Re: [PATCH v4 2/2] mm: Init page count in reserve_bootmem_region when MEMINIT_EARLY

From: Yajun Deng
Date: Mon Oct 09 2023 - 22:32:10 EST



On 2023/10/8 16:57, Yajun Deng wrote:

On 2023/10/2 16:30, David Hildenbrand wrote:
On 29.09.23 10:30, Mike Rapoport wrote:
On Thu, Sep 28, 2023 at 04:33:02PM +0800, Yajun Deng wrote:
memmap_init_range() would init page count of all pages, but the free
pages count would be reset in __free_pages_core(). There are opposite
operations. It's unnecessary and time-consuming when it's MEMINIT_EARLY
context.

Init page count in reserve_bootmem_region when in MEMINIT_EARLY context,
and check the page count before reset it.

At the same time, the INIT_LIST_HEAD in reserve_bootmem_region isn't
need, as it already done in __init_single_page.

The following data was tested on an x86 machine with 190GB of RAM.

before:
free_low_memory_core_early()    341ms

after:
free_low_memory_core_early()    285ms

Signed-off-by: Yajun Deng <yajun.deng@xxxxxxxxx>
---
v4: same with v2.
v3: same with v2.
v2: check page count instead of check context before reset it.
v1: https://lore.kernel.org/all/20230922070923.355656-1-yajun.deng@xxxxxxxxx/
---
  mm/mm_init.c    | 18 +++++++++++++-----
  mm/page_alloc.c | 20 ++++++++++++--------
  2 files changed, 25 insertions(+), 13 deletions(-)

diff --git a/mm/mm_init.c b/mm/mm_init.c
index 9716c8a7ade9..3ab8861e1ef3 100644
--- a/mm/mm_init.c
+++ b/mm/mm_init.c
@@ -718,7 +718,7 @@ static void __meminit init_reserved_page(unsigned long pfn, int nid)
          if (zone_spans_pfn(zone, pfn))
              break;
      }
-    __init_single_page(pfn_to_page(pfn), pfn, zid, nid, INIT_PAGE_COUNT);
+    __init_single_page(pfn_to_page(pfn), pfn, zid, nid, 0);
  }
  #else
  static inline void pgdat_set_deferred_range(pg_data_t *pgdat) {}
@@ -756,8 +756,8 @@ void __meminit reserve_bootmem_region(phys_addr_t start,
                init_reserved_page(start_pfn, nid);
  -            /* Avoid false-positive PageTail() */
-            INIT_LIST_HEAD(&page->lru);
+            /* Init page count for reserved region */

Please add a comment that describes _why_ we initialize the page count here.

+            init_page_count(page);
                /*
               * no need for atomic set_bit because the struct
@@ -888,9 +888,17 @@ void __meminit memmap_init_range(unsigned long size, int nid, unsigned long zone
          }
            page = pfn_to_page(pfn);
-        __init_single_page(page, pfn, zone, nid, INIT_PAGE_COUNT);
-        if (context == MEMINIT_HOTPLUG)
+
+        /* If the context is MEMINIT_EARLY, we will init page count and
+         * mark page reserved in reserve_bootmem_region, the free region
+         * wouldn't have page count and we will check the pages count
+         * in __free_pages_core.
+         */
+        __init_single_page(page, pfn, zone, nid, 0);
+        if (context == MEMINIT_HOTPLUG) {
+            init_page_count(page);
              __SetPageReserved(page);

Rather than calling init_page_count() and __SetPageReserved() for
MEMINIT_HOTPLUG you can set flags to INIT_PAGE_COUNT | INIT_PAGE_RESERVED
an call __init_single_page() after the check for MEMINIT_HOTPLUG.

But more generally, I wonder if we have to differentiate HOTPLUG here at all.
@David, can you comment please?

There are a lot of details to that, and I'll share some I can briefly think of.

1) __SetPageReserved()

I tried removing that a while ago, but there was a blocker (IIRC something about
ZONE_DEVICE). I still have the patches at [1] and I could probably take a look
if that blocker still exists (I recall that something changed at some point, but
I never had the time to follow up).

But once we stop setting the pages reserved, we might run into issues with ...


2) init_page_count()

virtio-mem, XEN balloon and HV-balloon add memory blocks that can contain holes.
set_online_page_callback() is used to intercept memory onlining and to expose
only the pages that are not holes to the buddy: calling generic_online_page() on !hole.

Holes are PageReserved but with an initialized page count. Memory offlining will fail on
PageReserved pages -- has_unmovable_pages().


At least virtio-mem clears the PageReserved flag of holes when onlining memory,
and currently relies in the page count to be reasonable (so memory offlining can work).

static void virtio_mem_set_fake_offline(unsigned long pfn,
                    unsigned long nr_pages, bool onlined)
{
    page_offline_begin();
    for (; nr_pages--; pfn++) {
        struct page *page = pfn_to_page(pfn);

        __SetPageOffline(page);
        if (!onlined) {
            SetPageDirty(page);
            /* FIXME: remove after cleanups */
            ClearPageReserved(page);
        }
    }
    page_offline_end();
}


For virtio-mem, we could initialize the page count there instead. The other PV drivers
might require a bit more thought.


[1] https://github.com/davidhildenbrand/linux/tree/online_reserved_cleanup


+        }
            /*
           * Usually, we want to mark the pageblock MIGRATE_MOVABLE,
diff --git a/mm/page_alloc.c b/mm/page_alloc.c
index 06be8821d833..b868caabe8dc 100644
--- a/mm/page_alloc.c
+++ b/mm/page_alloc.c
@@ -1285,18 +1285,22 @@ void __free_pages_core(struct page *page, unsigned int order)
      unsigned int loop;
        /*
-     * When initializing the memmap, __init_single_page() sets the refcount
-     * of all pages to 1 ("allocated"/"not free"). We have to set the
-     * refcount of all involved pages to 0.
+     * When initializing the memmap, memmap_init_range sets the refcount
+     * of all pages to 1 ("reserved" and "free") in hotplug context. We
+     * have to set the refcount of all involved pages to 0. Otherwise,
+     * we don't do it, as reserve_bootmem_region only set the refcount on
+     * reserve region ("reserved") in early context.
       */

Again, why hotplug and early init should be different?

-    prefetchw(p);
-    for (loop = 0; loop < (nr_pages - 1); loop++, p++) {
-        prefetchw(p + 1);
+    if (page_count(page)) {
+        prefetchw(p);
+        for (loop = 0; loop < (nr_pages - 1); loop++, p++) {
+            prefetchw(p + 1);
+            __ClearPageReserved(p);
+            set_page_count(p, 0);
+        }
          __ClearPageReserved(p);
          set_page_count(p, 0);

That looks wrong. if the page count would by pure luck be 0 already for hotplugged memory,
you wouldn't clear the reserved flag.

These changes make me a bit nervous.


Is 'if (page_count(page) || PageReserved(page))' be safer? Or do I need to do something else?


How about the following if statement? But it needs to add more patch like v1 ([PATCH 2/4] mm: Introduce MEMINIT_LATE context).

It'll be safer, but more complex. Please comment...

if (context != MEMINIT_EARLY || (page_count(page) || PageReserved(page)) {