Re: [PATCH RESEND v2 3/5] mm_zone: add function to check if managed dma zone exists

David Hildenbrand
2021-12-09T13:10:32Z

On 09.12.21 14:02, Baoquan He wrote:
> On 12/07/21 at 12:23pm, David Hildenbrand wrote:
>> On 07.12.21 04:07, Baoquan He wrote:
>>> In some places of the current kernel, it assumes that dma zone must have
>>> managed pages if CONFIG_ZONE_DMA is enabled. While this is not always true.
>>> E.g in kdump kernel of x86_64, only low 1M is presented and locked down
>>> at very early stage of boot, so that there's no managed pages at all in
>>> DMA zone. This exception will always cause page allocation failure if page
>>> is requested from DMA zone.
>>>
>>> Here add function has_managed_dma() and the relevant helper functions to
>>> check if there's DMA zone with managed pages. It will be used in later
>>> patches.
>>>
>>> Signed-off-by: Baoquan He <bhe@redhat.com>
>>> ---
>>> include/linux/mmzone.h | 21 +++++++++++++++++++++
>>> mm/page_alloc.c | 11 +++++++++++
>>> 2 files changed, 32 insertions(+)
>>>
>>> diff --git a/include/linux/mmzone.h b/include/linux/mmzone.h
>>> index 58e744b78c2c..82d23e13e0e5 100644
>>> --- a/include/linux/mmzone.h
>>> +++ b/include/linux/mmzone.h
>>> @@ -998,6 +998,18 @@ static inline bool zone_is_zone_device(struct zone *zone)
>>> }
>>> #endif
>>>
>>> +#ifdef CONFIG_ZONE_DMA
>>> +static inline bool zone_is_dma(struct zone *zone)
>>> +{
>>> + return zone_idx(zone) == ZONE_DMA;
>>> +}
>>> +#else
>>> +static inline bool zone_is_dma(struct zone *zone)
>>> +{
>>> + return false;
>>> +}
>>> +#endif
>>> +
>>> /*
>>> * Returns true if a zone has pages managed by the buddy allocator.
>>> * All the reclaim decisions have to use this function rather than
>>> @@ -1046,6 +1058,7 @@ static inline int is_highmem_idx(enum zone_type idx)
>>> #endif
>>> }
>>>
>>> +bool has_managed_dma(void);
>>> /**
>>> * is_highmem - helper function to quickly check if a struct zone is a
>>> * highmem zone or not. This is an attempt to keep references
>>> @@ -1131,6 +1144,14 @@ extern struct zone *next_zone(struct zone *zone);
>>> ; /* do nothing */ \
>>> else
>>>
>>> +#define for_each_managed_zone(zone) \
>>> + for (zone = (first_online_pgdat())->node_zones; \
>>> + zone; \
>>> + zone = next_zone(zone)) \
>>> + if (!managed_zone(zone)) \
>>> + ; /* do nothing */ \
>>> + else
>>> +
>>> static inline struct zone *zonelist_zone(struct zoneref *zoneref)
>>> {
>>> return zoneref->zone;
>>> diff --git a/mm/page_alloc.c b/mm/page_alloc.c
>>> index c5952749ad40..ac0ea42a4e5f 100644
>>> --- a/mm/page_alloc.c
>>> +++ b/mm/page_alloc.c
>>> @@ -9459,4 +9459,15 @@ bool take_page_off_buddy(struct page *page)
>>> spin_unlock_irqrestore(&zone->lock, flags);
>>> return ret;
>>> }
>>> +
>>> +bool has_managed_dma(void)
>>> +{
>>> + struct zone *zone;
>>> +
>>> + for_each_managed_zone(zone) {
>>> + if (zone_is_dma(zone))
>>> + return true;
>>> + }
>>> + return false;
>>> +}
>>
>> Wouldn't it be "easier/faster" to just iterate online nodes and directly
>> obtain the ZONE_DMA, checking if there are managed pages?
>
> Thanks, Dave.
>
> Please check for_each_managed_zone(), it is iterating online nodes and
> it's each managed zone.
>
> Is below what you are suggesting? The only difference is I introduced
> for_each_managed_zone() which can be reused later if needed. Not sure if
> I got your suggestion correctly.
>
> bool has_managed_dma(void)
> {
> struct pglist_data *pgdat;
> struct zone *zone;
> enum zone_type i, j;
>
> for_each_online_pgdat(pgdat) {
> for (i = 0; i < MAX_NR_ZONES - 1; i++) {
> struct zone *zone = &pgdat->node_zones[i];
> if (zone_is_dma(zone))
> return true;
> }
> }
> return false;
>
> }

Even simpler, no need to iterate over zones at all, only over nodes:

#ifdef CONFIG_ZONE_DMA
bool has_managed_dma(void)
{
struct pglist_data *pgdat;

for_each_online_pgdat(pgdat) {
struct zone *zone = &pgdat->node_zones[ZONE_DMA];

if (managed_zone(zone)
return true;
}
return false;
}
#endif /* CONFIG_ZONE_DMA */

Without CONFIG_ZONE_DMA, simply provide a dummy in the header that
returns false.

--
Thanks,

David / dhildenb