[RFC PATCH 1/1] mm: page_frag: account page_frag pages in vmstat and meminfo

[Yifan Li]

The page_frag allocator backs sk_buff->head and the frags portion of
skb_shared_info for the network stack and NIC drivers, but pages
obtained from it are not accounted by any memory statistics counter.
Unlike slab (NR_SLAB_*) or page_pool, page_frag pages are invisible
to tools that derive "LostRAM = MemTotal - (sum of accounted memory)",
which inflates LostRAM and makes memory diagnosis misleading.

Pages enter the cache through a single entry point,
__page_frag_cache_refill(), so account them there.  Decrement when the
cache page is freed via __page_frag_cache_drain() and page_frag_free().

Expose the counter, in pages, as "nr_page_frag" in /proc/vmstat and
"PageFrag" in /proc/meminfo so that page_frag memory is classified
instead of being counted as LostRAM.

Known limitation: a cache page handed over to an skb frags array is
released via put_page() (kfree_skb -> __skb_frag_unref ->
put_netmem -> put_page), bypassing page_frag_free(), so the counter
can over-count under network load.  Covering that open release path
requires marking the page and accounting at the common free path;
that will be posted separately for discussion.

Verified on an arm64 Android device with 4K pages: at boot, PageFrag
in /proc/meminfo reports 14432 kB, consistent with nr_page_frag
(3608 pages) in /proc/vmstat.

Signed-off-by: Yifan Li <liyifan21@xxxxxxxxxx>
---
 fs/proc/meminfo.c      |  1 +
 include/linux/mmzone.h |  1 +
 mm/page_frag_cache.c   | 14 ++++++++++++--
 mm/vmstat.c            |  1 +
 4 files changed, 15 insertions(+), 2 deletions(-)

diff --git a/fs/proc/meminfo.c b/fs/proc/meminfo.c
index b2813ff13..73241c3a3 100644
--- a/fs/proc/meminfo.c
+++ b/fs/proc/meminfo.c
@@ -108,6 +108,7 @@ static int meminfo_proc_show(struct seq_file *m, void *v)
        show_val_kb(m, "Slab:           ", sreclaimable + sunreclaim);
        show_val_kb(m, "SReclaimable:   ", sreclaimable);
        show_val_kb(m, "SUnreclaim:     ", sunreclaim);
+       show_val_kb(m, "PageFrag:       ", global_node_page_state(NR_PAGE_FRAG));
        seq_printf(m, "KernelStack:    %8lu kB\n",
                   global_node_page_state(NR_KERNEL_STACK_KB));
 #ifdef CONFIG_SHADOW_CALL_STACK
diff --git a/include/linux/mmzone.h b/include/linux/mmzone.h
index 94f9c3ff5..636f6ecb7 100644
--- a/include/linux/mmzone.h
+++ b/include/linux/mmzone.h
@@ -333,6 +333,7 @@ enum node_stat_item {
        NR_KERNEL_FILE_PAGES,
        NR_GPU_ACTIVE,  /* Pages assigned to GPU objects */
        NR_GPU_RECLAIM, /* Pages in shrinkable GPU pools */
+       NR_PAGE_FRAG,
        NR_VM_NODE_STAT_ITEMS
 };

diff --git a/mm/page_frag_cache.c b/mm/page_frag_cache.c
index e63efe78b..4f768093c 100644
--- a/mm/page_frag_cache.c
+++ b/mm/page_frag_cache.c
@@ -18,6 +18,7 @@
 #include <linux/init.h>
 #include <linux/mm.h>
 #include <linux/page_frag_cache.h>
+#include <linux/vmstat.h>
 #include "page_alloc.h"

 static unsigned long encoded_page_create(struct page *page, unsigned int order,
@@ -67,6 +68,9 @@ static struct page *__page_frag_cache_refill(struct page_frag_cache *nc,
        nc->encoded_page = page ?
                encoded_page_create(page, order, page_is_pfmemalloc(page)) : 0;

+       if (page)
+               mod_node_page_state(page_pgdat(page), NR_PAGE_FRAG,
+                                   1L << order);
        return page;
 }

@@ -85,8 +89,11 @@ void __page_frag_cache_drain(struct page *page, unsigned int count)
 {
        VM_BUG_ON_PAGE(page_ref_count(page) == 0, page);

-       if (page_ref_sub_and_test(page, count))
+       if (page_ref_sub_and_test(page, count)) {
+               mod_node_page_state(page_pgdat(page), NR_PAGE_FRAG,
+                                    -(1L << compound_order(page)));
                free_frozen_pages(page, compound_order(page));
+       }
 }
 EXPORT_SYMBOL(__page_frag_cache_drain);

@@ -165,7 +172,10 @@ void page_frag_free(void *addr)
 {
        struct page *page = virt_to_head_page(addr);

-       if (unlikely(put_page_testzero(page)))
+       if (unlikely(put_page_testzero(page))) {
+               mod_node_page_state(page_pgdat(page), NR_PAGE_FRAG,
+                                    -(1L << compound_order(page)));
                free_frozen_pages(page, compound_order(page));
+       }
 }
 EXPORT_SYMBOL(page_frag_free);
diff --git a/mm/vmstat.c b/mm/vmstat.c
index cb5771453..883137cb6 100644
--- a/mm/vmstat.c
+++ b/mm/vmstat.c
@@ -1311,6 +1311,7 @@ const char * const vmstat_text[] = {
        [I(NR_KERNEL_FILE_PAGES)]               = "nr_kernel_file_pages",
        [I(NR_GPU_ACTIVE)]                      = "nr_gpu_active",
        [I(NR_GPU_RECLAIM)]                     = "nr_gpu_reclaim",
+       [I(NR_PAGE_FRAG)]                       = "nr_page_frag",
 #undef I

        /* system-wide enum vm_stat_item counters */
--
2.46.0.windows.1

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