[PATCH v2 1/2] 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, there is at least
one occurrence where an out-of-tree driver that used
VM_HUGEPAGE|VM_MIXEDMAP with a vm_ops->huge_fault handler, was broken.

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>
---
Changed from v1[1]:
	- [Saurabhi] Allow ->huge_fault handler to handle fault, if it exists

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

---
 mm/huge_memory.c | 17 ++++++++++-------
 1 file changed, 10 insertions(+), 7 deletions(-)

diff --git a/mm/huge_memory.c b/mm/huge_memory.c
index eb3678360b97..cd379b2c077b 100644
--- a/mm/huge_memory.c
+++ b/mm/huge_memory.c
@@ -96,11 +96,11 @@ 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;
 
 	/*
@@ -128,12 +128,15 @@ 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))
-		return false;
+		return in_pf ?
+			/*
+			 * Trust that ->huge_fault() handlers know
+			 * what they are doing in fault path.
+			 */
+			!!vma->vm_ops->huge_fault :
+			/* Only regular file is valid in collapse path */
+			file_thp_enabled(vma);
 
 	if (vma_is_temporary_stack(vma))
 		return false;
-- 
2.42.0.rc1.204.g551eb34607-goog