linux-kernel - Re: [PATCH v3 1/1] selftests/mm/uffd: Refactor non-composite global vars into struct

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Message-ID: <DA5Z38N5WHO5.2FFOQZYC6WKMI@google.com>
Date: Mon, 26 May 2025 09:08:46 +0000
From: Brendan Jackman <jackmanb@...gle.com>
To: Ujwal Kundur <ujwal.kundur@...il.com>
Cc: <akpm@...ux-foundation.org>, <peterx@...hat.com>, <shuah@...nel.org>, 
	<linux-mm@...ck.org>, <linux-kselftest@...r.kernel.org>, 
	<linux-kernel@...r.kernel.org>
Subject: Re: [PATCH v3 1/1] selftests/mm/uffd: Refactor non-composite global
 vars into struct

On Sun May 25, 2025 at 7:19 PM UTC, Ujwal Kundur wrote:
>> I'm afraid I'm too ignorant of this code to be able to suggest something
>> good here. But, can we just remove the comment and plumb the gopts
>> through to uffd_poll_thread()->uffd_handle_page_fault()->__copy_page()?
>>
>> This is not pretty but it lets us remove the global vars which is
>> clearly a step in the right direction.
>
> Perhaps Andrew can weigh in? If I understood this correctly, we're
> trying to assert that retrying a successful UFFDIO_COPY operation
> always results in EEXIST. This is being done in a somewhat racy
> fashion where a flag (test_uffdio_copy_eexist) is set every 10 seconds
> using alarm(2). IMO this is a flaky test, we should either:
> - remove this variable and associated logic entirely (preferred)
> - use a probability function to set this a % of the time instead of
> every 10 seconds
> - use an async library that can replace the implementation without the
> use of global vars

Sorry I don't have an opinion on which of these is the best (I can try
to find some time to form an opionion on this later!), but:

Fixing the flakiness sounds great, but I would suggest decoupling that
from the refactoring. If it's practical, focus on removing the globals
first, while leaving the fundamental logic the same, even if it's bad.
Then as a separate series, fix the logic.