linux-kernel - Re: [PATCH v2 0/5] Allocate memmap from hotadded memory

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Date:   Wed, 26 Jun 2019 10:27:56 +0200
From:   Oscar Salvador <osalvador@...e.de>
To:     David Hildenbrand <david@...hat.com>
Cc:     akpm@...ux-foundation.org, mhocko@...e.com,
        dan.j.williams@...el.com, pasha.tatashin@...een.com,
        Jonathan.Cameron@...wei.com, anshuman.khandual@....com,
        vbabka@...e.cz, linux-mm@...ck.org, linux-kernel@...r.kernel.org
Subject: Re: [PATCH v2 0/5] Allocate memmap from hotadded memory

On Wed, Jun 26, 2019 at 10:15:16AM +0200, Oscar Salvador wrote:
> On Wed, Jun 26, 2019 at 10:11:06AM +0200, David Hildenbrand wrote:
> > Back then, I already mentioned that we might have some users that
> > remove_memory() they never added in a granularity it wasn't added. My
> > concerns back then were never fully sorted out.
> > 
> > arch/powerpc/platforms/powernv/memtrace.c
> > 
> > - Will remove memory in memory block size chunks it never added
> > - What if that memory resides on a DIMM added via MHP_MEMMAP_DEVICE?
> > 
> > Will it at least bail out? Or simply break?
> > 
> > IOW: I am not yet 100% convinced that MHP_MEMMAP_DEVICE is save to be
> > introduced.
> 
> Uhm, I will take a closer look and see if I can clear your concerns.
> TBH, I did not try to use arch/powerpc/platforms/powernv/memtrace.c
> yet.
> 
> I will get back to you once I tried it out.

On a second though, it would be quite trivial to implement a check in
remove_memory() that does not allow to remove memory used with MHP_MEMMAP_DEVICE
in a different granularity:

+static bool check_vmemmap_granularity(u64 start, u64 size);
+{
+	unsigned long pfn;
+	unsigned int nr_pages;
+	struct page *p;
+
+	pfn = PHYS_PFN(start);
+	p = pfn_to_page(pfn);
+	nr_pages = size >> PAGE_SIZE;
+
+	if (PageVmemmap(p)) {
+		struct page *h = vmemmap_get_head(p);
+		unsigned long sections = (unsigned long)h->private;
+
+		if (sections * PAGES_PER_SECTION > nr_pages)
+			fail;
+	}
+	no_fail;
+}
+		
+
 static int __ref try_remove_memory(int nid, u64 start, u64 size)
 {
 	int rc = 0;
 
 	BUG_ON(check_hotplug_memory_range(start, size));
 
 	mem_hotplug_begin();
 
+	rc = check_vmemmap_granularity(start, size);
+	if (rc)
+		goto done;


The above is quite hacky, but it gives an idea.
I will try the code from arch/powerpc/platforms/powernv/memtrace.c and see how
can I implement a check.

-- 
Oscar Salvador
SUSE L3