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:

quoted

 
+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.

-- 
Sincerely yours,
Mike.