Re: [PATCH RCFv2 1/7] mm: introduce and use PageOffline()

[David Hildenbrand]

On 30.04.2018 16:35, Pavel Tatashin wrote:
> Hi Dave,
> 
> A few comments below:
> 
>> +	for (i = 0; i < PAGES_PER_SECTION; i++) {
> 
> Performance wise, this is unfortunate that we have to add this loop for every hot-plug. But, I do like the finer hot-plug granularity that you achieve, and do not have a better suggestion how to avoid this loop. What I also like, is that you call init_single_page() only one time.

Thanks! Yes, unfortunately we cannot live with the single loop when
onlining pages for this feature.

> 
>> +		unsigned long pfn = phys_start_pfn + i;
>> +		struct page *page;
>> +		if (!pfn_valid(pfn))
>> +			continue;
>> +		page = pfn_to_page(pfn);
>> +
>> +		/* dummy zone, the actual one will be set when onlining pages */
>> +		init_single_page(page, pfn, ZONE_NORMAL, nid);
> 
> Is there a reason to use ZONE_NORMAL as a dummy zone? May be define some non-existent zone-id for that? I.e. __MAX_NR_ZONES? That might trigger some debugging checks of course..

Than it could happen that we consume more bits in pageflags than we
actually need. But it could be an opt-in debugging option later on, right?

> 
> In init_single_page() if WANT_PAGE_VIRTUAL is defined it is used to set virtual address.  Which is broken if we do not belong to ZONE_NORMAL.
> 

Grr, missed that. Thanks for your very good eyes!

> 1186	if (!is_highmem_idx(zone))
> 1187		set_page_address(page, __va(pfn << PAGE_SHIFT));
> 
> Otherwise, if you want to keep ZONE_NORMAL here, you could add a new function:
> 
> #ifdef WANT_PAGE_VIRTUAL
> static void set_page_virtual(struct page *page, and enum zone_type zone)
> {
> 	/* The shift won't overflow because ZONE_NORMAL is below 4G. */
> 	if (!is_highmem_idx(zone))
> 		set_page_address(page, __va(pfn << PAGE_SHIFT));
> }
> #else
> static inline void set_page_virtual(struct page *page, and enum zone_type zone)
> {}
> #endif
> 
> And call it from init_single_page(), and from __meminit memmap_init_zone() in "context == MEMMAP_HOTPLUG" if case.

Was thinking about moving it to set_page_zone() and conditionally
setting it to 0 or set_page_address(page, __va(pfn << PAGE_SHIFT)). What
do you prefer?

> 
>>
>> -static void __meminit __init_single_page(struct page *page, unsigned long pfn,
>> +extern void __meminit init_single_page(struct page *page, unsigned long pfn,
> 
> I've seen it in other places, but what is the point of having "extern" function in .c file?

I've seen it all over the place, that's why I am using it :) (as I
basically had the same question). Can somebody answer that?

> 
> 
>>  #ifdef CONFIG_MEMORY_HOTREMOVE
>> -/* Mark all memory sections within the pfn range as online */
>> +static bool all_pages_in_section_offline(unsigned long section_nr)
>> +{
>> +	unsigned long pfn = section_nr_to_pfn(section_nr);
>> +	struct page *page;
>> +	int i;
>> +
>> +	for (i = 0; i < PAGES_PER_SECTION; i++, pfn++) {
>> +		if (!pfn_valid(pfn))
>> +			continue;
>> +
>> +		page = pfn_to_page(pfn);
>> +		if (!PageOffline(page))
>> +			return false;
>> +	}
>> +	return true;
>> +}
> 
> Perhaps we could use some counter to keep track of number of subsections that are currently offlined? If section covers 128M of memory, and offline/online is 4M granularity, there are up-to 32 subsections in a section, and thus we need 5-bits to count them. I'm not sure if there is a space in mem_section for this counter. But, that would eliminate the loop above.

Yes, that would also be an optimization. At least I optimized it for now
so ordinary offline/online is not harmed. As we need PageOffline() also
for kdump (and maybe later also for safety checks when
onlining/offlining pages), we would right now store duplicate
information, so I would like to defer that.

Thanks a lot Pavel!

> 
> Thank you,
> Pavel
> 


-- 

Thanks,

David / dhildenb