mm: convert mm_lock_seq to a proper seqcount

Convert mm_lock_seq to be seqcount_t and change all mmap_write_lock
variants to increment it, in-line with the usual seqcount usage pattern.
This lets us check whether the mmap_lock is write-locked by checking
mm_lock_seq.sequence counter (odd=locked, even=unlocked). This will be
used when implementing mmap_lock speculation functions.
As a result vm_lock_seq is also change to be unsigned to match the type
of mm_lock_seq.sequence.

Link: https://lkml.kernel.org/r/20241122174416.1367052-2-surenb@google.com
Suggested-by: Peter Zijlstra <peterz@infradead.org>
Signed-off-by: Suren Baghdasaryan <surenb@google.com>
Reviewed-by: Liam R. Howlett <Liam.Howlett@Oracle.com>
Cc: Christian Brauner <brauner@kernel.org>
Cc: David Hildenbrand <david@redhat.com>
Cc: David Howells <dhowells@redhat.com>
Cc: Davidlohr Bueso <dave@stgolabs.net>
Cc: Hillf Danton <hdanton@sina.com>
Cc: Hugh Dickins <hughd@google.com>
Cc: Jann Horn <jannh@google.com>
Cc: Johannes Weiner <hannes@cmpxchg.org>
Cc: Jonathan Corbet <corbet@lwn.net>
Cc: Lorenzo Stoakes <lorenzo.stoakes@oracle.com>
Cc: Mateusz Guzik <mjguzik@gmail.com>
Cc: Matthew Wilcox <willy@infradead.org>
Cc: Mel Gorman <mgorman@techsingularity.net>
Cc: Michal Hocko <mhocko@suse.com>
Cc: Minchan Kim <minchan@google.com>
Cc: Oleg Nesterov <oleg@redhat.com>
Cc: Pasha Tatashin <pasha.tatashin@soleen.com>
Cc: Paul E. McKenney <paulmck@kernel.org>
Cc: Peter Xu <peterx@redhat.com>
Cc: Shakeel Butt <shakeel.butt@linux.dev>
Cc: Sourav Panda <souravpanda@google.com>
Cc: Vlastimil Babka <vbabka@suse.cz>
Cc: Wei Yang <richard.weiyang@gmail.com>
Signed-off-by: Andrew Morton <akpm@linux-foundation.org>

1 files changed, 5 insertions, 2 deletions

diff --git a/include/linux/mm_types.h b/include/linux/mm_types.h
index 7490d84af310..5f1b2dc788e2 100644
--- a/include/linux/mm_types.h
+++ b/include/linux/mm_types.h
@@ -729,7 +729,7 @@ struct vm_area_struct {
 	 * counter reuse can only lead to occasional unnecessary use of the
 	 * slowpath.
 	 */
-	int vm_lock_seq;
+	unsigned int vm_lock_seq;
 	/* Unstable RCU readers are allowed to read this. */
 	struct vma_lock *vm_lock;
 #endif
@@ -923,6 +923,9 @@ struct mm_struct {
 		 * Roughly speaking, incrementing the sequence number is
 		 * equivalent to releasing locks on VMAs; reading the sequence
 		 * number can be part of taking a read lock on a VMA.
+		 * Incremented every time mmap_lock is write-locked/unlocked.
+		 * Initialized to 0, therefore odd values indicate mmap_lock
+		 * is write-locked and even values that it's released.
 		 *
 		 * Can be modified under write mmap_lock using RELEASE
 		 * semantics.
@@ -931,7 +934,7 @@ struct mm_struct {
 		 * Can be read with ACQUIRE semantics if not holding write
 		 * mmap_lock.
 		 */
-		int mm_lock_seq;
+		seqcount_t mm_lock_seq;
 #endif