linux-kernel - [PATCH] Documentation: hugetlb: Update hugetlb options

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Date:   Thu,  9 Apr 2020 17:58:00 -0400
From:   Peter Xu <peterx@...hat.com>
To:     linux-mm@...ck.org, linux-kernel@...r.kernel.org
Cc:     Nitesh Narayan Lal <nitesh@...hat.com>, peterx@...hat.com,
        Andrew Morton <akpm@...ux-foundation.org>,
        Mike Kravetz <mike.kravetz@...cle.com>
Subject: [PATCH] Documentation: hugetlb: Update hugetlb options

The hugepage options are not documented clearly.

Firstly, default_hugepagesz= should always be specified after the
declaration of the same type of huge page using hugepagesz=.  For
example, if we boot a x86_64 system with kernel cmdline
"default_hugepagesz=2M", we'll get a very funny error message:

"HugeTLB: unsupported default_hugepagesz 2097152. Reverting to 2097152"

It's understandable from code-wise because when hugetlb_init() we
didn't have the 2M page hstate registered, so it's unsupported.
However 2M is actually the default huge page size on x86_64, so we'll
register it right after the error message.  However it's very
confusing if without these knowledges.

Secondly, hugepages= option must be used _after_ another hugepagesz=.
The word "interleave" is fine but it didn't declare the fact that
each of the hugepages= option will be applied to the previous parsed
hugepagesz= option.

State all these clear.

Signed-off-by: Peter Xu <peterx@...hat.com>
---
 .../admin-guide/kernel-parameters.txt         | 19 ++++++++++++-------
 1 file changed, 12 insertions(+), 7 deletions(-)

diff --git a/Documentation/admin-guide/kernel-parameters.txt b/Documentation/admin-guide/kernel-parameters.txt
index 86aae1fa099a..2a77b2b01e5e 100644
--- a/Documentation/admin-guide/kernel-parameters.txt
+++ b/Documentation/admin-guide/kernel-parameters.txt
@@ -839,7 +839,8 @@
 			the legacy /proc/ hugepages APIs, used for SHM, and
 			default size when mounting hugetlbfs filesystems.
 			Defaults to the default architecture's huge page size
-			if not specified.
+			if not specified.  This option can only be used with a
+			pre-defined "hugepagesz=" of the same size.
 
 	deferred_probe_timeout=
 			[KNL] Debugging option to set a timeout in seconds for
@@ -1475,13 +1476,17 @@
 	hpet_mmap=	[X86, HPET_MMAP] Allow userspace to mmap HPET
 			registers.  Default set by CONFIG_HPET_MMAP_DEFAULT.
 
+	hugepagesz=	[HW,IA-64,PPC,X86-64] Declare one type of HugeTLB pages
+			with the size specified.  Valid pages sizes on x86-64
+			are 2M (when the CPU supports "pse") and 1G (when the
+			CPU supports the "pdpe1gb" cpuinfo flag).
+
 	hugepages=	[HW,X86-32,IA-64] HugeTLB pages to allocate at boot.
-	hugepagesz=	[HW,IA-64,PPC,X86-64] The size of the HugeTLB pages.
-			On x86-64 and powerpc, this option can be specified
-			multiple times interleaved with hugepages= to reserve
-			huge pages of different sizes. Valid pages sizes on
-			x86-64 are 2M (when the CPU supports "pse") and 1G
-			(when the CPU supports the "pdpe1gb" cpuinfo flag).
+			On x86-64 and powerpc, this option must be used after
+			one hugepagesz= to allocate huge pages of that specific
+			size.  This option can also be used multiple times
+			pairing with hugepagesz= to allocate huge pages for
+			different sizes.
 
 	hung_task_panic=
 			[KNL] Should the hung task detector generate panics.
-- 
2.24.1