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>

2 files changed, 4 insertions, 4 deletions

diff --git a/tools/testing/vma/vma.c b/tools/testing/vma/vma.c
index 891d87a9ad6bc..920fba58884e3 100644
--- a/tools/testing/vma/vma.c
+++ b/tools/testing/vma/vma.c
@@ -100,7 +100,7 @@ static struct vm_area_struct *alloc_and_link_vma(struct mm_struct *mm,
 	 * begun. Linking to the tree will have caused this to be incremented,
 	 * which means we will get a false positive otherwise.
 	 */
-	vma->vm_lock_seq = -1;
+	vma->vm_lock_seq = UINT_MAX;
 
 	return vma;
 }
@@ -225,7 +225,7 @@ static bool vma_write_started(struct vm_area_struct *vma)
 	int seq = vma->vm_lock_seq;
 
 	/* We reset after each check. */
-	vma->vm_lock_seq = -1;
+	vma->vm_lock_seq = UINT_MAX;
 
 	/* The vma_start_write() stub simply increments this value. */
 	return seq > -1;
diff --git a/tools/testing/vma/vma_internal.h b/tools/testing/vma/vma_internal.h
index a7de59a0d694d..b973b3e41c834 100644
--- a/tools/testing/vma/vma_internal.h
+++ b/tools/testing/vma/vma_internal.h
@@ -281,7 +281,7 @@ 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;
 	struct vma_lock *vm_lock;
 #endif
 
@@ -467,7 +467,7 @@ static inline bool vma_lock_alloc(struct vm_area_struct *vma)
 		return false;
 
 	init_rwsem(&vma->vm_lock->lock);
-	vma->vm_lock_seq = -1;
+	vma->vm_lock_seq = UINT_MAX;
 
 	return true;
 }