Re: [RFC PATCH 1/1] idr: do not create idr if new id would be outside given range

[Matthew Wilcox <willy@infradead.org>]

On Thu, Nov 27, 2025 at 10:27:32AM +0100, Jan Sokolowski wrote:
> A scenario was found where trying to add id in range 0,1
> would return an id of 2, which is outside the range and thus
> now what the user would expect.

Can you do a bit better with this bug report?  Under what circumstances
does this happen?  Preferably answer in the form of a test case for the
IDR test suite.  Here's my attempt to recreate your situation based on
what I read in that thread.  It doesn't show a problem, so clearly I got
something wrong.

To run the test suite, apply this patch, then

$ make -C tools/testing/radix-tree
$ ./tools/testing/radix-tree/idr-test

diff --git a/tools/testing/radix-tree/idr-test.c b/tools/testing/radix-tree/idr-test.c
index 2f830ff8396c..774c0c9c141f 100644
--- a/tools/testing/radix-tree/idr-test.c
+++ b/tools/testing/radix-tree/idr-test.c
@@ -57,6 +57,21 @@ void idr_alloc_test(void)
 	idr_destroy(&idr);
 }
 
+void idr_alloc2_test(void)
+{
+	int id;
+	DEFINE_IDR(idr);
+
+	id = idr_alloc(&idr, idr_alloc2_test, 0, 1, GFP_KERNEL);
+	printf("id = %d\n", id);
+	assert(id == 0);
+	id = idr_alloc(&idr, idr_alloc2_test, 0, 1, GFP_KERNEL);
+	printf("id = %d\n", id);
+	assert(id == -ENOSPC);
+
+	idr_destroy(&idr);
+}
+
 void idr_replace_test(void)
 {
 	DEFINE_IDR(idr);
@@ -409,6 +424,7 @@ void idr_checks(void)
 
 	idr_replace_test();
 	idr_alloc_test();
+	idr_alloc2_test();
 	idr_null_test();
 	idr_nowait_test();
 	idr_get_next_test(0);