Re: [PATCH v5 07/14] mm: shmem: allow THP support determination at folio allocation time

[Luiz Capitulino <luizcap@redhat.com>]

On 2026-06-01 08:09, Baolin Wang wrote:
> 
> 
> On 5/29/26 10:55 PM, 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.
>>
>> 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 88bc0c292f1a..108dc969b92a 100644
>> --- a/mm/shmem.c
>> +++ b/mm/shmem.c
>> @@ -689,7 +689,7 @@ 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;
>> @@ -1303,6 +1303,8 @@ static int shmem_getattr(struct mnt_idmap *idmap,
>>       generic_fillattr(idmap, request_mask, inode, stat);
>>       orders = shmem_huge_global_enabled(inode, 0, 0, false, NULL, 0);
>> +    if (!pgtable_has_pmd_leaves())
>> +        orders &= ~BIT(PMD_ORDER);
> 
> Ah, you did in this patch, but this change should belong in patch 14?

If we move it, then this patch will add a temporary issue until patch 14
where shmem_getattr() may report PMD-size for blksize even when the CPU
doesn't support PMD-sized pages.