mm/hugetlb.c: fix incorrect proc nr_hugepages value

Vaishali Thakkar · torvalds · commit f8b74815a452 · 2016-02-18T16:23:24.000-08:00
Currently incorrect default hugepage pool size is reported by proc
nr_hugepages when number of pages for the default huge page size is
specified twice.

When multiple huge page sizes are supported, /proc/sys/vm/nr_hugepages
indicates the current number of pre-allocated huge pages of the default
size.  Basically /proc/sys/vm/nr_hugepages displays default_hstate-&gt;
max_huge_pages and after boot time pre-allocation, max_huge_pages should
equal the number of pre-allocated pages (nr_hugepages).

Test case:

Note that this is specific to x86 architecture.

Boot the kernel with command line option 'default_hugepagesz=1G
hugepages=X hugepagesz=2M hugepages=Y hugepagesz=1G hugepages=Z'.  After
boot, 'cat /proc/sys/vm/nr_hugepages' and 'sysctl -a | grep hugepages'
returns the value X.  However, dmesg output shows that Z huge pages were
pre-allocated.

So, the root cause of the problem here is that the global variable
default_hstate_max_huge_pages is set if a default huge page size is
specified (directly or indirectly) on the command line.  After the command
line processing in hugetlb_init, if default_hstate_max_huge_pages is set,
the value is assigned to default_hstae.max_huge_pages.  However,
default_hstate.max_huge_pages may have already been set based on the
number of pre-allocated huge pages of default_hstate size.

The solution to this problem is if hstate-&gt;max_huge_pages is already set
then it should not set as a result of global max_huge_pages value.
Basically if the value of the variable hugepages is set multiple times on
a command line for a specific supported hugepagesize then proc layer
should consider the last specified value.

Signed-off-by: Vaishali Thakkar &lt;vaishali.thakkar@oracle.com&gt;
Reviewed-by: Naoya Horiguchi &lt;n-horiguchi@ah.jp.nec.com&gt;
Cc: Mike Kravetz &lt;mike.kravetz@oracle.com&gt;
Cc: Hillf Danton &lt;hillf.zj@alibaba-inc.com&gt;
Cc: Kirill A. Shutemov &lt;kirill.shutemov@linux.intel.com&gt;
Cc: Dave Hansen &lt;dave.hansen@linux.intel.com&gt;
Cc: Paul Gortmaker &lt;paul.gortmaker@windriver.com&gt;
Signed-off-by: Andrew Morton &lt;akpm@linux-foundation.org&gt;
Signed-off-by: Linus Torvalds &lt;torvalds@linux-foundation.org&gt;
diff --git a/mm/hugetlb.c b/mm/hugetlb.c
@@ -2630,8 +2630,10 @@ static int __init hugetlb_init(void)
 			hugetlb_add_hstate(HUGETLB_PAGE_ORDER);
 	}
 	default_hstate_idx = hstate_index(size_to_hstate(default_hstate_size));
-	if (default_hstate_max_huge_pages)
-		default_hstate.max_huge_pages = default_hstate_max_huge_pages;
+	if (default_hstate_max_huge_pages) {
+		if (!default_hstate.max_huge_pages)
+			default_hstate.max_huge_pages = default_hstate_max_huge_pages;
+	}
 
 	hugetlb_init_hstates();
 	gather_bootmem_prealloc();
