Messages in this thread | | | Date | Thu, 21 Apr 2022 09:38:11 +0100 | From | Mel Gorman <> | Subject | Re: [PATCH 1/6] mm/page_alloc: Add page->buddy_list and page->pcp_list |
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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
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