[PATCH 7.2 v4 06/12] mm: fs: remove filemap_nr_thps*() functions and their users

Zi Yan
2026-04-24T02:52:45Z

They are used by READ_ONLY_THP_FOR_FS to handle writes to FSes without
large folio support, so that read-only THPs created in these FSes are not
seen by the FSes when the underlying fd becomes writable. Now read-only PMD
THPs only appear in a FS with large folio support and the supported orders
include PMD_ORDRE.

READ_ONLY_THP_FOR_FS was using mapping->nr_thps, inode->i_writecount, and
smp_mb() to prevent writes to a read-only THP and collapsing writable
folios into a THP. In collapse_file(), mapping->nr_thps is increased, then
smp_mb(), and if inode->i_writecount > 0, collapse is stopped, while
do_dentry_open() first increases inode->i_writecount, then a full memory
fence, and if mapping->nr_thps > 0, all read-only THPs are truncated.

Now this mechanism can be removed along with READ_ONLY_THP_FOR_FS code,
since a dirty folio check has been added after try_to_unmap() in
collapse_file() to make sure no writable folio can be collapsed.

Signed-off-by: Zi Yan <ziy@nvidia.com>
Reviewed-by: Matthew Wilcox (Oracle) <willy@infradead.org>
Acked-by: David Hildenbrand (Arm) <david@kernel.org>
Reviewed-by: Baolin Wang <baolin.wang@linux.alibaba.com>
---
fs/open.c | 27 ---------------------------
include/linux/pagemap.h | 29 -----------------------------
mm/filemap.c | 1 -
mm/huge_memory.c | 1 -
mm/khugepaged.c | 28 ----------------------------
5 files changed, 86 deletions(-)

diff --git a/fs/open.c b/fs/open.c
index 681d405bc61e..c321b80027f1 100644
--- a/fs/open.c
+++ b/fs/open.c
@@ -968,33 +968,6 @@ static int do_dentry_open(struct file *f,
if ((f->f_flags & O_DIRECT) && !(f->f_mode & FMODE_CAN_ODIRECT))
return -EINVAL;

- /*
- * XXX: Huge page cache doesn't support writing yet. Drop all page
- * cache for this file before processing writes.
- */
- if (f->f_mode & FMODE_WRITE) {
- /*
- * Depends on full fence from get_write_access() to synchronize
- * against collapse_file() regarding i_writecount and nr_thps
- * updates. Ensures subsequent insertion of THPs into the page
- * cache will fail.
- */
- if (filemap_nr_thps(inode->i_mapping)) {
- struct address_space *mapping = inode->i_mapping;
-
- filemap_invalidate_lock(inode->i_mapping);
- /*
- * unmap_mapping_range just need to be called once
- * here, because the private pages is not need to be
- * unmapped mapping (e.g. data segment of dynamic
- * shared libraries here).
- */
- unmap_mapping_range(mapping, 0, 0, 0);
- truncate_inode_pages(mapping, 0);
- filemap_invalidate_unlock(inode->i_mapping);
- }
- }
-
return 0;

cleanup_all:
diff --git a/include/linux/pagemap.h b/include/linux/pagemap.h
index 5b4313d91137..88e58ca79bb5 100644
--- a/include/linux/pagemap.h
+++ b/include/linux/pagemap.h
@@ -528,35 +528,6 @@ static inline size_t mapping_max_folio_size(const struct address_space *mapping)
return PAGE_SIZE << mapping_max_folio_order(mapping);
}

-static inline int filemap_nr_thps(const struct address_space *mapping)
-{
-#ifdef CONFIG_READ_ONLY_THP_FOR_FS
- return atomic_read(&mapping->nr_thps);
-#else
- return 0;
-#endif
-}
-
-static inline void filemap_nr_thps_inc(struct address_space *mapping)
-{
-#ifdef CONFIG_READ_ONLY_THP_FOR_FS
- if (!mapping_large_folio_support(mapping))
- atomic_inc(&mapping->nr_thps);
-#else
- WARN_ON_ONCE(mapping_large_folio_support(mapping) == 0);
-#endif
-}
-
-static inline void filemap_nr_thps_dec(struct address_space *mapping)
-{
-#ifdef CONFIG_READ_ONLY_THP_FOR_FS
- if (!mapping_large_folio_support(mapping))
- atomic_dec(&mapping->nr_thps);
-#else
- WARN_ON_ONCE(mapping_large_folio_support(mapping) == 0);
-#endif
-}
-
struct address_space *folio_mapping(const struct folio *folio);

/**
diff --git a/mm/filemap.c b/mm/filemap.c
index 4e636647100c..d3cd4d2f3734 100644
--- a/mm/filemap.c
+++ b/mm/filemap.c
@@ -189,7 +189,6 @@ static void filemap_unaccount_folio(struct address_space *mapping,
lruvec_stat_mod_folio(folio, NR_SHMEM_THPS, -nr);
} else if (folio_test_pmd_mappable(folio)) {
lruvec_stat_mod_folio(folio, NR_FILE_THPS, -nr);
- filemap_nr_thps_dec(mapping);
}
if (test_bit(AS_KERNEL_FILE, &folio->mapping->flags))
mod_node_page_state(folio_pgdat(folio),
diff --git a/mm/huge_memory.c b/mm/huge_memory.c
index f0db1390a18f..8b85a3e58b00 100644
--- a/mm/huge_memory.c
+++ b/mm/huge_memory.c
@@ -3937,7 +3937,6 @@ static int __folio_freeze_and_split_unmapped(struct folio *folio, unsigned int n
} else {
lruvec_stat_mod_folio(folio,
NR_FILE_THPS, -nr);
- filemap_nr_thps_dec(mapping);
}
}
}
diff --git a/mm/khugepaged.c b/mm/khugepaged.c
index cdd4b37e4a68..4de7e30c4b71 100644
--- a/mm/khugepaged.c
+++ b/mm/khugepaged.c
@@ -2469,21 +2469,6 @@ static enum scan_result collapse_file(struct mm_struct *mm, unsigned long addr,
goto xa_unlocked;
}

- if (!is_shmem) {
- filemap_nr_thps_inc(mapping);
- /*
- * Paired with the fence in do_dentry_open() -> get_write_access()
- * to ensure i_writecount is up to date and the update to nr_thps
- * is visible. Ensures the page cache will be truncated if the
- * file is opened writable.
- */
- smp_mb();
- if (inode_is_open_for_write(mapping->host)) {
- result = SCAN_FAIL;
- filemap_nr_thps_dec(mapping);
- }
- }
-
xa_locked:
xas_unlock_irq(&xas);
xa_unlocked:
@@ -2661,19 +2646,6 @@ static enum scan_result collapse_file(struct mm_struct *mm, unsigned long addr,
folio_putback_lru(folio);
folio_put(folio);
}
- /*
- * Undo the updates of filemap_nr_thps_inc for non-SHMEM
- * file only. This undo is not needed unless failure is
- * due to SCAN_COPY_MC.
- */
- if (!is_shmem && result == SCAN_COPY_MC) {
- filemap_nr_thps_dec(mapping);
- /*
- * Paired with the fence in do_dentry_open() -> get_write_access()
- * to ensure the update to nr_thps is visible.
- */
- smp_mb();
- }

new_folio->mapping = NULL;

--
2.43.0