* Re: [PATCH] arm64/mm: add fallback option to allocate virtually contiguous memory
@ 2020-09-10 6:45 ` Anshuman Khandual
0 siblings, 0 replies; 20+ messages in thread
From: Anshuman Khandual @ 2020-09-10 6:45 UTC (permalink / raw)
To: Sudarshan Rajagopalan, linux-arm-kernel, linux-kernel
Cc: Mark Rutland, Will Deacon, David Hildenbrand, Catalin Marinas,
Steven Price, Andrew Morton, Logan Gunthorpe
Hello Sudarshan,
On 09/10/2020 11:35 AM, Sudarshan Rajagopalan wrote:
> When section mappings are enabled, we allocate vmemmap pages from physically
> continuous memory of size PMD_SZIE using vmemmap_alloc_block_buf(). Section> mappings are good to reduce TLB pressure. But when system is highly fragmented
> and memory blocks are being hot-added at runtime, its possible that such
> physically continuous memory allocations can fail. Rather than failing the
Did you really see this happen on a system ?
> memory hot-add procedure, add a fallback option to allocate vmemmap pages from
> discontinuous pages using vmemmap_populate_basepages().
Which could lead to a mixed page size mapping in the VMEMMAP area.
Allocation failure in vmemmap_populate() should just cleanly fail
the memory hot add operation, which can then be retried. Why the
retry has to be offloaded to kernel ?
>
> Signed-off-by: Sudarshan Rajagopalan <sudaraja@codeaurora.org>
> Cc: Catalin Marinas <catalin.marinas@arm.com>
> Cc: Will Deacon <will@kernel.org>
> Cc: Anshuman Khandual <anshuman.khandual@arm.com>
> Cc: Mark Rutland <mark.rutland@arm.com>
> Cc: Logan Gunthorpe <logang@deltatee.com>
> Cc: David Hildenbrand <david@redhat.com>
> Cc: Andrew Morton <akpm@linux-foundation.org>
> Cc: Steven Price <steven.price@arm.com>
> ---
> arch/arm64/mm/mmu.c | 15 ++++++++++++---
> 1 file changed, 12 insertions(+), 3 deletions(-)
>
> diff --git a/arch/arm64/mm/mmu.c b/arch/arm64/mm/mmu.c
> index 75df62f..a46c7d4 100644
> --- a/arch/arm64/mm/mmu.c
> +++ b/arch/arm64/mm/mmu.c
> @@ -1100,6 +1100,7 @@ int __meminit vmemmap_populate(unsigned long start, unsigned long end, int node,
> p4d_t *p4dp;
> pud_t *pudp;
> pmd_t *pmdp;
> + int ret = 0;
>
> do {
> next = pmd_addr_end(addr, end);
> @@ -1121,15 +1122,23 @@ int __meminit vmemmap_populate(unsigned long start, unsigned long end, int node,
> void *p = NULL;
>
> p = vmemmap_alloc_block_buf(PMD_SIZE, node, altmap);
> - if (!p)
> - return -ENOMEM;
> + if (!p) {
> +#ifdef CONFIG_MEMORY_HOTPLUG
> + vmemmap_free(start, end, altmap);
> +#endif
The mapping was never created in the first place, as the allocation
failed. vmemmap_free() here will free an unmapped area !
> + ret = -ENOMEM;
> + break;
> + }
>
> pmd_set_huge(pmdp, __pa(p), __pgprot(PROT_SECT_NORMAL));
> } else
> vmemmap_verify((pte_t *)pmdp, node, addr, next);
> } while (addr = next, addr != end);
>
> - return 0;
> + if (ret)
> + return vmemmap_populate_basepages(start, end, node, altmap);
> + else
> + return ret;
> }
> #endif /* !ARM64_SWAPPER_USES_SECTION_MAPS */
> void vmemmap_free(unsigned long start, unsigned long end,
>
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^ permalink raw reply [flat|nested] 20+ messages in thread* Re: [PATCH] arm64/mm: add fallback option to allocate virtually contiguous memory
2020-09-10 6:45 ` Anshuman Khandual
@ 2020-09-10 8:08 ` David Hildenbrand
-1 siblings, 0 replies; 20+ messages in thread
From: David Hildenbrand @ 2020-09-10 8:08 UTC (permalink / raw)
To: Anshuman Khandual, Sudarshan Rajagopalan, linux-arm-kernel, linux-kernel
Cc: Catalin Marinas, Will Deacon, Mark Rutland, Logan Gunthorpe,
Andrew Morton, Steven Price
On 10.09.20 08:45, Anshuman Khandual wrote:
> Hello Sudarshan,
>
> On 09/10/2020 11:35 AM, Sudarshan Rajagopalan wrote:
>> When section mappings are enabled, we allocate vmemmap pages from physically
>> continuous memory of size PMD_SZIE using vmemmap_alloc_block_buf(). Section> mappings are good to reduce TLB pressure. But when system is highly fragmented
>> and memory blocks are being hot-added at runtime, its possible that such
>> physically continuous memory allocations can fail. Rather than failing the
>
> Did you really see this happen on a system ?
>
>> memory hot-add procedure, add a fallback option to allocate vmemmap pages from
>> discontinuous pages using vmemmap_populate_basepages().
>
> Which could lead to a mixed page size mapping in the VMEMMAP area.
Right, with gives you a slight performance hit - nobody really cares,
especially if it happens in corner cases only.
At least x86_64 (see vmemmap_populate_hugepages()) and s390x (added
recently by me) implement that behavior.
Assume you run in a virtualized environment where your hypervisor tries
to do some smart dynamic guest resizing - like monitoring the guest
memory consumption and adding more memory on demand. You much rather
want hotadd to succeed (in these corner cases) that failing just because
you weren't able to grab a huge page in one instance.
Examples include XEN balloon, Hyper-V balloon, and virtio-mem. We might
see some of these for arm64 as well (if don't already do).
> Allocation failure in vmemmap_populate() should just cleanly fail
> the memory hot add operation, which can then be retried. Why the
> retry has to be offloaded to kernel ?
(not sure what "offloaded to kernel" really means here - add_memory() is
also just triggered from the kernel) I disagree, we should try our best
to add memory and make it available, especially when short on memory
already.
--
Thanks,
David / dhildenb
^ permalink raw reply [flat|nested] 20+ messages in thread
* Re: [PATCH] arm64/mm: add fallback option to allocate virtually contiguous memory
@ 2020-09-10 8:08 ` David Hildenbrand
0 siblings, 0 replies; 20+ messages in thread
From: David Hildenbrand @ 2020-09-10 8:08 UTC (permalink / raw)
To: Anshuman Khandual, Sudarshan Rajagopalan, linux-arm-kernel, linux-kernel
Cc: Mark Rutland, Will Deacon, Catalin Marinas, Steven Price,
Andrew Morton, Logan Gunthorpe
On 10.09.20 08:45, Anshuman Khandual wrote:
> Hello Sudarshan,
>
> On 09/10/2020 11:35 AM, Sudarshan Rajagopalan wrote:
>> When section mappings are enabled, we allocate vmemmap pages from physically
>> continuous memory of size PMD_SZIE using vmemmap_alloc_block_buf(). Section> mappings are good to reduce TLB pressure. But when system is highly fragmented
>> and memory blocks are being hot-added at runtime, its possible that such
>> physically continuous memory allocations can fail. Rather than failing the
>
> Did you really see this happen on a system ?
>
>> memory hot-add procedure, add a fallback option to allocate vmemmap pages from
>> discontinuous pages using vmemmap_populate_basepages().
>
> Which could lead to a mixed page size mapping in the VMEMMAP area.
Right, with gives you a slight performance hit - nobody really cares,
especially if it happens in corner cases only.
At least x86_64 (see vmemmap_populate_hugepages()) and s390x (added
recently by me) implement that behavior.
Assume you run in a virtualized environment where your hypervisor tries
to do some smart dynamic guest resizing - like monitoring the guest
memory consumption and adding more memory on demand. You much rather
want hotadd to succeed (in these corner cases) that failing just because
you weren't able to grab a huge page in one instance.
Examples include XEN balloon, Hyper-V balloon, and virtio-mem. We might
see some of these for arm64 as well (if don't already do).
> Allocation failure in vmemmap_populate() should just cleanly fail
> the memory hot add operation, which can then be retried. Why the
> retry has to be offloaded to kernel ?
(not sure what "offloaded to kernel" really means here - add_memory() is
also just triggered from the kernel) I disagree, we should try our best
to add memory and make it available, especially when short on memory
already.
--
Thanks,
David / dhildenb
_______________________________________________
linux-arm-kernel mailing list
linux-arm-kernel@lists.infradead.org
http://lists.infradead.org/mailman/listinfo/linux-arm-kernel
^ permalink raw reply [flat|nested] 20+ messages in thread
* Re: [PATCH] arm64/mm: add fallback option to allocate virtually contiguous memory
2020-09-10 8:08 ` David Hildenbrand
@ 2020-09-10 10:58 ` Anshuman Khandual
-1 siblings, 0 replies; 20+ messages in thread
From: Anshuman Khandual @ 2020-09-10 10:58 UTC (permalink / raw)
To: David Hildenbrand, Sudarshan Rajagopalan, linux-arm-kernel, linux-kernel
Cc: Catalin Marinas, Will Deacon, Mark Rutland, Logan Gunthorpe,
Andrew Morton, Steven Price
On 09/10/2020 01:38 PM, David Hildenbrand wrote:
> On 10.09.20 08:45, Anshuman Khandual wrote:
>> Hello Sudarshan,
>>
>> On 09/10/2020 11:35 AM, Sudarshan Rajagopalan wrote:
>>> When section mappings are enabled, we allocate vmemmap pages from physically
>>> continuous memory of size PMD_SZIE using vmemmap_alloc_block_buf(). Section> mappings are good to reduce TLB pressure. But when system is highly fragmented
>>> and memory blocks are being hot-added at runtime, its possible that such
>>> physically continuous memory allocations can fail. Rather than failing the
>>
>> Did you really see this happen on a system ?
>>
>>> memory hot-add procedure, add a fallback option to allocate vmemmap pages from
>>> discontinuous pages using vmemmap_populate_basepages().
>>
>> Which could lead to a mixed page size mapping in the VMEMMAP area.
>
> Right, with gives you a slight performance hit - nobody really cares,
> especially if it happens in corner cases only.
On performance impact, will probably let Catalin and others comment from
arm64 platform perspective, because I might not have all information here.
But will do some more audit regarding possible impact of a mixed page size
vmemmap mapping.
>
> At least x86_64 (see vmemmap_populate_hugepages()) and s390x (added
> recently by me) implement that behavior.
>
> Assume you run in a virtualized environment where your hypervisor tries
> to do some smart dynamic guest resizing - like monitoring the guest
> memory consumption and adding more memory on demand. You much rather
> want hotadd to succeed (in these corner cases) that failing just because
> you weren't able to grab a huge page in one instance.
>
> Examples include XEN balloon, Hyper-V balloon, and virtio-mem. We might
> see some of these for arm64 as well (if don't already do).
Makes sense.
>
>> Allocation failure in vmemmap_populate() should just cleanly fail
>> the memory hot add operation, which can then be retried. Why the
>> retry has to be offloaded to kernel ?
>
> (not sure what "offloaded to kernel" really means here - add_memory() is
Offloaded here referred to the responsibility to retry or just fallback.
If the situation can be resolved by user retrying hot add operation till
it succeeds, compared to kernel falling back on allocating normal pages.
> also just triggered from the kernel) I disagree, we should try our best
> to add memory and make it available, especially when short on memory
> already.
Okay.
^ permalink raw reply [flat|nested] 20+ messages in thread
* Re: [PATCH] arm64/mm: add fallback option to allocate virtually contiguous memory
@ 2020-09-10 10:58 ` Anshuman Khandual
0 siblings, 0 replies; 20+ messages in thread
From: Anshuman Khandual @ 2020-09-10 10:58 UTC (permalink / raw)
To: David Hildenbrand, Sudarshan Rajagopalan, linux-arm-kernel, linux-kernel
Cc: Mark Rutland, Will Deacon, Catalin Marinas, Steven Price,
Andrew Morton, Logan Gunthorpe
On 09/10/2020 01:38 PM, David Hildenbrand wrote:
> On 10.09.20 08:45, Anshuman Khandual wrote:
>> Hello Sudarshan,
>>
>> On 09/10/2020 11:35 AM, Sudarshan Rajagopalan wrote:
>>> When section mappings are enabled, we allocate vmemmap pages from physically
>>> continuous memory of size PMD_SZIE using vmemmap_alloc_block_buf(). Section> mappings are good to reduce TLB pressure. But when system is highly fragmented
>>> and memory blocks are being hot-added at runtime, its possible that such
>>> physically continuous memory allocations can fail. Rather than failing the
>>
>> Did you really see this happen on a system ?
>>
>>> memory hot-add procedure, add a fallback option to allocate vmemmap pages from
>>> discontinuous pages using vmemmap_populate_basepages().
>>
>> Which could lead to a mixed page size mapping in the VMEMMAP area.
>
> Right, with gives you a slight performance hit - nobody really cares,
> especially if it happens in corner cases only.
On performance impact, will probably let Catalin and others comment from
arm64 platform perspective, because I might not have all information here.
But will do some more audit regarding possible impact of a mixed page size
vmemmap mapping.
>
> At least x86_64 (see vmemmap_populate_hugepages()) and s390x (added
> recently by me) implement that behavior.
>
> Assume you run in a virtualized environment where your hypervisor tries
> to do some smart dynamic guest resizing - like monitoring the guest
> memory consumption and adding more memory on demand. You much rather
> want hotadd to succeed (in these corner cases) that failing just because
> you weren't able to grab a huge page in one instance.
>
> Examples include XEN balloon, Hyper-V balloon, and virtio-mem. We might
> see some of these for arm64 as well (if don't already do).
Makes sense.
>
>> Allocation failure in vmemmap_populate() should just cleanly fail
>> the memory hot add operation, which can then be retried. Why the
>> retry has to be offloaded to kernel ?
>
> (not sure what "offloaded to kernel" really means here - add_memory() is
Offloaded here referred to the responsibility to retry or just fallback.
If the situation can be resolved by user retrying hot add operation till
it succeeds, compared to kernel falling back on allocating normal pages.
> also just triggered from the kernel) I disagree, we should try our best
> to add memory and make it available, especially when short on memory
> already.
Okay.
_______________________________________________
linux-arm-kernel mailing list
linux-arm-kernel@lists.infradead.org
http://lists.infradead.org/mailman/listinfo/linux-arm-kernel
^ permalink raw reply [flat|nested] 20+ messages in thread
* RE: [PATCH] arm64/mm: add fallback option to allocate virtually contiguous memory
2020-09-10 6:45 ` Anshuman Khandual
(?)
(?)
@ 2020-09-10 8:27 ` sudaraja
2020-09-10 11:16 ` Anshuman Khandual
-1 siblings, 1 reply; 20+ messages in thread
From: sudaraja @ 2020-09-10 8:27 UTC (permalink / raw)
To: 'Anshuman Khandual', linux-arm-kernel, linux-kernel
Cc: 'Mark Rutland', 'Will Deacon',
'David Hildenbrand', 'Catalin Marinas',
'Steven Price', 'Andrew Morton',
'Logan Gunthorpe',
pratikp
Hello Anshuman,
>On 09/10/2020 11:35 AM, Sudarshan Rajagopalan wrote:
>> When section mappings are enabled, we allocate vmemmap pages from
>> physically continuous memory of size PMD_SZIE using
>> vmemmap_alloc_block_buf(). Section> mappings are good to reduce TLB
>> pressure. But when system is highly fragmented and memory blocks are
>> being hot-added at runtime, its possible that such physically
>> continuous memory allocations can fail. Rather than failing the
>
>Did you really see this happen on a system ?
Thanks for the response.
Yes, this happened on a system with very low RAM (size ~120MB) where no free order-9 pages were present. Pasting below few kernel logs. On systems with low RAM, its high probability where memory is fragmented and no higher order pages are free. On such scenarios, vmemmap alloc would fail for PMD_SIZE of contiguous memory.
We have a usecase for memory sharing between VMs where one of the VM uses add_memory() to add the memory that was donated by the other VM. This uses something similar to VirtIO-Mem. And this requires memory to be _guaranteed_ to be added in the VM so that the usecase can run without any failure.
vmemmap alloc failure: order:9, mode:0x4cc0(GFP_KERNEL|__GFP_RETRY_MAYFAIL), nodemask=(null),cpuset=/,mems_allowed=0
CPU: 1 PID: 294 Comm: -------- Tainted: G S 5.4.50 #1
Call trace:
dump_stack+0xa4/0xdc
warn_alloc+0x104/0x160
vmemmap_alloc_block+0xe4/0xf4
vmemmap_alloc_block_buf+0x34/0x38
vmemmap_populate+0xc8/0x224
__populate_section_memmap+0x34/0x54
sparse_add_section+0x16c/0x254
__add_pages+0xd0/0x138
arch_add_memory+0x114/0x1a8
DMA32: 2627*4kB (UMC) 23*8kB (UME) 6*16kB (UM) 8*32kB (UME) 2*64kB (ME) 2*128kB (UE) 1*256kB (M) 2*512kB (ME) 1*1024kB (M) 0*2048kB 0*4096kB = 13732kB
30455 pages RAM
But keeping this usecase aside, won’t this be problematic on any systems with low RAM where order-9 alloc would fail on a fragmented system, and any memory hot-adding would fail? Or other similar users of VirtIO-Mem which uses arch_add_memory.
>
>> memory hot-add procedure, add a fallback option to allocate vmemmap
>> pages from discontinuous pages using vmemmap_populate_basepages().
>
>Which could lead to a mixed page size mapping in the VMEMMAP area.
Would this be problematic? We would only lose one section mapping per failure and increases slight TLB pressure. Also, we would anyway do discontinuous pages alloc for systems having non-4K pages (ARM64_SWAPPER_USES_SECTION_MAPS will be 0). I only see a small cost to performance due to slight TLB pressure.
>Allocation failure in vmemmap_populate() should just cleanly fail the memory hot add operation, which can then be retried. Why the retry has to be offloaded to kernel ?
While a retry can attempted again, but it won’t help in cases where there are no order-9 pages available and any retry would just not succeed again until a order-9 page gets free'ed. Here we are just falling back to use discontinuous pages allocation to help succeed memory hot-add as best as possible.
Thanks and Regards,
Sudarshan
--
Qualcomm Innovation Center, Inc. is a member of Code Aurora Forum, a Linux Foundation Collaborative Project
-----Original Message-----
From: Anshuman Khandual <anshuman.khandual@arm.com>
Sent: Wednesday, September 9, 2020 11:45 PM
To: Sudarshan Rajagopalan <sudaraja@codeaurora.org>; linux-arm-kernel@lists.infradead.org; linux-kernel@vger.kernel.org
Cc: Catalin Marinas <catalin.marinas@arm.com>; Will Deacon <will@kernel.org>; Mark Rutland <mark.rutland@arm.com>; Logan Gunthorpe <logang@deltatee.com>; David Hildenbrand <david@redhat.com>; Andrew Morton <akpm@linux-foundation.org>; Steven Price <steven.price@arm.com>
Subject: Re: [PATCH] arm64/mm: add fallback option to allocate virtually contiguous memory
Hello Sudarshan,
On 09/10/2020 11:35 AM, Sudarshan Rajagopalan wrote:
> When section mappings are enabled, we allocate vmemmap pages from
> physically continuous memory of size PMD_SZIE using
> vmemmap_alloc_block_buf(). Section> mappings are good to reduce TLB
> pressure. But when system is highly fragmented and memory blocks are
> being hot-added at runtime, its possible that such physically
> continuous memory allocations can fail. Rather than failing the
Did you really see this happen on a system ?
> memory hot-add procedure, add a fallback option to allocate vmemmap
> pages from discontinuous pages using vmemmap_populate_basepages().
Which could lead to a mixed page size mapping in the VMEMMAP area.
Allocation failure in vmemmap_populate() should just cleanly fail the memory hot add operation, which can then be retried. Why the retry has to be offloaded to kernel ?
>
> Signed-off-by: Sudarshan Rajagopalan <sudaraja@codeaurora.org>
> Cc: Catalin Marinas <catalin.marinas@arm.com>
> Cc: Will Deacon <will@kernel.org>
> Cc: Anshuman Khandual <anshuman.khandual@arm.com>
> Cc: Mark Rutland <mark.rutland@arm.com>
> Cc: Logan Gunthorpe <logang@deltatee.com>
> Cc: David Hildenbrand <david@redhat.com>
> Cc: Andrew Morton <akpm@linux-foundation.org>
> Cc: Steven Price <steven.price@arm.com>
> ---
> arch/arm64/mm/mmu.c | 15 ++++++++++++---
> 1 file changed, 12 insertions(+), 3 deletions(-)
>
> diff --git a/arch/arm64/mm/mmu.c b/arch/arm64/mm/mmu.c index
> 75df62f..a46c7d4 100644
> --- a/arch/arm64/mm/mmu.c
> +++ b/arch/arm64/mm/mmu.c
> @@ -1100,6 +1100,7 @@ int __meminit vmemmap_populate(unsigned long start, unsigned long end, int node,
> p4d_t *p4dp;
> pud_t *pudp;
> pmd_t *pmdp;
> + int ret = 0;
>
> do {
> next = pmd_addr_end(addr, end);
> @@ -1121,15 +1122,23 @@ int __meminit vmemmap_populate(unsigned long start, unsigned long end, int node,
> void *p = NULL;
>
> p = vmemmap_alloc_block_buf(PMD_SIZE, node, altmap);
> - if (!p)
> - return -ENOMEM;
> + if (!p) {
> +#ifdef CONFIG_MEMORY_HOTPLUG
> + vmemmap_free(start, end, altmap); #endif
The mapping was never created in the first place, as the allocation failed. vmemmap_free() here will free an unmapped area !
> + ret = -ENOMEM;
> + break;
> + }
>
> pmd_set_huge(pmdp, __pa(p), __pgprot(PROT_SECT_NORMAL));
> } else
> vmemmap_verify((pte_t *)pmdp, node, addr, next);
> } while (addr = next, addr != end);
>
> - return 0;
> + if (ret)
> + return vmemmap_populate_basepages(start, end, node, altmap);
> + else
> + return ret;
> }
> #endif /* !ARM64_SWAPPER_USES_SECTION_MAPS */
> void vmemmap_free(unsigned long start, unsigned long end,
>
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^ permalink raw reply [flat|nested] 20+ messages in thread* Re: [PATCH] arm64/mm: add fallback option to allocate virtually contiguous memory
2020-09-10 8:27 ` sudaraja
@ 2020-09-10 11:16 ` Anshuman Khandual
0 siblings, 0 replies; 20+ messages in thread
From: Anshuman Khandual @ 2020-09-10 11:16 UTC (permalink / raw)
To: sudaraja, linux-arm-kernel, linux-kernel
Cc: 'Mark Rutland', 'Will Deacon',
'David Hildenbrand', 'Catalin Marinas',
'Steven Price', 'Andrew Morton',
'Logan Gunthorpe',
pratikp
On 09/10/2020 01:57 PM, sudaraja@codeaurora.org wrote:
> Hello Anshuman,
>
>> On 09/10/2020 11:35 AM, Sudarshan Rajagopalan wrote:
>>> When section mappings are enabled, we allocate vmemmap pages from
>>> physically continuous memory of size PMD_SZIE using
>>> vmemmap_alloc_block_buf(). Section> mappings are good to reduce TLB
>>> pressure. But when system is highly fragmented and memory blocks are
>>> being hot-added at runtime, its possible that such physically
>>> continuous memory allocations can fail. Rather than failing the
>>
>> Did you really see this happen on a system ?
>
> Thanks for the response.
There seems to be some text alignment problem in your response on this
thread, please have a look.
>
> Yes, this happened on a system with very low RAM (size ~120MB) where no free order-9 pages were present. Pasting below few kernel logs. On systems with low RAM, its high probability where memory is fragmented and no higher order pages are free. On such scenarios, vmemmap alloc would fail for PMD_SIZE of contiguous memory.
>
> We have a usecase for memory sharing between VMs where one of the VM uses add_memory() to add the memory that was donated by the other VM. This uses something similar to VirtIO-Mem. And this requires memory to be _guaranteed_ to be added in the VM so that the usecase can run without any failure.
>
> vmemmap alloc failure: order:9, mode:0x4cc0(GFP_KERNEL|__GFP_RETRY_MAYFAIL), nodemask=(null),cpuset=/,mems_allowed=0
> CPU: 1 PID: 294 Comm: -------- Tainted: G S 5.4.50 #1
> Call trace:
> dump_stack+0xa4/0xdc
> warn_alloc+0x104/0x160
> vmemmap_alloc_block+0xe4/0xf4
> vmemmap_alloc_block_buf+0x34/0x38
> vmemmap_populate+0xc8/0x224
> __populate_section_memmap+0x34/0x54
> sparse_add_section+0x16c/0x254
> __add_pages+0xd0/0x138
> arch_add_memory+0x114/0x1a8
>
> DMA32: 2627*4kB (UMC) 23*8kB (UME) 6*16kB (UM) 8*32kB (UME) 2*64kB (ME) 2*128kB (UE) 1*256kB (M) 2*512kB (ME) 1*1024kB (M) 0*2048kB 0*4096kB = 13732kB
> 30455 pages RAM
>
> But keeping this usecase aside, won’t this be problematic on any systems with low RAM where order-9 alloc would fail on a fragmented system, and any memory hot-adding would fail? Or other similar users of VirtIO-Mem which uses arch_add_memory.
>
>>
>>> memory hot-add procedure, add a fallback option to allocate vmemmap
>>> pages from discontinuous pages using vmemmap_populate_basepages().
>>
>> Which could lead to a mixed page size mapping in the VMEMMAP area.
>
> Would this be problematic? We would only lose one section mapping per failure and increases slight TLB pressure. Also, we would anyway do discontinuous pages alloc for systems having non-4K pages (ARM64_SWAPPER_USES_SECTION_MAPS will be 0). I only see a small cost to performance due to slight TLB pressure.
>
>> Allocation failure in vmemmap_populate() should just cleanly fail the memory hot add operation, which can then be retried. Why the retry has to be offloaded to kernel ?
>
> While a retry can attempted again, but it won’t help in cases where there are no order-9 pages available and any retry would just not succeed again until a order-9 page gets free'ed. Here we are just falling back to use discontinuous pages allocation to help succeed memory hot-add as best as possible.
Understood, seems like there is enough potential use cases and scenarios
right now, to consider this fallback mechanism and a possible mixed page
size vmemmap. But I would let others weigh in, on the performance impact.
>
> Thanks and Regards,
> Sudarshan
>
> --
> Qualcomm Innovation Center, Inc. is a member of Code Aurora Forum, a Linux Foundation Collaborative Project
>
> -----Original Message-----
> From: Anshuman Khandual <anshuman.khandual@arm.com>
> Sent: Wednesday, September 9, 2020 11:45 PM
> To: Sudarshan Rajagopalan <sudaraja@codeaurora.org>; linux-arm-kernel@lists.infradead.org; linux-kernel@vger.kernel.org
> Cc: Catalin Marinas <catalin.marinas@arm.com>; Will Deacon <will@kernel.org>; Mark Rutland <mark.rutland@arm.com>; Logan Gunthorpe <logang@deltatee.com>; David Hildenbrand <david@redhat.com>; Andrew Morton <akpm@linux-foundation.org>; Steven Price <steven.price@arm.com>
> Subject: Re: [PATCH] arm64/mm: add fallback option to allocate virtually contiguous memory
>
> Hello Sudarshan,
>
> On 09/10/2020 11:35 AM, Sudarshan Rajagopalan wrote:
>> When section mappings are enabled, we allocate vmemmap pages from
>> physically continuous memory of size PMD_SZIE using
>> vmemmap_alloc_block_buf(). Section> mappings are good to reduce TLB
>> pressure. But when system is highly fragmented and memory blocks are
>> being hot-added at runtime, its possible that such physically
>> continuous memory allocations can fail. Rather than failing the
>
> Did you really see this happen on a system ?
>
>> memory hot-add procedure, add a fallback option to allocate vmemmap
>> pages from discontinuous pages using vmemmap_populate_basepages().
>
> Which could lead to a mixed page size mapping in the VMEMMAP area.
> Allocation failure in vmemmap_populate() should just cleanly fail the memory hot add operation, which can then be retried. Why the retry has to be offloaded to kernel ?
>
>>
>> Signed-off-by: Sudarshan Rajagopalan <sudaraja@codeaurora.org>
>> Cc: Catalin Marinas <catalin.marinas@arm.com>
>> Cc: Will Deacon <will@kernel.org>
>> Cc: Anshuman Khandual <anshuman.khandual@arm.com>
>> Cc: Mark Rutland <mark.rutland@arm.com>
>> Cc: Logan Gunthorpe <logang@deltatee.com>
>> Cc: David Hildenbrand <david@redhat.com>
>> Cc: Andrew Morton <akpm@linux-foundation.org>
>> Cc: Steven Price <steven.price@arm.com>
>> ---
>> arch/arm64/mm/mmu.c | 15 ++++++++++++---
>> 1 file changed, 12 insertions(+), 3 deletions(-)
>>
>> diff --git a/arch/arm64/mm/mmu.c b/arch/arm64/mm/mmu.c index
>> 75df62f..a46c7d4 100644
>> --- a/arch/arm64/mm/mmu.c
>> +++ b/arch/arm64/mm/mmu.c
>> @@ -1100,6 +1100,7 @@ int __meminit vmemmap_populate(unsigned long start, unsigned long end, int node,
>> p4d_t *p4dp;
>> pud_t *pudp;
>> pmd_t *pmdp;
>> + int ret = 0;
>>
>> do {
>> next = pmd_addr_end(addr, end);
>> @@ -1121,15 +1122,23 @@ int __meminit vmemmap_populate(unsigned long start, unsigned long end, int node,
>> void *p = NULL;
>>
>> p = vmemmap_alloc_block_buf(PMD_SIZE, node, altmap);
>> - if (!p)
>> - return -ENOMEM;
>> + if (!p) {
>> +#ifdef CONFIG_MEMORY_HOTPLUG
>> + vmemmap_free(start, end, altmap); #endif
>
> The mapping was never created in the first place, as the allocation failed. vmemmap_free() here will free an unmapped area !
>
>> + ret = -ENOMEM;
>> + break;
>> + }
>>
>> pmd_set_huge(pmdp, __pa(p), __pgprot(PROT_SECT_NORMAL));
>> } else
>> vmemmap_verify((pte_t *)pmdp, node, addr, next);
>> } while (addr = next, addr != end);
>>
>> - return 0;
>> + if (ret)
>> + return vmemmap_populate_basepages(start, end, node, altmap);
>> + else
>> + return ret;
>> }
>> #endif /* !ARM64_SWAPPER_USES_SECTION_MAPS */
>> void vmemmap_free(unsigned long start, unsigned long end,
>>
>
>
> _______________________________________________
> linux-arm-kernel mailing list
> linux-arm-kernel@lists.infradead.org
> http://lists.infradead.org/mailman/listinfo/linux-arm-kernel
>
^ permalink raw reply [flat|nested] 20+ messages in thread* Re: [PATCH] arm64/mm: add fallback option to allocate virtually contiguous memory
@ 2020-09-10 11:16 ` Anshuman Khandual
0 siblings, 0 replies; 20+ messages in thread
From: Anshuman Khandual @ 2020-09-10 11:16 UTC (permalink / raw)
To: sudaraja, linux-arm-kernel, linux-kernel
Cc: 'Mark Rutland', 'David Hildenbrand',
'Catalin Marinas', 'Steven Price',
'Logan Gunthorpe', 'Andrew Morton',
'Will Deacon',
pratikp
On 09/10/2020 01:57 PM, sudaraja@codeaurora.org wrote:
> Hello Anshuman,
>
>> On 09/10/2020 11:35 AM, Sudarshan Rajagopalan wrote:
>>> When section mappings are enabled, we allocate vmemmap pages from
>>> physically continuous memory of size PMD_SZIE using
>>> vmemmap_alloc_block_buf(). Section> mappings are good to reduce TLB
>>> pressure. But when system is highly fragmented and memory blocks are
>>> being hot-added at runtime, its possible that such physically
>>> continuous memory allocations can fail. Rather than failing the
>>
>> Did you really see this happen on a system ?
>
> Thanks for the response.
There seems to be some text alignment problem in your response on this
thread, please have a look.
>
> Yes, this happened on a system with very low RAM (size ~120MB) where no free order-9 pages were present. Pasting below few kernel logs. On systems with low RAM, its high probability where memory is fragmented and no higher order pages are free. On such scenarios, vmemmap alloc would fail for PMD_SIZE of contiguous memory.
>
> We have a usecase for memory sharing between VMs where one of the VM uses add_memory() to add the memory that was donated by the other VM. This uses something similar to VirtIO-Mem. And this requires memory to be _guaranteed_ to be added in the VM so that the usecase can run without any failure.
>
> vmemmap alloc failure: order:9, mode:0x4cc0(GFP_KERNEL|__GFP_RETRY_MAYFAIL), nodemask=(null),cpuset=/,mems_allowed=0
> CPU: 1 PID: 294 Comm: -------- Tainted: G S 5.4.50 #1
> Call trace:
> dump_stack+0xa4/0xdc
> warn_alloc+0x104/0x160
> vmemmap_alloc_block+0xe4/0xf4
> vmemmap_alloc_block_buf+0x34/0x38
> vmemmap_populate+0xc8/0x224
> __populate_section_memmap+0x34/0x54
> sparse_add_section+0x16c/0x254
> __add_pages+0xd0/0x138
> arch_add_memory+0x114/0x1a8
>
> DMA32: 2627*4kB (UMC) 23*8kB (UME) 6*16kB (UM) 8*32kB (UME) 2*64kB (ME) 2*128kB (UE) 1*256kB (M) 2*512kB (ME) 1*1024kB (M) 0*2048kB 0*4096kB = 13732kB
> 30455 pages RAM
>
> But keeping this usecase aside, won’t this be problematic on any systems with low RAM where order-9 alloc would fail on a fragmented system, and any memory hot-adding would fail? Or other similar users of VirtIO-Mem which uses arch_add_memory.
>
>>
>>> memory hot-add procedure, add a fallback option to allocate vmemmap
>>> pages from discontinuous pages using vmemmap_populate_basepages().
>>
>> Which could lead to a mixed page size mapping in the VMEMMAP area.
>
> Would this be problematic? We would only lose one section mapping per failure and increases slight TLB pressure. Also, we would anyway do discontinuous pages alloc for systems having non-4K pages (ARM64_SWAPPER_USES_SECTION_MAPS will be 0). I only see a small cost to performance due to slight TLB pressure.
>
>> Allocation failure in vmemmap_populate() should just cleanly fail the memory hot add operation, which can then be retried. Why the retry has to be offloaded to kernel ?
>
> While a retry can attempted again, but it won’t help in cases where there are no order-9 pages available and any retry would just not succeed again until a order-9 page gets free'ed. Here we are just falling back to use discontinuous pages allocation to help succeed memory hot-add as best as possible.
Understood, seems like there is enough potential use cases and scenarios
right now, to consider this fallback mechanism and a possible mixed page
size vmemmap. But I would let others weigh in, on the performance impact.
>
> Thanks and Regards,
> Sudarshan
>
> --
> Qualcomm Innovation Center, Inc. is a member of Code Aurora Forum, a Linux Foundation Collaborative Project
>
> -----Original Message-----
> From: Anshuman Khandual <anshuman.khandual@arm.com>
> Sent: Wednesday, September 9, 2020 11:45 PM
> To: Sudarshan Rajagopalan <sudaraja@codeaurora.org>; linux-arm-kernel@lists.infradead.org; linux-kernel@vger.kernel.org
> Cc: Catalin Marinas <catalin.marinas@arm.com>; Will Deacon <will@kernel.org>; Mark Rutland <mark.rutland@arm.com>; Logan Gunthorpe <logang@deltatee.com>; David Hildenbrand <david@redhat.com>; Andrew Morton <akpm@linux-foundation.org>; Steven Price <steven.price@arm.com>
> Subject: Re: [PATCH] arm64/mm: add fallback option to allocate virtually contiguous memory
>
> Hello Sudarshan,
>
> On 09/10/2020 11:35 AM, Sudarshan Rajagopalan wrote:
>> When section mappings are enabled, we allocate vmemmap pages from
>> physically continuous memory of size PMD_SZIE using
>> vmemmap_alloc_block_buf(). Section> mappings are good to reduce TLB
>> pressure. But when system is highly fragmented and memory blocks are
>> being hot-added at runtime, its possible that such physically
>> continuous memory allocations can fail. Rather than failing the
>
> Did you really see this happen on a system ?
>
>> memory hot-add procedure, add a fallback option to allocate vmemmap
>> pages from discontinuous pages using vmemmap_populate_basepages().
>
> Which could lead to a mixed page size mapping in the VMEMMAP area.
> Allocation failure in vmemmap_populate() should just cleanly fail the memory hot add operation, which can then be retried. Why the retry has to be offloaded to kernel ?
>
>>
>> Signed-off-by: Sudarshan Rajagopalan <sudaraja@codeaurora.org>
>> Cc: Catalin Marinas <catalin.marinas@arm.com>
>> Cc: Will Deacon <will@kernel.org>
>> Cc: Anshuman Khandual <anshuman.khandual@arm.com>
>> Cc: Mark Rutland <mark.rutland@arm.com>
>> Cc: Logan Gunthorpe <logang@deltatee.com>
>> Cc: David Hildenbrand <david@redhat.com>
>> Cc: Andrew Morton <akpm@linux-foundation.org>
>> Cc: Steven Price <steven.price@arm.com>
>> ---
>> arch/arm64/mm/mmu.c | 15 ++++++++++++---
>> 1 file changed, 12 insertions(+), 3 deletions(-)
>>
>> diff --git a/arch/arm64/mm/mmu.c b/arch/arm64/mm/mmu.c index
>> 75df62f..a46c7d4 100644
>> --- a/arch/arm64/mm/mmu.c
>> +++ b/arch/arm64/mm/mmu.c
>> @@ -1100,6 +1100,7 @@ int __meminit vmemmap_populate(unsigned long start, unsigned long end, int node,
>> p4d_t *p4dp;
>> pud_t *pudp;
>> pmd_t *pmdp;
>> + int ret = 0;
>>
>> do {
>> next = pmd_addr_end(addr, end);
>> @@ -1121,15 +1122,23 @@ int __meminit vmemmap_populate(unsigned long start, unsigned long end, int node,
>> void *p = NULL;
>>
>> p = vmemmap_alloc_block_buf(PMD_SIZE, node, altmap);
>> - if (!p)
>> - return -ENOMEM;
>> + if (!p) {
>> +#ifdef CONFIG_MEMORY_HOTPLUG
>> + vmemmap_free(start, end, altmap); #endif
>
> The mapping was never created in the first place, as the allocation failed. vmemmap_free() here will free an unmapped area !
>
>> + ret = -ENOMEM;
>> + break;
>> + }
>>
>> pmd_set_huge(pmdp, __pa(p), __pgprot(PROT_SECT_NORMAL));
>> } else
>> vmemmap_verify((pte_t *)pmdp, node, addr, next);
>> } while (addr = next, addr != end);
>>
>> - return 0;
>> + if (ret)
>> + return vmemmap_populate_basepages(start, end, node, altmap);
>> + else
>> + return ret;
>> }
>> #endif /* !ARM64_SWAPPER_USES_SECTION_MAPS */
>> void vmemmap_free(unsigned long start, unsigned long end,
>>
>
>
> _______________________________________________
> linux-arm-kernel mailing list
> linux-arm-kernel@lists.infradead.org
> http://lists.infradead.org/mailman/listinfo/linux-arm-kernel
>
_______________________________________________
linux-arm-kernel mailing list
linux-arm-kernel@lists.infradead.org
http://lists.infradead.org/mailman/listinfo/linux-arm-kernel
^ permalink raw reply [flat|nested] 20+ messages in thread
* RE: [PATCH] arm64/mm: add fallback option to allocate virtually contiguous memory
2020-09-10 6:45 ` Anshuman Khandual
` (2 preceding siblings ...)
(?)
@ 2020-09-10 8:27 ` sudaraja
-1 siblings, 0 replies; 20+ messages in thread
From: sudaraja @ 2020-09-10 8:27 UTC (permalink / raw)
To: 'Anshuman Khandual', linux-arm-kernel, linux-kernel
Cc: 'Catalin Marinas', 'Will Deacon',
'Mark Rutland', 'Logan Gunthorpe',
'David Hildenbrand', 'Andrew Morton',
'Steven Price',
pratikp
Hello Anshuman,
>On 09/10/2020 11:35 AM, Sudarshan Rajagopalan wrote:
>> When section mappings are enabled, we allocate vmemmap pages from
>> physically continuous memory of size PMD_SZIE using
>> vmemmap_alloc_block_buf(). Section> mappings are good to reduce TLB
>> pressure. But when system is highly fragmented and memory blocks are
>> being hot-added at runtime, its possible that such physically
>> continuous memory allocations can fail. Rather than failing the
>
>Did you really see this happen on a system ?
Thanks for the response.
Yes, this happened on a system with very low RAM (size ~120MB) where no free order-9 pages were present. Pasting below few kernel logs. On systems with low RAM, its high probability where memory is fragmented and no higher order pages are free. On such scenarios, vmemmap alloc would fail for PMD_SIZE of contiguous memory.
We have a usecase for memory sharing between VMs where one of the VM uses add_memory() to add the memory that was donated by the other VM. This uses something similar to VirtIO-Mem. And this requires memory to be _guaranteed_ to be added in the VM so that the usecase can run without any failure.
vmemmap alloc failure: order:9, mode:0x4cc0(GFP_KERNEL|__GFP_RETRY_MAYFAIL), nodemask=(null),cpuset=/,mems_allowed=0
CPU: 1 PID: 294 Comm: -------- Tainted: G S 5.4.50 #1
Call trace:
dump_stack+0xa4/0xdc
warn_alloc+0x104/0x160
vmemmap_alloc_block+0xe4/0xf4
vmemmap_alloc_block_buf+0x34/0x38
vmemmap_populate+0xc8/0x224
__populate_section_memmap+0x34/0x54
sparse_add_section+0x16c/0x254
__add_pages+0xd0/0x138
arch_add_memory+0x114/0x1a8
DMA32: 2627*4kB (UMC) 23*8kB (UME) 6*16kB (UM) 8*32kB (UME) 2*64kB (ME) 2*128kB (UE) 1*256kB (M) 2*512kB (ME) 1*1024kB (M) 0*2048kB 0*4096kB = 13732kB
30455 pages RAM
But keeping this usecase aside, won’t this be problematic on any systems with low RAM where order-9 alloc would fail on a fragmented system, and any memory hot-adding would fail? Or other similar users of VirtIO-Mem which uses arch_add_memory.
>
>> memory hot-add procedure, add a fallback option to allocate vmemmap
>> pages from discontinuous pages using vmemmap_populate_basepages().
>
>Which could lead to a mixed page size mapping in the VMEMMAP area.
Would this be problematic? We would only lose one section mapping per failure and increases slight TLB pressure. Also, we would anyway do discontinuous pages alloc for systems having non-4K pages (ARM64_SWAPPER_USES_SECTION_MAPS will be 0). I only see a small cost to performance due to slight TLB pressure.
>Allocation failure in vmemmap_populate() should just cleanly fail the memory hot add operation, which can then be retried. Why the retry has to be offloaded to kernel ?
While a retry can attempted again, but it won’t help in cases where there are no order-9 pages available and any retry would just not succeed again until a order-9 page gets free'ed. Here we are just falling back to use discontinuous pages allocation to help succeed memory hot-add as best as possible.
Thanks and Regards,
Sudarshan
--
Qualcomm Innovation Center, Inc. is a member of Code Aurora Forum, a Linux Foundation Collaborative Project
-----Original Message-----
From: Anshuman Khandual <anshuman.khandual@arm.com>
Sent: Wednesday, September 9, 2020 11:45 PM
To: Sudarshan Rajagopalan <sudaraja@codeaurora.org>; linux-arm-kernel@lists.infradead.org; linux-kernel@vger.kernel.org
Cc: Catalin Marinas <catalin.marinas@arm.com>; Will Deacon <will@kernel.org>; Mark Rutland <mark.rutland@arm.com>; Logan Gunthorpe <logang@deltatee.com>; David Hildenbrand <david@redhat.com>; Andrew Morton <akpm@linux-foundation.org>; Steven Price <steven.price@arm.com>
Subject: Re: [PATCH] arm64/mm: add fallback option to allocate virtually contiguous memory
Hello Sudarshan,
On 09/10/2020 11:35 AM, Sudarshan Rajagopalan wrote:
> When section mappings are enabled, we allocate vmemmap pages from
> physically continuous memory of size PMD_SZIE using
> vmemmap_alloc_block_buf(). Section> mappings are good to reduce TLB
> pressure. But when system is highly fragmented and memory blocks are
> being hot-added at runtime, its possible that such physically
> continuous memory allocations can fail. Rather than failing the
Did you really see this happen on a system ?
> memory hot-add procedure, add a fallback option to allocate vmemmap
> pages from discontinuous pages using vmemmap_populate_basepages().
Which could lead to a mixed page size mapping in the VMEMMAP area.
Allocation failure in vmemmap_populate() should just cleanly fail the memory hot add operation, which can then be retried. Why the retry has to be offloaded to kernel ?
>
> Signed-off-by: Sudarshan Rajagopalan <sudaraja@codeaurora.org>
> Cc: Catalin Marinas <catalin.marinas@arm.com>
> Cc: Will Deacon <will@kernel.org>
> Cc: Anshuman Khandual <anshuman.khandual@arm.com>
> Cc: Mark Rutland <mark.rutland@arm.com>
> Cc: Logan Gunthorpe <logang@deltatee.com>
> Cc: David Hildenbrand <david@redhat.com>
> Cc: Andrew Morton <akpm@linux-foundation.org>
> Cc: Steven Price <steven.price@arm.com>
> ---
> arch/arm64/mm/mmu.c | 15 ++++++++++++---
> 1 file changed, 12 insertions(+), 3 deletions(-)
>
> diff --git a/arch/arm64/mm/mmu.c b/arch/arm64/mm/mmu.c index
> 75df62f..a46c7d4 100644
> --- a/arch/arm64/mm/mmu.c
> +++ b/arch/arm64/mm/mmu.c
> @@ -1100,6 +1100,7 @@ int __meminit vmemmap_populate(unsigned long start, unsigned long end, int node,
> p4d_t *p4dp;
> pud_t *pudp;
> pmd_t *pmdp;
> + int ret = 0;
>
> do {
> next = pmd_addr_end(addr, end);
> @@ -1121,15 +1122,23 @@ int __meminit vmemmap_populate(unsigned long start, unsigned long end, int node,
> void *p = NULL;
>
> p = vmemmap_alloc_block_buf(PMD_SIZE, node, altmap);
> - if (!p)
> - return -ENOMEM;
> + if (!p) {
> +#ifdef CONFIG_MEMORY_HOTPLUG
> + vmemmap_free(start, end, altmap); #endif
The mapping was never created in the first place, as the allocation failed. vmemmap_free() here will free an unmapped area !
> + ret = -ENOMEM;
> + break;
> + }
>
> pmd_set_huge(pmdp, __pa(p), __pgprot(PROT_SECT_NORMAL));
> } else
> vmemmap_verify((pte_t *)pmdp, node, addr, next);
> } while (addr = next, addr != end);
>
> - return 0;
> + if (ret)
> + return vmemmap_populate_basepages(start, end, node, altmap);
> + else
> + return ret;
> }
> #endif /* !ARM64_SWAPPER_USES_SECTION_MAPS */
> void vmemmap_free(unsigned long start, unsigned long end,
>
^ permalink raw reply [flat|nested] 20+ messages in thread