Re: [PATCH v6 08/10] mm/memory_hotplug: Don't check for "all holes" in shrink_zone_span()

[Baoquan He]

On 02/06/20 at 06:56am, Wei Yang wrote:
> On Wed, Feb 05, 2020 at 10:48:11PM +0800, Baoquan He wrote:
> >Hi Wei Yang,
> >
> >On 02/05/20 at 05:59pm, Wei Yang wrote:
> >> >diff --git a/mm/memory_hotplug.c b/mm/memory_hotplug.c
> >> >index f294918f7211..8dafa1ba8d9f 100644
> >> >--- a/mm/memory_hotplug.c
> >> >+++ b/mm/memory_hotplug.c
> >> >@@ -393,6 +393,9 @@ static void shrink_zone_span(struct zone *zone, 
> >> >unsigned long start_pfn,
> >> >          if (pfn) {
> >> >                  zone->zone_start_pfn = pfn;
> >> >                  zone->spanned_pages = zone_end_pfn - pfn;
> >> >+         } else {
> >> >+                 zone->zone_start_pfn = 0;
> >> >+                 zone->spanned_pages = 0;
> >> >          }
> >> >  } else if (zone_end_pfn == end_pfn) {
> >> >          /*
> >> >@@ -405,34 +408,11 @@ static void shrink_zone_span(struct zone *zone, 
> >> >unsigned long start_pfn,
> >> >                                         start_pfn);
> >> >          if (pfn)
> >> >                  zone->spanned_pages = pfn - zone_start_pfn + 1;
> >> >+         else {
> >> >+                 zone->zone_start_pfn = 0;
> >> >+                 zone->spanned_pages = 0;
> >> >+         }
> >> >  }
> >> 
> >> If it is me, I would like to take out these two similar logic out.
> >
> >I also like this style. 
> >> 
> >> For example:
> >> 
> >>    if () {
> >>    } else if () {
> >>    } else {
> >>            goto out;
> >Here the last else is unnecessary, right?
> >
> 
> I am afraid not.
> 
> If the range is not the first or last, we would leave pfn not initialized.

Ah, you are right. I forgot that one. Then pfn can be assigned the
zone_start_pfn as the old code. Then the following logic is the same
as the original code, find_smallest_section_pfn()/find_biggest_section_pfn() 
have done the iteration the old for loop was doing.

        unsigned long pfn = zone_start_pfn;     
        if () {
        } else if () {
        } 

        /* The zone has no valid section */
        if (!pfn) {
                zone->zone_start_pfn = 0;
                zone->spanned_pages = 0;
        }