Reading hot folios without a reference defers the free of their memory
by an RCU grace period.  A workload that loses on that trade, or a bug
in the lockless read, has no way to turn it off: CONFIG_PAGE_RCU_FREE is
on in every 64-bit kernel.

Add lockless_page_cache_read to debugfs.  Writing 0 stops filemap_read()
and shmem_file_read_iter() from setting PG_rcu_free and from copying
without a reference.  A static key keeps the test off the read path.
debugfs, not a sysctl: the switch is not ABI and can go once the reads
have proven themselves.

Folios that have the flag keep it, and their free stays deferred: a read
that tested the key before the switch may still be copying from them.

Assisted-by: LLM
Signed-off-by: Kiryl Shutsemau (Meta) <kas@kernel.org>
---
 mm/Kconfig   |  3 +++
 mm/filemap.c | 51 ++++++++++++++++++++++++++++++++++++++++++++++++---
 2 files changed, 51 insertions(+), 3 deletions(-)

diff --git a/mm/Kconfig b/mm/Kconfig
index 5af00db23f90..1ad1cb7969e7 100644
--- a/mm/Kconfig
+++ b/mm/Kconfig
@@ -730,6 +730,9 @@ config PAGE_RCU_FREE
 	  64-bit only: it takes a page flag, and those are scarce on
 	  32-bit.
 
+	  Writing 0 to lockless_page_cache_read in debugfs turns these
+	  reads off.
+
 config PHYS_ADDR_T_64BIT
 	def_bool 64BIT
 
diff --git a/mm/filemap.c b/mm/filemap.c
index cde7bc68b32d..7021ed2ab6c9 100644
--- a/mm/filemap.c
+++ b/mm/filemap.c
@@ -48,6 +48,7 @@
 #include <linux/rcupdate_wait.h>
 #include <linux/sched/mm.h>
 #include <linux/sysctl.h>
+#include <linux/debugfs.h>
 #include <linux/pgalloc.h>
 
 #include <asm/tlbflush.h>
@@ -2765,6 +2766,50 @@ static inline bool pos_same_folio(loff_t pos1, loff_t pos2, struct folio *folio)
 	return (pos1 >> shift == pos2 >> shift);
 }
 
+#ifdef CONFIG_PAGE_RCU_FREE
+/* Writing 0 to lockless_page_cache_read in debugfs turns the reads off */
+static DEFINE_STATIC_KEY_TRUE(lockless_read_key);
+
+static bool lockless_read_enabled(void)
+{
+	return static_branch_likely(&lockless_read_key);
+}
+
+#ifdef CONFIG_DEBUG_FS
+static int lockless_read_get(void *data, u64 *val)
+{
+	*val = static_key_enabled(&lockless_read_key);
+	return 0;
+}
+
+static int lockless_read_set(void *data, u64 val)
+{
+	if (val > 1)
+		return -EINVAL;
+	if (val)
+		static_branch_enable(&lockless_read_key);
+	else
+		static_branch_disable(&lockless_read_key);
+	return 0;
+}
+DEFINE_DEBUGFS_ATTRIBUTE(lockless_read_fops, lockless_read_get,
+			 lockless_read_set, "%llu\n");
+
+static int __init lockless_read_debugfs_init(void)
+{
+	debugfs_create_file_unsafe("lockless_page_cache_read", 0644, NULL, NULL,
+				   &lockless_read_fops);
+	return 0;
+}
+late_initcall(lockless_read_debugfs_init);
+#endif
+#else
+static bool lockless_read_enabled(void)
+{
+	return false;
+}
+#endif
+
 /*
  * Reads keep coming back to @folio: free it only after an RCU grace period
  * from now on.  Not if its next read needs a reference anyway.  Not if it
@@ -2775,7 +2820,7 @@ static void filemap_mark_hot(struct folio *folio)
 {
 	struct address_space *mapping;
 
-	if (!IS_ENABLED(CONFIG_PAGE_RCU_FREE))
+	if (!lockless_read_enabled())
 		return;
 	if (folio_test_rcu_free(folio) || !folio_test_uptodate(folio) ||
 	    folio_test_dropbehind(folio) || folio_test_unevictable(folio))
@@ -3025,8 +3070,8 @@ size_t filemap_read_rcu(struct kiocb *iocb, struct iov_iter *iter,
 	pgoff_t last_index = (last_pos - 1) >> PAGE_SHIFT;
 	size_t copied = 0;
 
-	/* Not on an inode that never had a folio flagged */
-	if (!IS_ENABLED(CONFIG_PAGE_RCU_FREE) ||
+	/* Not when switched off, or on an inode that never had a folio flagged */
+	if (!lockless_read_enabled() ||
 	    !mapping_rcu_read(iocb->ki_filp->f_mapping))
 		return 0;
 
-- 
2.54.0