[PATCH v2] samples/damon/mtier: fail early if address range parameters are invalid

[Zenghui Yu]

The comment on top of `struct damon_region` clearly says that

    For any use case, @ar should be non-zero positive size.

which is now verified in damon_verify_new_region() if the kernel is built
with DAMON_DEBUG_SANITY.

The WARN_ONCE() can be triggered if the mtier sample module is enabled
before node{0,1}_{start,end}_addr have been properly initialized, which is
obviously not good.

 ------------[ cut here ]------------
 start 0 >= end 0
 WARNING: mm/damon/core.c:217 at damon_new_region+0xf4/0x118, CPU#59: bash/341468
 Call trace:
  damon_new_region+0xf4/0x118 (P)
  damon_set_regions+0xfc/0x3c0
  damon_sample_mtier_build_ctx+0xe8/0x3a8
  damon_sample_mtier_start+0x1c/0x90
  damon_sample_mtier_enable_store+0x98/0xb0
  param_attr_store+0xb4/0x128
  module_attr_store+0x2c/0x50
  sysfs_kf_write+0x58/0x90
  kernfs_fop_write_iter+0x16c/0x238
  vfs_write+0x2c0/0x370
  ksys_write+0x74/0x118
  __arm64_sys_write+0x24/0x38
  invoke_syscall+0xa8/0x118
  el0_svc_common.constprop.0+0x48/0xf0
  do_el0_svc+0x24/0x38
  el0_svc+0x54/0x370
  el0t_64_sync_handler+0xa0/0xe8
  el0t_64_sync+0x1ac/0x1b0
 ---[ end trace 0000000000000000 ]---

Note that the same issue can happen if detect_node_addresses is true, and
node 0 or 1 is memoryless. Fix it together by checking the validity of
parameters right before damon_new_region() and fail early if they're
invalid.

Signed-off-by: Zenghui Yu <yuzenghui@xxxxxxxxxx>
---
* From v1 [1]:
  - take into account the detect_node_addresses case (Sashiko)
  - based on mm-new (SeongJae)

[1] https://lore.kernel.org/r/20260608111534.264-1-yuzenghui@xxxxxxxxxx

 samples/damon/mtier.c | 3 +++
 1 file changed, 3 insertions(+)

diff --git a/samples/damon/mtier.c b/samples/damon/mtier.c
index eb1143de8df1..3785b0c7ffb1 100644
--- a/samples/damon/mtier.c
+++ b/samples/damon/mtier.c
@@ -120,6 +120,9 @@ static struct damon_ctx *damon_sample_mtier_build_ctx(bool promote)
 		addr.end = promote ? node1_end_addr : node0_end_addr;
 	}
 
+	if (addr.start >= addr.end)
+		goto free_out;
+
 	range.start = addr.start;
 	range.end = addr.end;
 
-- 
2.33.0