linux-kernel - Re: [PATCH 1/6] mm/page_alloc: Add page->buddy_list and page->pcp

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Message-ID: <20220421083811.GA4105@techsingularity.net>
Date:   Thu, 21 Apr 2022 09:38:11 +0100
From:   Mel Gorman <mgorman@...hsingularity.net>
To:     Matthew Wilcox <willy@...radead.org>
Cc:     Nicolas Saenz Julienne <nsaenzju@...hat.com>,
        Marcelo Tosatti <mtosatti@...hat.com>,
        Vlastimil Babka <vbabka@...e.cz>,
        Michal Hocko <mhocko@...nel.org>,
        LKML <linux-kernel@...r.kernel.org>,
        Linux-MM <linux-mm@...ck.org>
Subject: Re: [PATCH 1/6] mm/page_alloc: Add page->buddy_list and
 page->pcp_list

On Wed, Apr 20, 2022 at 09:43:05PM +0100, Matthew Wilcox wrote:
> On Wed, Apr 20, 2022 at 10:59:01AM +0100, Mel Gorman wrote:
> > The page allocator uses page->lru for storing pages on either buddy or
> > PCP lists. Create page->buddy_list and page->pcp_list as a union with
> > page->lru. This is simply to clarify what type of list a page is on
> > in the page allocator.
> 
> Hi Mel,
> 
> No objection to this change, and I certainly don't want to hold up
> fixing this (or any other) problem in the page allocator. 

Minimally, I think the patch makes it easier to implement your suggestion
and it can happen before or after the rest of the series.

> I would
> like to talk about splitting out free page management from struct page.
> Maybe you'd like to discuss that in person at LSFMM, but a quick
> sketch of a data structure might look like ...
> 

Unfortunately, I am unable to attend LSF/MM (or any other conference)
physically but I have no objection to splitting this out as a separate
structure. I assume the basis for the change would be for type checking.

> struct free_mem {
> 	unsigned long __page_flags;

page->flags, ok

> 	union {
> 		struct list_head buddy_list;
> 		struct list_head pcp_list;
> 	};

page->lru, ok

> 	unsigned long __rsvd4;

page->mapping, we need that

> 	unsigned long pcp_migratetype_and_order;

page->index, ok but more on this later

> 	unsigned long buddy_order;

page->private, ok.

> 	unsigned int __page_type;

page->_mapcount, we need that too.

> 	atomic_t _refcount;

page->_refcount, ok.

> };
> 
> Am I missing anything there?
> 

s/__page_flags/flags/	The allocator checks and modifies these bits so
	it has awareness of page->flags

s/__rsvd4/mapping/	The mapping is checked for debugging and the
	allocator is responsible for clearing page->mapping

s/pcp_migratetype_and_order/pcp_migratetype/
	Commit 8b10b465d0e1 ("mm/page_alloc: free pages in a single pass
	during bulk free") removed the migratetype and order stuffing
	in page->index. The order is inferred from the array index via
	order_to_pindex and pindex_to_order but migratetype is still
	stored in page->index by set_pcppage_migratetype

s/__page_type/_mapcount/ because _mapcount if checked for debugging

memcg_data needs to be accessible for debugging checks

_last_cpupid needs to be accessible as it's reset during prepare via
	page_cpupid_reset_last. Rather than putting in a dummy field
	for virtual, maybe virtual can move.

> (Would you like to use separate types for pcp and buddy?  That might be
> overkill, or it might help keep the different stages of "free" memory
> separate from each other)

I think it's overkill because there is too much overlap between a PCP
page and buddy page due to page checks and preparation. The distinguishing
factor between a free pcp page and free buddy page is the PageBuddy bit.

-- 
Mel Gorman
SUSE Labs