Re: [PATCHv7 02/14] mm: Add support for unaccepted memory

Dave Hansen
2022-08-05T14:41:42Z

On 8/5/22 06:38, Vlastimil Babka wrote:
>> I'm sure we could optimize for the !unaccepted memory via static keys
>> also in this version with some checks at the right places if we find
>> this to hurt performance?
> It would be great if we would at least somehow hit the necessary code only
> when dealing with a >=pageblock size block. The bitmap approach and
> accepting everything smaller uprofront actually seems rather compatible. Yet
> in the current patch we e.g. check PageUnaccepted(buddy) on every buddy size
> while merging.

Needing to check PageUnaccepted() during the merge is fallout from
moving the acceptance to post_alloc_hook(). I _think_ an earlier
version of this did page acceptance under the zone lock, closer to where
the page comes off the 2M/4M lists.

But, page acceptance is horribly slow, so I asked Kirill to move it out
from under the zone lock. Doing it in post_alloc_hook() (after the zone
lock is dropped) makes a lot of sense since we do zeroing in there and
zeroing is also nice and slow.

But, post_alloc_hook() is long after the 2M page has been split and that
means that we have to deal with potentially unaccepted pages during merges.

I think there are three basic options:

1. This patch: Do acceptance after the zone lock is dropped and deal
with mixed-acceptance merges
2. Do acceptance under the zone lock as pages come off the 2M/4M lists,
but before the page is split.
3. Pull the page off the 2M/4M lists, drop the zone lock, accept it,
then put it back.

I'm not sure any of those other options are better.