[to-be-updated] mm-khugepaged-remove-read_only_thp_for_fs-check.patch removed from -mm tree

[Andrew Morton <akpm@linux-foundation.org>]

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The quilt patch titled
     Subject: mm/khugepaged: remove READ_ONLY_THP_FOR_FS check
has been removed from the -mm tree.  Its filename was
     mm-khugepaged-remove-read_only_thp_for_fs-check.patch

This patch was dropped because an updated version will be issued

------------------------------------------------------
From: Zi Yan <ziy@nvidia.com>
Subject: mm/khugepaged: remove READ_ONLY_THP_FOR_FS check
Date: Thu, 23 Apr 2026 22:49:04 -0400

Patch series "Remove read-only THP support for FSes without large folio
support", v4.

This patchset removes READ_ONLY_THP_FOR_FS Kconfig and enables creating
read-only THPs for FSes with large folio support (the supported orders
need to include PMD_ORDER) by default.  It is on top of mm-new.

Before the patchset, the status of creating read-only THPs is below:

                            |    PF     | MADV_COLLAPSE | khugepaged |
                            |-----------|---------------|------------|
 large folio FSes only      |     ✓     |       x       |      x     |
 READ_ONLY_THP_FOR_FS only  |     x     |       ✓       |      ✓     |
 both                       |     ✓     |       ✓       |      ✓     |

where READ_ONLY_THP_FOR_FS implies no large folio FSes.


Now without READ_ONLY_THP_FOR_FS:

                           |    PF     | MADV_COLLAPSE | khugepaged |
                           |-----------|---------------|------------|
 large folio FSes          |     ✓     |       ✓       |      ✓     |
 no large folio FSes       |     x     |       x       |      x     |

This means no large folio FSes need to add large folio support (the
supported orders need to include PMD_ORDER), so that they can leverage
read-only THP creation function.

To prevent breaking read-only THP support for large folio FSes,
1. first 4 patches enables the support, so that without READ_ONLY_THP_FOR_FS,
   read-only THP still works for large folio FSes,
2. Patch 5 removes READ_ONLY_THP_FOR_FS Kconfig,
3. the rest of patches remove code related to READ_ONLY_THP_FOR_FS.

NOTE: collapsing writable MAP_PRIVATE pagecache folios is not supported,
since:
1. PMD THP CoW only faults in at PTE level to avoid long CoW latency,
2. the first check, due to 1, in file_backed_vma_is_retractable() prevents it.


The overview of the changes is:

1. collapse_file() checks for to-be-collapsed folio dirtiness after they
   are locked, unmapped to make sure no new write happens. Before,
   mapping->nr_thps and inode->i_writecount are used to cause read-only
   THP truncation before a fd becomes writable.

2. hugepage_enabled() is true for anon, shmem, and file-backed cases
   if the global khugepaged control is on, otherwise, khugepaged for
   file-backed case is turned off and anon and shmem depend on per-size
   control knobs.

3. collapse_file() from mm/khugepaged.c, instead of checking
   CONFIG_READ_ONLY_THP_FOR_FS, makes sure the mapping_max_folio_order()
   of struct address_space of the file is at least PMD_ORDER.

4. file_thp_enabled() also checks mapping_max_folio_order() instead and
   no longer checks if the input file is opened as read-only (Change 1
   handles read-write files).

5. truncate_inode_partial_folio() calls folio_split() directly instead
   of the removed try_folio_split_to_order(), since large folios can
   only show up on a FS with large folio support.

6. nr_thps is removed from struct address_space, since it is no longer
   needed to drop all read-only THPs from a FS without large folio
   support when the fd becomes writable. Its related filemap_nr_thps*()
   are removed too.

7. folio_check_splittable() no longer checks READ_ONLY_THP_FOR_FS.

8. Updated comments in various places.


This patch (of 12):

collapse_file() requires FSes supporting large folio with at least
PMD_ORDER, so replace the READ_ONLY_THP_FOR_FS check with that. 
MADV_COLLAPSE ignores shmem huge config, so exclude the check for shmem.

While at it, replace VM_BUG_ON with VM_WARN_ON_ONCE.

Add a helper function mapping_pmd_thp_support() for FSes supporting large
folio with at least PMD_ORDER.

Link: https://lore.kernel.org/20260424024915.28758-1-ziy@nvidia.com
Link: https://lore.kernel.org/20260424024915.28758-2-ziy@nvidia.com
Signed-off-by: Zi Yan <ziy@nvidia.com>
Reviewed-by: Lance Yang <lance.yang@linux.dev>
Reviewed-by: Baolin Wang <baolin.wang@linux.alibaba.com>
Cc: Al Viro <viro@zeniv.linux.org.uk>
Cc: Barry Song <baohua@kernel.org>
Cc: Chris Mason <clm@fb.com>
Cc: Christian Brauner <brauner@kernel.org>
Cc: David Hildenbrand <david@kernel.org>
Cc: David Sterba <dsterba@suse.com>
Cc: Dev Jain <dev.jain@arm.com>
Cc: Jan Kara <jack@suse.cz>
Cc: Liam Howlett <liam@infradead.org>
Cc: Lorenzo Stoakes <ljs@kernel.org>
Cc: Matthew Wilcox (Oracle) <willy@infradead.org>
Cc: Michal Hocko <mhocko@suse.com>
Cc: Mike Rapoport <rppt@kernel.org>
Cc: Nico Pache <npache@redhat.com>
Cc: Ryan Roberts <ryan.roberts@arm.com>
Cc: Shuah Khan <shuah@kernel.org>
Cc: Song Liu <songliubraving@fb.com>
Cc: Suren Baghdasaryan <surenb@google.com>
Signed-off-by: Andrew Morton <akpm@linux-foundation.org>
---

 include/linux/pagemap.h |    9 +++++++++
 mm/khugepaged.c         |   10 ++++++++--
 2 files changed, 17 insertions(+), 2 deletions(-)

--- a/include/linux/pagemap.h~mm-khugepaged-remove-read_only_thp_for_fs-check
+++ a/include/linux/pagemap.h
@@ -513,6 +513,15 @@ static inline bool mapping_large_folio_s
 	return mapping_max_folio_order(mapping) > 0;
 }
 
+static inline bool mapping_pmd_thp_support(const struct address_space *mapping)
+{
+	/* AS_FOLIO_ORDER is only reasonable for pagecache folios */
+	VM_WARN_ON_ONCE((unsigned long)mapping & FOLIO_MAPPING_ANON);
+
+	return mapping_max_folio_order(mapping) >= PMD_ORDER;
+}
+
+
 /* Return the maximum folio size for this pagecache mapping, in bytes. */
 static inline size_t mapping_max_folio_size(const struct address_space *mapping)
 {
--- a/mm/khugepaged.c~mm-khugepaged-remove-read_only_thp_for_fs-check
+++ a/mm/khugepaged.c
@@ -2235,8 +2235,14 @@ static enum scan_result collapse_file(st
 	int nr_none = 0;
 	bool is_shmem = shmem_file(file);
 
-	VM_BUG_ON(!IS_ENABLED(CONFIG_READ_ONLY_THP_FOR_FS) && !is_shmem);
-	VM_BUG_ON(start & (HPAGE_PMD_NR - 1));
+	/*
+	 * MADV_COLLAPSE ignores shmem huge config, so do not check shmem
+	 *
+	 * TODO: once shmem always calls mapping_set_large_folios() on its
+	 * mapping, the shmem check can be removed.
+	 */
+	VM_WARN_ON_ONCE(!is_shmem && !mapping_pmd_thp_support(mapping));
+	VM_WARN_ON_ONCE(start & (HPAGE_PMD_NR - 1));
 
 	result = alloc_charge_folio(&new_folio, mm, cc, HPAGE_PMD_ORDER);
 	if (result != SCAN_SUCCEED)
_

Patches currently in -mm which might be from ziy@nvidia.com are

mm-khugepaged-add-folio-dirty-check-after-try_to_unmap.patch
mm-huge_memory-remove-read_only_thp_for_fs-from-file_thp_enabled.patch
mm-khugepaged-remove-read_only_thp_for_fs-check-in-hugepage_enabled.patch
mm-remove-read_only_thp_for_fs-kconfig-option.patch
mm-fs-remove-filemap_nr_thps-functions-and-their-users.patch
fs-remove-nr_thps-from-struct-address_space.patch
mm-huge_memory-remove-folio-split-check-for-read_only_thp_for_fs.patch
mm-truncate-use-folio_split-in-truncate_inode_partial_folio.patch
fs-btrfs-remove-a-comment-referring-to-read_only_thp_for_fs.patch
selftests-mm-remove-read_only_thp_for_fs-in-khugepaged.patch
selftests-mm-remove-read_only_thp_for_fs-code-from-guard-regions.patch