io_uring: hold mmap_sem for mm->locked_vm manipulation

The kernel doesn't seem to have clear rules around this, but various
spots are using the mmap_sem to serialize access to modifying the
locked_vm count. Play it safe and lock the mm for write when accounting
or unaccounting locked memory.
Signed-off-by: Jens Axboe <axboe@kernel.dk>

fs/io_uring.c

@@ -8157,11 +8157,14 @@ static void io_unaccount_mem(struct io_ring_ctx *ctx, unsigned long nr_pages,
 		__io_unaccount_mem(ctx->user, nr_pages);
 
 	if (ctx->mm_account) {
-		if (acct == ACCT_LOCKED)
+		if (acct == ACCT_LOCKED) {
+			mmap_write_lock(ctx->mm_account);
 			ctx->mm_account->locked_vm -= nr_pages;
-		else if (acct == ACCT_PINNED)
+			mmap_write_unlock(ctx->mm_account);
+		}else if (acct == ACCT_PINNED) {
 			atomic64_sub(nr_pages, &ctx->mm_account->pinned_vm);
 		}
+	}
 }
 
 static int io_account_mem(struct io_ring_ctx *ctx, unsigned long nr_pages,
@@ -8176,11 +8179,14 @@ static int io_account_mem(struct io_ring_ctx *ctx, unsigned long nr_pages,
 	}
 
 	if (ctx->mm_account) {
-		if (acct == ACCT_LOCKED)
+		if (acct == ACCT_LOCKED) {
+			mmap_write_lock(ctx->mm_account);
 			ctx->mm_account->locked_vm += nr_pages;
-		else if (acct == ACCT_PINNED)
+			mmap_write_unlock(ctx->mm_account);
+		} else if (acct == ACCT_PINNED) {
 			atomic64_add(nr_pages, &ctx->mm_account->pinned_vm);
 		}
+	}
 
 	return 0;
 }