linux-kernel - [PATCH] mm/hugetlb: Fix incorrect proc nr

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Date:	Wed, 17 Feb 2016 01:13:26 +0530
From:	Vaishali Thakkar <vaishali.thakkar@...cle.com>
To:	Andrew Morton <akpm@...ux-foundation.org>
Cc:	n-horiguchi@...jp.nec.com, mike.kravetz@...cle.com,
	hillf.zj@...baba-inc.com, kirill.shutemov@...ux.intel.com,
	dave.hansen@...ux.intel.com, paul.gortmaker@...driver.com,
	linux-mm@...ck.org, linux-kernel@...r.kernel.org,
	Vaishali Thakkar <vaishali.thakkar@...cle.com>
Subject: [PATCH] mm/hugetlb: Fix incorrect proc nr_hugepages value

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->
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->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 <vaishali.thakkar@...cle.com>
---
The patch contains one line over 80 characters as I think limiting that
line to 80 characters makes code look bit ugly. But if anyone is having
issue with that then I am fine with limiting it to 80 chracters.
---
 mm/hugetlb.c | 6 ++++--
 1 file changed, 4 insertions(+), 2 deletions(-)

diff --git a/mm/hugetlb.c b/mm/hugetlb.c
index 06ae13e..01f2b48 100644
--- 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();
-- 
2.1.4