[PATCH v4] mm/thp: fix "mm: thp: kill __transhuge_page_enabled()"

["Zach O'Keefe" <zokeefe@google.com>]

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.

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>
---
I've updated the changelog to reflect discussions in [1] -- leaving
ack to David / Matthew on whether to take the patch.

Changed from v3[1]:
	- [akpm / David H. / M. Wilcox] Updated log to capture email discussion
Changed from v2[2]:
	- Fixed false negative in smaps check when !dax && ->huge_fault
Changed from v1[3]:
	- [Saurabhi] Allow ->huge_fault handler to handle fault, if it exists

[1] https://lore.kernel.org/linux-mm/20230821234844.699818-1-zokeefe@google.com/
[2] https://lore.kernel.org/linux-mm/20230818211533.2523697-1-zokeefe@google.com/
[3] https://lore.kernel.org/linux-mm/CAAa6QmQw+F=o6htOn=6ADD6mwvMO=Ow_67f3ifBv3GpXx9Xg_g@mail.gmail.com/

---
 mm/huge_memory.c | 20 +++++++++++++-------
 1 file changed, 13 insertions(+), 7 deletions(-)

diff --git a/mm/huge_memory.c b/mm/huge_memory.c
index 0f93a73115f73..797fe617e51ab 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;
-- 
2.42.0.515.g380fc7ccd1-goog