lkml.org 
[lkml]   [2018]   [Mar]   [11]   [last100]   RSS Feed
Views: [wrap][no wrap]   [headers]  [forward] 
 
Messages in this thread
/
From
Date
SubjectRe: [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@infradead.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.

\
 
 \ /
  Last update: 2018-03-11 18:17    [W:0.102 / U:0.280 seconds]
©2003-2020 Jasper Spaans|hosted at Digital Ocean and TransIP|Read the blog|Advertise on this site