When invoking thp_vma_allowable_orders(), if the TVA_ENFORCE_SYSFS flag is not
specified, we will ignore the THP sysfs settings. And the MADV_COLLAPSE is an
example of such a case.

The MADV_COLLAPSE will ignore the system-wide shmem THP sysfs settings, which
means that even though we have disabled the shmem THP configuration, MADV_COLLAPSE
will still attempt to collapse into a shmem THP. This violates the rule we have
agreed upon: never means never.

Another rule for madvise, referring to David's suggestion: “allowing for collapsing
in a VM without VM_HUGEPAGE in the "madvise" mode would be fine".

To fix the MADV_COLLAPSE issue for shmem, then the current strategy should be:

For shmem, if none of ‘always’, ‘madvise’, ‘within_size’, and ‘inherit’ have enabled
PMD-sized THP, then MADV_COLLAPSE will be prohibited from collapsing PMD-sized THP.
Similarly, if 'deny' is set, it will also prohibit MADV_COLLAPSE.

For tmpfs, if the mount option is set with the 'huge=never' parameter, then
MADV_COLLAPSE will be prohibited from collapsing PMD-sized THP.

Meanwhile, we should fix the khugepaged selftest for shmem MADV_COLLAPSE by enabling
shmem THP like anonymous pages collapse.

Acked-by: Zi Yan <ziy@nvidia.com>
Reviewed-by: Lorenzo Stoakes <lorenzo.stoakes@oracle.com>
Signed-off-by: Baolin Wang <baolin.wang@linux.alibaba.com>
---
 mm/shmem.c                              | 6 +++---
 tools/testing/selftests/mm/khugepaged.c | 2 +-
 2 files changed, 4 insertions(+), 4 deletions(-)

diff --git a/mm/shmem.c b/mm/shmem.c
index 2b19965d27df..e3f51fab2b7d 100644
--- a/mm/shmem.c
+++ b/mm/shmem.c
@@ -637,7 +637,7 @@ static unsigned int shmem_huge_global_enabled(struct inode *inode, pgoff_t index
 		return 0;
 	if (shmem_huge == SHMEM_HUGE_DENY)
 		return 0;
-	if (shmem_huge_force || shmem_huge == SHMEM_HUGE_FORCE)
+	if (shmem_huge == SHMEM_HUGE_FORCE)
 		return maybe_pmd_order;
 
 	/*
@@ -672,7 +672,7 @@ static unsigned int shmem_huge_global_enabled(struct inode *inode, pgoff_t index
 
 		fallthrough;
 	case SHMEM_HUGE_ADVISE:
-		if (vm_flags & VM_HUGEPAGE)
+		if (shmem_huge_force || (vm_flags & VM_HUGEPAGE))
 			return maybe_pmd_order;
 		fallthrough;
 	default:
@@ -1806,7 +1806,7 @@ unsigned long shmem_allowable_huge_orders(struct inode *inode,
 	/* Allow mTHP that will be fully within i_size. */
 	mask |= shmem_get_orders_within_size(inode, within_size_orders, index, 0);
 
-	if (vm_flags & VM_HUGEPAGE)
+	if (shmem_huge_force || (vm_flags & VM_HUGEPAGE))
 		mask |= READ_ONCE(huge_shmem_orders_madvise);
 
 	if (global_orders > 0)
diff --git a/tools/testing/selftests/mm/khugepaged.c b/tools/testing/selftests/mm/khugepaged.c
index 85bfff53dba6..9517ed99c382 100644
--- a/tools/testing/selftests/mm/khugepaged.c
+++ b/tools/testing/selftests/mm/khugepaged.c
@@ -502,7 +502,7 @@ static void __madvise_collapse(const char *msg, char *p, int nr_hpages,
 	printf("%s...", msg);
 
 	settings.thp_enabled = THP_ALWAYS;
-	settings.shmem_enabled = SHMEM_NEVER;
+	settings.shmem_enabled = SHMEM_ALWAYS;
 	thp_push_settings(&settings);
 
 	/* Clear VM_NOHUGEPAGE */
-- 
2.43.5