Re: [PATCH v2 10/35] mm: add per-mm mmap sequence counter for speculative page fault handling.

The counter's write side is hooked into the existing mmap locking API:
mmap_write_lock() increments the counter to the next (odd) value, and
mmap_write_unlock() increments it again to the next (even) value.

The counter's speculative read side is supposed to be used as follows:

seq = mmap_seq_read_start(mm);
if (seq & 1)
	goto fail;
.... speculative handling here ....
if (!mmap_seq_read_check(mm, seq)
	goto fail;

This API guarantees that, if none of the "fail" tests abort
speculative execution, the speculative code section did not run
concurrently with any mmap writer.

This is very similar to a seqlock, but both the writer and speculative
readers are allowed to block. In the fail case, the speculative reader
does not spin on the sequence counter; instead it should fall back to
a different mechanism such as grabbing the mmap lock read side.

Signed-off-by: Michel Lespinasse <redacted>
---
 include/linux/mm_types.h  |  4 +++
 include/linux/mmap_lock.h | 58 +++++++++++++++++++++++++++++++++++++--
 2 files changed, 60 insertions(+), 2 deletions(-)

diff --git a/include/linux/mm_types.h b/include/linux/mm_types.h
index 0ae3bf854aad..e4965a6f34f2 100644
--- a/include/linux/mm_types.h
+++ b/include/linux/mm_types.h

@@ -523,6 +523,10 @@ struct mm_struct {
 		 * cacheline.
 		 */
 		struct rw_semaphore mmap_lock;
+#ifdef CONFIG_SPECULATIVE_PAGE_FAULT
+		unsigned long mmap_seq;
+#endif
+