linux-kernel - Re: [PATCH 1/4] selftests/mm: remove flaky header check

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Message-ID: <c59eb8d7-c6bb-4618-b7d4-2ce5df88b5f1@arm.com>
Date: Wed, 17 Dec 2025 10:58:48 +0100
From: Kevin Brodsky <kevin.brodsky@....com>
To: Yunsheng Lin <linyunsheng@...wei.com>, linux-mm@...ck.org,
 linux-kselftest@...r.kernel.org
Cc: linux-kernel@...r.kernel.org, Andrew Morton <akpm@...ux-foundation.org>,
 David Hildenbrand <david@...nel.org>,
 Lorenzo Stoakes <lorenzo.stoakes@...cle.com>, Mark Brown
 <broonie@...nel.org>, Ryan Roberts <ryan.roberts@....com>,
 Shuah Khan <shuah@...nel.org>, Paolo Abeni <pabeni@...hat.com>
Subject: Re: [PATCH 1/4] selftests/mm: remove flaky header check

On 17/12/2025 04:18, Yunsheng Lin wrote:
> On 2025/12/16 22:26, Kevin Brodsky wrote:
>> Commit 96ed62ea0298 ("mm: page_frag: fix a compile error when kernel
>> is not compiled") introduced a check to avoid attempting to build
>> the page_frag module if <linux/page_frag_cache.h> is missing.
>>
>> Unfortunately this check only works if KDIR points to
>> /lib/modules/... or an in-tree kernel build. It always fails if KDIR
>> points to an out-of-tree build (i.e. when the kernel was built with
>> O=$KDIR make) because only generated headers are present under
>> $KDIR/include/ in that case.
>>
>> <linux/page_frag_cache.h> was added more than a year ago (v6.13) so
>> we can probably live without that check.
> As some commercial OS still uses v6.6, I am wondering if we need that

Fair point, I hadn't considered that kselftests are supposed to be
buildable against older stable kernels.

> check for a little longer, is it possible to do something like below to
> avoid the flaky check?
>
> @@ -46,7 +46,8 @@ CFLAGS += -U_FORTIFY_SOURCE
>
>  KDIR ?= /lib/modules/$(shell uname -r)/build
>  ifneq (,$(wildcard $(KDIR)/Module.symvers))
> -ifneq (,$(wildcard $(KDIR)/include/linux/page_frag_cache.h))
> +KSRC := $(shell readlink -f $(KDIR)/source 2>/dev/null || echo $(KDIR))
> +ifneq (,$(wildcard $(KSRC)/include/linux/page_frag_cache.h))
>  TEST_GEN_MODS_DIR := page_frag
>  else
>  PAGE_FRAG_WARNING = "missing page_frag_cache.h, please use a newer kernel"

That seems reasonable, and it works for my out-of-tree setup.

Will do that in v2, shall I add your Suggested-by, or maybe Co-developed-by?

- Kevin