linux-kernel - Re: [PATCH v2 1/2] seqlock: Do the lockdep annotation before locking in do_write_seqcount_begin

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Date:   Fri, 4 Aug 2023 22:27:22 +0900
From:   Tetsuo Handa <penguin-kernel@...ove.SAKURA.ne.jp>
To:     Michal Hocko <mhocko@...e.com>,
        Sebastian Andrzej Siewior <bigeasy@...utronix.de>,
        Andrew Morton <akpm@...ux-foundation.org>
Cc:     Petr Mladek <pmladek@...e.com>, linux-mm@...ck.org,
        linux-kernel@...r.kernel.org,
        "Luis Claudio R. Goncalves" <lgoncalv@...hat.com>,
        Boqun Feng <boqun.feng@...il.com>,
        Ingo Molnar <mingo@...hat.com>,
        John Ogness <john.ogness@...utronix.de>,
        Mel Gorman <mgorman@...hsingularity.net>,
        Peter Zijlstra <peterz@...radead.org>,
        Thomas Gleixner <tglx@...utronix.de>,
        Waiman Long <longman@...hat.com>, Will Deacon <will@...nel.org>
Subject: Re: [PATCH v2 1/2] seqlock: Do the lockdep annotation before locking
 in do_write_seqcount_begin_nested()

On 2023/08/03 23:49, Michal Hocko wrote:
> On Thu 03-08-23 22:18:10, Tetsuo Handa wrote:
>> On 2023/07/31 23:25, Michal Hocko wrote:
>>> On Sat 29-07-23 20:05:43, Tetsuo Handa wrote:
>>>> On 2023/07/29 14:31, Tetsuo Handa wrote:
>>>>> On 2023/07/28 0:10, Sebastian Andrzej Siewior wrote:
>>>>>> On 2023-06-28 21:14:16 [+0900], Tetsuo Handa wrote:
>>>>>>>> Anyway, please do not do this change only because of printk().
>>>>>>>> IMHO, the current ordering is more logical and the printk() problem
>>>>>>>> should be solved another way.
>>>>>>>
>>>>>>> Then, since [PATCH 1/2] cannot be applied, [PATCH 2/2] is automatically
>>>>>>> rejected.
>>>>>>
>>>>>> My understanding is that this patch gets applied and your objection will
>>>>>> be noted.
>>>>>
>>>>> My preference is that zonelist_update_seq is not checked by !__GFP_DIRECT_RECLAIM
>>>>> allocations, which is a low-hanging fruit towards GFP_LOCKLESS mentioned at
>>>>> https://lkml.kernel.org/r/ZG3+l4qcCWTPtSMD@dhcp22.suse.cz and
>>>>> https://lkml.kernel.org/r/ZJWWpGZMJIADQvRS@dhcp22.suse.cz .
>>>>>
>>>>> Maybe we can defer checking zonelist_update_seq till retry check like below,
>>>>> for this is really an infrequent event.
>>>>>
>>>>
>>>> An updated version with comments added.
>>>
>>> Seriously, don't you see how hairy all this is? And for what? Nitpicking
>>> something that doesn't seem to be a real problem in the first place?
>>
>> Seriously, can't you find "zonelist_update_seq is not checked by !__GFP_DIRECT_RECLAIM
>> allocations, which is a low-hanging fruit towards GFP_LOCKLESS" !?
> 
> I do not think we have concluded that we want to support GFP_LOCKLESS.
> This might be trivial straightforward now but it imposes some constrains
> for future maintainability. So far we haven't heard about many usecases
> where this would be needed and a single one is not sufficient IMHO.

When you introduced a word GFP_LOCKLESS in the link above, I was wondering the meaning
of "LESS" part. Since we know that it is difficult to achieve "hold 0 lock during memory
allocation", "hold least locks during memory allocation" will be at best. Therefore,
GFP_LOCKLESS is as misleading name as GFP_ATOMIC. GFP_LOCK_LEAST or GFP_LEAST_LOCKS will
represent the real behavior better.

Like I said

  I consider that memory allocations which do not do direct reclaim should be geared
  towards less locking dependency.

in the thread above, I still believe that this what-you-call-hairy version (which
matches "hold least locks during memory allocation" direction) is better than
"[PATCH v3 2/2] mm/page_alloc: Use write_seqlock_irqsave() instead write_seqlock() + local_irq_save()."
(which does not match "hold least locks during memory allocation"). My version not
only avoids possibility of deadlock, but also makes zonelist_iter_begin() faster and
simpler.

Not holding zonelist_update_seq lock is trivially doable compared to removing
__GFP_KSWAPD_RECLAIM from GFP_ATOMIC. Please give me feedback about which line of my
proposal is technically unsafe, instead of discarding my proposal with negative words
like "hairy".