From: Christophe Leroy <christophe.leroy@csgroup.eu>
To: Dave Hansen <dave.hansen@intel.com>,
Kefeng Wang <wangkefeng.wang@huawei.com>,
Jonathan Corbet <corbet@lwn.net>,
Andrew Morton <akpm@linux-foundation.org>,
"linuxppc-dev@lists.ozlabs.org" <linuxppc-dev@lists.ozlabs.org>,
"linux-doc@vger.kernel.org" <linux-doc@vger.kernel.org>,
"linux-kernel@vger.kernel.org" <linux-kernel@vger.kernel.org>,
"linux-mm@kvack.org" <linux-mm@kvack.org>,
"x86@kernel.org" <x86@kernel.org>,
"linux-arm-kernel@lists.infradead.org"
<linux-arm-kernel@lists.infradead.org>
Cc: Nicholas Piggin <npiggin@gmail.com>,
Catalin Marinas <catalin.marinas@arm.com>,
Will Deacon <will@kernel.org>,
Thomas Gleixner <tglx@linutronix.de>,
Ingo Molnar <mingo@redhat.com>, Borislav Petkov <bp@alien8.de>,
Dave Hansen <dave.hansen@linux.intel.com>,
"H. Peter Anvin" <hpa@zytor.com>,
Michael Ellerman <mpe@ellerman.id.au>,
Benjamin Herrenschmidt <benh@kernel.crashing.org>,
Paul Mackerras <paulus@samba.org>,
Matthew Wilcox <willy@infradead.org>
Subject: Re: [PATCH v2 3/3] x86: Support huge vmalloc mappings
Date: Sat, 15 Jan 2022 10:15:34 +0000 [thread overview]
Message-ID: <0ffb59f0-3ce9-99f3-8184-44dc2d56d57f@csgroup.eu> (raw)
In-Reply-To: <31a75f95-6e6e-b640-2d95-08a95ea8cf51@intel.com>
Le 28/12/2021 à 17:14, Dave Hansen a écrit :
> On 12/28/21 2:26 AM, Kefeng Wang wrote:
>>>> There are some disadvantages about this feature[2], one of the main
>>>> concerns is the possible memory fragmentation/waste in some scenarios,
>>>> also archs must ensure that any arch specific vmalloc allocations that
>>>> require PAGE_SIZE mappings(eg, module alloc with STRICT_MODULE_RWX)
>>>> use the VM_NO_HUGE_VMAP flag to inhibit larger mappings.
>>> That just says that x86 *needs* PAGE_SIZE allocations. But, what
>>> happens if VM_NO_HUGE_VMAP is not passed (like it was in v1)? Will the
>>> subsequent permission changes just fragment the 2M mapping?
>>
>> Yes, without VM_NO_HUGE_VMAP, it could fragment the 2M mapping.
>>
>> When module alloc with STRICT_MODULE_RWX on x86, it calls
>> __change_page_attr()
>>
>> from set_memory_ro/rw/nx which will split large page, so there is no
>> need to make
>>
>> module alloc with HUGE_VMALLOC.
>
> This all sounds very fragile to me. Every time a new architecture would
> get added for huge vmalloc() support, the developer needs to know to go
> find that architecture's module_alloc() and add this flag. They next
> guy is going to forget, just like you did.
That's not correct from my point of view.
When powerpc added that, a clear comment explains why:
+ /*
+ * Don't do huge page allocations for modules yet until more testing
+ * is done. STRICT_MODULE_RWX may require extra work to support this
+ * too.
+ */
So as you can see, this is something specific to powerpc and temporary.
>
> Considering that this is not a hot path, a weak function would be a nice
> choice:
>
> /* vmalloc() flags used for all module allocations. */
> unsigned long __weak arch_module_vm_flags()
> {
> /*
> * Modules use a single, large vmalloc(). Different
> * permissions are applied later and will fragment
> * huge mappings. Avoid using huge pages for modules.
> */
Why ? Not everybody use STRICT_MODULES_RWX.
Even if you do so, you can still benefit from huge pages for modules.
Why make what was initially a temporary precaution for powerpc become a
definitive default limitation for all ?
> return VM_NO_HUGE_VMAP;
> }
>
> Stick that in some the common module code, next to:
>
>> void * __weak module_alloc(unsigned long size)
>> {
>> return __vmalloc_node_range(size, 1, VMALLOC_START, VMALLOC_END,
> ...
>
> Then, put arch_module_vm_flags() in *all* of the module_alloc()
> implementations, including the generic one. That way (even with a new
> architecture) whoever copies-and-pastes their module_alloc()
> implementation is likely to get it right. The next guy who just does a
> "select HAVE_ARCH_HUGE_VMALLOC" will hopefully just work.
>
> VM_FLUSH_RESET_PERMS could probably be dealt with in the same way.
next prev parent reply other threads:[~2022-01-15 10:15 UTC|newest]
Thread overview: 25+ messages / expand[flat|nested] mbox.gz Atom feed top
2021-12-27 14:59 [PATCH v2 0/3] mm: support huge vmalloc mapping on arm64/x86 Kefeng Wang
2021-12-27 14:59 ` [PATCH v2 1/3] mm: vmalloc: Let user to control huge vmalloc default behavior Kefeng Wang
2022-01-18 2:52 ` Nicholas Piggin
2022-01-19 12:57 ` Kefeng Wang
2022-01-19 13:22 ` Matthew Wilcox
2022-01-19 13:44 ` Kefeng Wang
2022-01-19 13:48 ` Matthew Wilcox
2021-12-27 14:59 ` [PATCH v2 2/3] arm64: Support huge vmalloc mappings Kefeng Wang
2021-12-27 17:35 ` (No subject) William Kucharski
2021-12-28 1:36 ` Kefeng Wang
2022-01-15 10:05 ` [PATCH v2 2/3] arm64: Support huge vmalloc mappings Christophe Leroy
2021-12-27 14:59 ` [PATCH v2 3/3] x86: " Kefeng Wang
2021-12-27 15:56 ` Dave Hansen
2021-12-28 10:26 ` Kefeng Wang
2021-12-28 16:14 ` Dave Hansen
2021-12-29 11:01 ` Kefeng Wang
2022-01-15 10:17 ` Christophe Leroy
2022-01-15 10:15 ` Christophe Leroy [this message]
2022-01-18 2:46 ` Nicholas Piggin
2022-01-18 17:28 ` Dave Hansen
2022-01-19 4:17 ` Nicholas Piggin
2022-01-19 13:32 ` Kefeng Wang
2022-01-15 10:11 ` Christophe Leroy
2022-01-15 10:06 ` Christophe Leroy
2022-01-15 10:07 ` [PATCH v2 0/3] mm: support huge vmalloc mapping on arm64/x86 Christophe Leroy
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