linux-kernel - Re: [PATCH v5 08/11] wait_bit: introduce {wait_on,wake_up}_atomic

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Date:   Mon, 12 Mar 2018 12:32:37 -0700
From:   Dan Williams <dan.j.williams@...el.com>
To:     Peter Zijlstra <peterz@...radead.org>
Cc:     linux-nvdimm <linux-nvdimm@...ts.01.org>,
        Ingo Molnar <mingo@...hat.com>, Christoph Hellwig <hch@....de>,
        david <david@...morbit.com>,
        linux-xfs <linux-xfs@...r.kernel.org>,
        linux-fsdevel <linux-fsdevel@...r.kernel.org>,
        Jan Kara <jack@...e.cz>,
        Ross Zwisler <ross.zwisler@...ux.intel.com>,
        Linux Kernel Mailing List <linux-kernel@...r.kernel.org>
Subject: Re: [PATCH v5 08/11] wait_bit: introduce {wait_on,wake_up}_atomic_one

On Sun, Mar 11, 2018 at 10:15 AM, Dan Williams <dan.j.williams@...el.com> wrote:
> On Sun, Mar 11, 2018 at 4:27 AM, Peter Zijlstra <peterz@...radead.org> wrote:
>> On Fri, Mar 09, 2018 at 10:55:32PM -0800, Dan Williams wrote:
>>> Add a generic facility for awaiting an atomic_t to reach a value of 1.
>>>
>>> Page reference counts typically need to reach 0 to be considered a
>>> free / inactive page. However, ZONE_DEVICE pages allocated via
>>> devm_memremap_pages() are never 'onlined', i.e. the put_page() typically
>>> done at init time to assign pages to the page allocator is skipped.
>>>
>>> These pages will have their reference count elevated > 1 by
>>> get_user_pages() when they are under DMA. In order to coordinate DMA to
>>> these pages vs filesytem operations like hole-punch and truncate the
>>> filesystem-dax implementation needs to capture the DMA-idle event i.e.
>>> the 2 to 1 count transition).
>>>
>>> For now, this implementation does not have functional behavior change,
>>> follow-on patches will add waiters for these page-idle events.
>>
>> Argh, no no no.. That whole wait_for_atomic_t thing is a giant
>> trainwreck already and now you're making it worse still.
>>
>> Please have a look here:
>>
>>   https://lkml.kernel.org/r/20171101190644.chwhfpoz3ywxx2m7@hirez.programming.kicks-ass.net
>
> That thread seems to be worried about the object disappearing the
> moment it's reference count reaches a target. That isn't the case with
> the memmap / struct page objects for ZONE_DEVICE pages. I understand
> wait_for_atomic_one() is broken in the general case, but as far as I
> can see it works fine specifically for ZONE_DEVICE page busy tracking,
> just not generic object lifetime.

Ok, that thread is also concerned with cleaning up the
wait_for_atomic_* pattern to also do something more idiomatic with
wait_event(). I agree that would be better, but I'm running short of
time to go refactor this aou for 4.17 inclusion, especially as I
expect another couple rounds of review on this more urgent data
corruption fix series that depends on this new api. I think the
addition of wait_for_atomic_one() makes it clear that we need a way to
pass a conditional expression rather than create a variant api for
each different condition. Can you help me out with an attempt of your
own, or at least point in a direction that you would accept for
solving the "Except the current wait_event() doesn't do the whole key
part that makes the hash-table 'work'."  problem that you highlighted?