mm/thp: fix "mm: thp: kill __transhuge_page_enabled()"

The 6.0 commits:

commit 9fec51689ff6 ("mm: thp: kill transparent_hugepage_active()")
commit 7da4e2cb8b1f ("mm: thp: kill __transhuge_page_enabled()")

merged "can we have THPs in this VMA?" logic that was previously done
separately by fault-path, khugepaged, and smaps "THPeligible" checks.

During the process, the semantics of the fault path check changed in two
ways:

1) A VM_NO_KHUGEPAGED check was introduced (also added to smaps path).
2) We no longer checked if non-anonymous memory had a vm_ops->huge_fault
   handler that could satisfy the fault.  Previously, this check had been
   done in create_huge_pud() and create_huge_pmd() routines, but after
   the changes, we never reach those routines.

During the review of the above commits, it was determined that in-tree
users weren't affected by the change; most notably, since the only
relevant user (in terms of THP) of VM_MIXEDMAP or ->huge_fault is DAX,
which is explicitly approved early in approval logic.  However, this was a
bad assumption to make as it assumes the only reason to support
->huge_fault was for DAX (which is not true in general).

Remove the VM_NO_KHUGEPAGED check when not in collapse path and give any
->huge_fault handler a chance to handle the fault.  Note that we don't
validate the file mode or mapping alignment, which is consistent with the
behavior before the aforementioned commits.

Link: https://lkml.kernel.org/r/20230925200110.1979606-1-zokeefe@google.com
Fixes: 7da4e2cb8b1f ("mm: thp: kill __transhuge_page_enabled()")
Reported-by: Saurabh Singh Sengar <ssengar@microsoft.com>
Signed-off-by: Zach O'Keefe <zokeefe@google.com>
Cc: Yang Shi <shy828301@gmail.com>
Cc: Matthew Wilcox <willy@infradead.org>
Cc: David Hildenbrand <david@redhat.com>
Cc: Ryan Roberts <ryan.roberts@arm.com>
Signed-off-by: Andrew Morton <akpm@linux-foundation.org>

1 files changed, 13 insertions, 7 deletions

diff --git a/mm/huge_memory.c b/mm/huge_memory.c
index 51a66eb48938..c9cbcbf6697e 100644
--- a/mm/huge_memory.c
+++ b/mm/huge_memory.c
@@ -100,11 +100,11 @@ bool hugepage_vma_check(struct vm_area_struct *vma, unsigned long vm_flags,
 		return in_pf;
 
 	/*
-	 * Special VMA and hugetlb VMA.
+	 * khugepaged special VMA and hugetlb VMA.
 	 * Must be checked after dax since some dax mappings may have
 	 * VM_MIXEDMAP set.
 	 */
-	if (vm_flags & VM_NO_KHUGEPAGED)
+	if (!in_pf && !smaps && (vm_flags & VM_NO_KHUGEPAGED))
 		return false;
 
 	/*
@@ -132,12 +132,18 @@ bool hugepage_vma_check(struct vm_area_struct *vma, unsigned long vm_flags,
 					   !hugepage_flags_always())))
 		return false;
 
-	/* Only regular file is valid */
-	if (!in_pf && file_thp_enabled(vma))
-		return true;
-
-	if (!vma_is_anonymous(vma))
+	if (!vma_is_anonymous(vma)) {
+		/*
+		 * Trust that ->huge_fault() handlers know what they are doing
+		 * in fault path.
+		 */
+		if (((in_pf || smaps)) && vma->vm_ops->huge_fault)
+			return true;
+		/* Only regular file is valid in collapse path */
+		if (((!in_pf || smaps)) && file_thp_enabled(vma))
+			return true;
 		return false;
+	}
 
 	if (vma_is_temporary_stack(vma))
 		return false;