Re: [PATCH v4] selftests/mm: add process_madvise() tests

[wang lian <lianux.mm@gmail.com>]

Hi Lorenzo Stoakes,

>> + *
>> + * This test deterministically validates process_madvise() with MADV_COLLAPSE
>> + * on a remote process, other advices are difficult to verify reliably.
>> + *
>> + * The test verifies that a memory region in a child process, initially
>> + * backed by small pages, can be collapsed into a Transparent Huge Page by a
>> + * request from the parent. The result is verified by parsing the child's
>> + * /proc/<pid>/smaps file.
>> + */

> This is clever and you've put a lot of effort in, but this just seems
> absolutely prone to flaking and you're essentially testing something that's
> highly automated.

> I think you're also going way outside of the realms of testing
> process_madvise() and are getting into testing essentially MADV_COLLAPSE
> here.

> We have to try to keep the test specific to what it is you're testing -
> which is process_madvise() itself.

> So for me, and I realise you've put a ton of work into this and I'm really
> sorry to say it, I think you should drop this specific test.

> For me simply testing the remote MADV_DONTNEED is enough.

My motivation for this complex test came from the need to verify that
the process_madvise operation was actually successful. Without checking
the outcome, the test would only validate that the syscall returns the
correct number of bytes, not that the advice truly took effect on the
target process's memory.

For remote calls, process_madvise is intentionally limited to
non-destructive advice: MADV_COLD, MADV_PAGEOUT, MADV_WILLNEED,
and MADV_COLLAPSE. However, verifying the effects of COLD, PAGEOUT,
and WILLNEED is very difficult to do reliably in a selftest. This left
MADV_COLLAPSE as what seemed to be the only verifiable option.

But, as you correctly pointed out, MADV_COLLAPSE is too dependent on
the system's THP state and prone to races with khugepaged. This is the
very issue I tried to work around in v4 after the v3 test failures. 
So I think this test is necessary. 
As for your other opinions, I completely agree.



Best regards,
Wang Lian