmem_parse_huge(const char *str)
  	else
  		return -EINVAL;
  
-	if (!IS_ENABLED(CONFIG_TRANSPARENT_HUGEPAGE) &&
-	    huge != SHMEM_HUGE_NEVER && huge != SHMEM_HUGE_DENY)
-		return -EINVAL;
-
  	/* Do not override huge allocation policy with non-PMD sized mTHP */
  	if (huge == SHMEM_HUGE_FORCE &&
  	    huge_shmem_orders_inherit != BIT(HPAGE_PMD_ORDER))