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

[Luiz Capitulino]

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@xxxxxxxxxxxxxxxxx>
> > Signed-off-by: Luiz Capitulino <luizcap@xxxxxxxxxx>
> > ---
> >   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@xxxxxxxxxx>

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))