Re: [PATCH v2 2/2] mm,page_owner: Fix accounting of pages when migrating

[Oscar Salvador]

On Thu, Mar 21, 2024 at 11:50:36AM +0100, Vlastimil Babka wrote:
> Yeah I think we could keep that logic.

I am all for keeping it.

> But we could also simply subtract the refcount of the old handle (the
> "allocated for migration") in __folio_copy_owner() no? Then we wouldn't need
> the extra migrate_handle.

Since new_page will have the old handle pointing to the old stack after
the call, we
could uncharge the old_page to the migrate_stack, which new_page->_handle holds
before it gets changed.

So we basically swap it.

It could work, but I kinda have a bittersweet feeling here.
I am trying to work towards to reduce the number of lookups in the
hlist, but for the approach described above I would need to lookup
the stack for new_page->handle in order to substract the page.

OTHO, I understand that adding migrate_handle kinda wasted memory.
16MB for 16GB of memory.

> Also we might have more issues here. Most page owner code takes care to set
> everything for all pages within a folio, but __folio_copy_owner() and
> __set_page_owner_migrate_reason() don't.

I did not check deeply but do not we split the folio upon migration
in case it is large?
Which means we should reach split_page_owner() before the copy takes
place. Do I get it right?

-- 
Oscar Salvador
SUSE Labs