Re: [PATCH v4 2/4] memblock: update initialization of reserved pages

[David Woodhouse]

On Mon, 2025-03-31 at 17:50 +0300, Mike Rapoport wrote:
> On Mon, Mar 31, 2025 at 01:50:33PM +0100, David Woodhouse wrote:
> > On Tue, 2021-05-11 at 13:05 +0300, Mike Rapoport wrote:
> > >  
> > > +static void __init memmap_init_reserved_pages(void)
> > > +{
> > > +	struct memblock_region *region;
> > > +	phys_addr_t start, end;
> > > +	u64 i;
> > > +
> > > +	/* initialize struct pages for the reserved regions */
> > > +	for_each_reserved_mem_range(i, &start, &end)
> > > +		reserve_bootmem_region(start, end);
> > > +
> > > +	/* and also treat struct pages for the NOMAP regions as PageReserved */
> > > +	for_each_mem_region(region) {
> > > +		if (memblock_is_nomap(region)) {
> > > +			start = region->base;
> > > +			end = start + region->size;
> > > +			reserve_bootmem_region(start, end);
> > > +		}
> > > +	}
> > > +}
> > > +
> > 
> > In some cases, that whole call to reserve_bootmem_region() may be a no-
> > op because pfn_valid() is not true for *any* address in that range.
> > 
> > But reserve_bootmem_region() spends a long time iterating of them all,
> > and eventually doing nothing:
> > 
> > void __meminit reserve_bootmem_region(phys_addr_t start,
> >                                       phys_addr_t end, int nid)
> > {
> >         unsigned long start_pfn = PFN_DOWN(start);
> >         unsigned long end_pfn = PFN_UP(end);
> > 
> >         for (; start_pfn < end_pfn; start_pfn++) {
> >                 if (pfn_valid(start_pfn)) {
> >                         struct page *page = pfn_to_page(start_pfn);
> > 
> >                         init_reserved_page(start_pfn, nid);
> > 
> >                         /*
> >                          * no need for atomic set_bit because the struct
> >                          * page is not visible yet so nobody should
> >                          * access it yet.
> >                          */
> >                         __SetPageReserved(page);
> >                 }
> >         }
> > }
> > 
> > On platforms with large NOMAP regions (e.g. which are actually reserved
> > for guest memory to keep it out of the Linux address map and allow for
> > kexec-based live update of the hypervisor), this pointless loop ends up
> > taking a significant amount of time which is visible as guest steal
> > time during the live update.
> > 
> > Can reserve_bootmem_region() skip the loop *completely* if no PFN in
> > the range from start to end is valid? Or tweak the loop itself to have
> > an 'else' case which skips to the next valid PFN? Something like
> > 
> >  for(...) {
> >     if (pfn_valid(start_pfn)) {
> >        ...
> >     } else {
> >        start_pfn = next_valid_pfn(start_pfn);
> >     }
> >  }
> 
> My understanding is that you have large reserved NOMAP ranges that don't
> appear as memory at all, so no memory map for them is created and so
> pfn_valid() is false for pfns in those ranges.
> 
> If this is the case one way indeed would be to make
> reserve_bootmem_region() skip ranges with no valid pfns.
> 
> Another way could be to memblock_reserved_mark_noinit() such ranges and
> then reserve_bootmem_region() won't even get called, but that would require
> firmware to pass that information somehow.

I was thinking along these lines (not even build tested)...

I don't much like the (unsigned long)-1 part. I might make the helper
'static inline bool first_valid_pfn (unsigned long *pfn)' and return
success or failure. But that's an implementation detail.

index 6d1fb6162ac1..edd27ba3e908 100644
--- a/include/asm-generic/memory_model.h
+++ b/include/asm-generic/memory_model.h
@@ -29,8 +29,43 @@ static inline int pfn_valid(unsigned long pfn)
        return pfn >= pfn_offset && (pfn - pfn_offset) < max_mapnr;
 }
 #define pfn_valid pfn_valid
+
+static inline unsigned long first_valid_pfn(unsigned long pfn)
+{
+       /* avoid <linux/mm.h> include hell */
+       extern unsigned long max_mapnr;
+       unsigned long pfn_offset = ARCH_PFN_OFFSET;
+
+       if (pfn < pfn_offset)
+               return pfn_offset;
+
+       if ((pfn - pfn_offset) < max_mapnr)
+               return pfn;
+
+       return (unsigned long)(-1);
+}
+
+#ifndef for_each_valid_pfn
+#define for_each_valid_pfn(pfn, start_pfn, end_pfn)            \
+       /* Sanity check on the end condition */                 \
+       BUG_ON(end_pfn == (unsigned long)-1);                   \
+       for (pfn = first_valid_pfn(pfn); pfn < end_pfn;         \
+            pfn = first_valid_pfn(pfn + 1))
+#endif
+#endif
+
+/*
+ * If the architecture provides its own pfn_valid(), it can either
+ * provide a matching for_each_valid_pfn() or use the fallback which
+ * just iterates over them *all*, calling pfn_valid() for each.
+ */
+#ifndef for_each_valid_pfn
+#define for_each_valid_pfn(pfn, start_pfn, end_pfn)            \
+       for (pfn = start_pfn; pfn < end_pfn, pfn++) {           \
+               if (pfn_valid(pfn))
 #endif
 
+
 #elif defined(CONFIG_SPARSEMEM_VMEMMAP)
 
 /* memmap is virtually contiguous.  */

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