Re: [PATCH RFC 7/9] mm/gup: Decrement head page once for group of subpages

[Joao Martins <joao.m.martins@oracle.com>]

On 12/19/20 2:06 AM, John Hubbard wrote:
> On 12/17/20 12:05 PM, Jason Gunthorpe wrote:
>> On Thu, Dec 17, 2020 at 07:05:37PM +0000, Joao Martins wrote:
>>>> No reason not to fix set_page_dirty_lock() too while you are here.
>>>
>>> The wack of atomics you mentioned earlier you referred to, I suppose it
>>> ends being account_page_dirtied(). See partial diff at the end.
>>
>> Well, even just eliminating the lock_page, page_mapping, PageDirty,
>> etc is already a big win.
>>
>> If mapping->a_ops->set_page_dirty() needs to be called multiple times
>> on the head page I'd probably just suggest:
>>
>>    while (ntails--)
>>          rc |= (*spd)(head);
> 
> I think once should be enough. There is no counter for page dirtiness,
> and this kind of accounting is always tracked in the head page, so there
> is no reason to repeatedly call set_page_dirty() from the same
> spot.
> 
I think that's what we do even today, considering the Dirty bit is only set on the
compound head (regardless of accounting). Even without this patch,
IIUC we don't call a second set_page_dirty(head) after the first time
we dirty it. So probably there's no optimization to do here, as you say.

	Joao