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

Baoquan He
2021-12-09T13:02:46Z

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;

}