Date:   Fri, 16 Oct 2020 16:00:46 +0800
From:   Wei Yang <richard.weiyang@...ux.alibaba.com>
To:     Wei Yang <richard.weiyang@...ux.alibaba.com>
Cc:     David Hildenbrand <david@...hat.com>, linux-kernel@...r.kernel.org,
        linux-mm@...ck.org, virtualization@...ts.linux-foundation.org,
        Andrew Morton <akpm@...ux-foundation.org>,
        "Michael S . Tsirkin" <mst@...hat.com>,
        Jason Wang <jasowang@...hat.com>,
        Pankaj Gupta <pankaj.gupta.linux@...il.com>
Subject: Re: [PATCH v1 13/29] virtio-mem: factor out handling of fake-offline
 pages in memory notifier

On Fri, Oct 16, 2020 at 03:15:03PM +0800, Wei Yang wrote:
>On Mon, Oct 12, 2020 at 02:53:07PM +0200, David Hildenbrand wrote:
>>Let's factor out the core pieces and place the implementation next to
>>virtio_mem_fake_offline(). We'll reuse this functionality soon.
>>
>>Cc: "Michael S. Tsirkin" <mst@...hat.com>
>>Cc: Jason Wang <jasowang@...hat.com>
>>Cc: Pankaj Gupta <pankaj.gupta.linux@...il.com>
>>Signed-off-by: David Hildenbrand <david@...hat.com>
>>---
>> drivers/virtio/virtio_mem.c | 73 +++++++++++++++++++++++++------------
>> 1 file changed, 50 insertions(+), 23 deletions(-)
>>
>>diff --git a/drivers/virtio/virtio_mem.c b/drivers/virtio/virtio_mem.c
>>index d132bc54ef57..a2124892e510 100644
>>--- a/drivers/virtio/virtio_mem.c
>>+++ b/drivers/virtio/virtio_mem.c
>>@@ -168,6 +168,10 @@ static LIST_HEAD(virtio_mem_devices);
>> 
>> static void virtio_mem_online_page_cb(struct page *page, unsigned int order);
>> static void virtio_mem_retry(struct virtio_mem *vm);
>>+static void virtio_mem_fake_offline_going_offline(unsigned long pfn,
>>+						  unsigned long nr_pages);
>>+static void virtio_mem_fake_offline_cancel_offline(unsigned long pfn,
>>+						   unsigned long nr_pages);
>> 
>> /*
>>  * Register a virtio-mem device so it will be considered for the online_page
>>@@ -604,27 +608,15 @@ static void virtio_mem_notify_going_offline(struct virtio_mem *vm,
>> 					    unsigned long mb_id)
>> {
>> 	const unsigned long nr_pages = PFN_DOWN(vm->subblock_size);
>>-	struct page *page;
>> 	unsigned long pfn;
>>-	int sb_id, i;
>>+	int sb_id;
>> 
>> 	for (sb_id = 0; sb_id < vm->nb_sb_per_mb; sb_id++) {
>> 		if (virtio_mem_mb_test_sb_plugged(vm, mb_id, sb_id, 1))
>> 			continue;
>>-		/*
>>-		 * Drop our reference to the pages so the memory can get
>>-		 * offlined and add the unplugged pages to the managed
>>-		 * page counters (so offlining code can correctly subtract
>>-		 * them again).
>>-		 */
>> 		pfn = PFN_DOWN(virtio_mem_mb_id_to_phys(mb_id) +
>> 			       sb_id * vm->subblock_size);
>>-		adjust_managed_page_count(pfn_to_page(pfn), nr_pages);
>
>One question about the original code, why we want to adjust count here?
>
>The code flow is
>
>    __offline_pages()
>        memory_notify(MEM_GOING_OFFLINE, &arg)
>	    virtio_mem_notify_going_offline(vm, mb_id)
>	        adjust_managed_page_count(pfn_to_page(pfn), nr_pages)
>	adjust_managed_page_count(pfn_to_page(start_pfn), -offlined_pages)
>
>Do we adjust the count twice?
>

Ah, I got the reason why we need to adjust count for *unplugged* sub-blocks.

>>-		for (i = 0; i < nr_pages; i++) {
>>-			page = pfn_to_page(pfn + i);
>>-			if (WARN_ON(!page_ref_dec_and_test(page)))

Another question is when we grab a refcount for the unpluged pages? The one
you mentioned in virtio_mem_set_fake_offline().

>>-				dump_page(page, "unplugged page referenced");
>>-		}
>>+		virtio_mem_fake_offline_going_offline(pfn, nr_pages);
>> 	}
>> }
>> 
>>@@ -633,21 +625,14 @@ static void virtio_mem_notify_cancel_offline(struct virtio_mem *vm,
>> {
>> 	const unsigned long nr_pages = PFN_DOWN(vm->subblock_size);
>> 	unsigned long pfn;
>>-	int sb_id, i;
>>+	int sb_id;
>> 
>> 	for (sb_id = 0; sb_id < vm->nb_sb_per_mb; sb_id++) {
>> 		if (virtio_mem_mb_test_sb_plugged(vm, mb_id, sb_id, 1))
>> 			continue;
>>-		/*
>>-		 * Get the reference we dropped when going offline and
>>-		 * subtract the unplugged pages from the managed page
>>-		 * counters.
>>-		 */
>> 		pfn = PFN_DOWN(virtio_mem_mb_id_to_phys(mb_id) +
>> 			       sb_id * vm->subblock_size);
>>-		adjust_managed_page_count(pfn_to_page(pfn), -nr_pages);
>>-		for (i = 0; i < nr_pages; i++)
>>-			page_ref_inc(pfn_to_page(pfn + i));
>>+		virtio_mem_fake_offline_cancel_offline(pfn, nr_pages);
>> 	}
>> }
>> 
>>@@ -853,6 +838,48 @@ static int virtio_mem_fake_offline(unsigned long pfn, unsigned long nr_pages)
>> 	return 0;
>> }
>> 
>>+/*
>>+ * Handle fake-offline pages when memory is going offline - such that the
>>+ * pages can be skipped by mm-core when offlining.
>>+ */
>>+static void virtio_mem_fake_offline_going_offline(unsigned long pfn,
>>+						  unsigned long nr_pages)
>>+{
>>+	struct page *page;
>>+	unsigned long i;
>>+
>>+	/*
>>+	 * Drop our reference to the pages so the memory can get offlined
>>+	 * and add the unplugged pages to the managed page counters (so
>>+	 * offlining code can correctly subtract them again).
>>+	 */
>>+	adjust_managed_page_count(pfn_to_page(pfn), nr_pages);
>>+	/* Drop our reference to the pages so the memory can get offlined. */
>>+	for (i = 0; i < nr_pages; i++) {
>>+		page = pfn_to_page(pfn + i);
>>+		if (WARN_ON(!page_ref_dec_and_test(page)))
>>+			dump_page(page, "fake-offline page referenced");
>>+	}
>>+}
>>+
>>+/*
>>+ * Handle fake-offline pages when memory offlining is canceled - to undo
>>+ * what we did in virtio_mem_fake_offline_going_offline().
>>+ */
>>+static void virtio_mem_fake_offline_cancel_offline(unsigned long pfn,
>>+						   unsigned long nr_pages)
>>+{
>>+	unsigned long i;
>>+
>>+	/*
>>+	 * Get the reference we dropped when going offline and subtract the
>>+	 * unplugged pages from the managed page counters.
>>+	 */
>>+	adjust_managed_page_count(pfn_to_page(pfn), -nr_pages);
>>+	for (i = 0; i < nr_pages; i++)
>>+		page_ref_inc(pfn_to_page(pfn + i));
>>+}
>>+
>> static void virtio_mem_online_page_cb(struct page *page, unsigned int order)
>> {
>> 	const unsigned long addr = page_to_phys(page);
>>-- 
>>2.26.2
>
>-- 
>Wei Yang
>Help you, Help me

-- 
Wei Yang
Help you, Help me

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