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

[Kevin Brodsky <kevin.brodsky@arm.com>]

On 17/12/2025 11:04, Mark Brown wrote:
> On Tue, Dec 16, 2025 at 02:26:30PM +0000, 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.
> More generally building selftests with random older kernel versions
> isn't really something that's expected to be robust:

I suppose that Documentation/dev-tools/kselftest.rst talks about
*running* against older kernels, not *building* against them. That said,
we are dealing with an out-of-tree kernel module here, so the two are
essentially the same... Yunsheng suggested an updated check that I think
is reasonable, maybe it is a reasonable compromise?

- Kevin