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

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Date:   Sun, 11 Mar 2018 10:15:55 -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 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.