[PATCH v6] arm64: mm: Populate vmemmap/linear at the page level for hotplugged sections

[PATCH v6] arm64: mm: Populate vmemmap/linear at the page level for hotplugged sections

[Zhenhua Huang]

On the arm64 platform with 4K base page config, SECTION_SIZE_BITS is set
to 27, making one section 128M. The related page struct which vmemmap
points to is 2M then.
Commit c1cc1552616d ("arm64: MMU initialisation") optimizes the
vmemmap to populate at the PMD section level which was suitable
initially since hot plug granule is always one section(128M). However,
commit ba72b4c8cf60 ("mm/sparsemem: support sub-section hotplug")
introduced a 2M(SUBSECTION_SIZE) hot plug granule, which disrupted the
existing arm64 assumptions.

Considering the vmemmap_free -> unmap_hotplug_pmd_range path, when
pmd_sect() is true, the entire PMD section is cleared, even if there is
other effective subsection. For example page_struct_map1 and
page_strcut_map2 are part of a single PMD entry and they are hot-added
sequentially. Then page_struct_map1 is removed, vmemmap_free() will clear
the entire PMD entry freeing the struct page map for the whole section,
even though page_struct_map2 is still active. Similar problem exists
with linear mapping as well, for 16K base page(PMD size = 32M) or 64K
base page(PMD = 512M), their block mappings exceed SUBSECTION_SIZE.
Tearing down the entire PMD mapping too will leave other subsections
unmapped in the linear mapping.

To address the issue, we need to prevent PMD/PUD/CONT mappings for both
linear and vmemmap for non-boot sections if corresponding size on the
given base page exceeds SUBSECTION_SIZE(2MB now).

Cc: <stable@vger.kernel.org> # v5.4+
Fixes: ba72b4c8cf60 ("mm/sparsemem: support sub-section hotplug")
Reviewed-by: Catalin Marinas <catalin.marinas@arm.com>
Signed-off-by: Zhenhua Huang <quic_zhenhuah@quicinc.com>
---
 arch/arm64/mm/mmu.c | 46 ++++++++++++++++++++++++++++++++++++---------
 1 file changed, 37 insertions(+), 9 deletions(-)

diff --git a/arch/arm64/mm/mmu.c b/arch/arm64/mm/mmu.c
index b4df5bc5b1b8..b1089365f3e7 100644
--- a/arch/arm64/mm/mmu.c
+++ b/arch/arm64/mm/mmu.c
@@ -42,9 +42,13 @@
 #include <asm/pgalloc.h>
 #include <asm/kfence.h>
 
-#define NO_BLOCK_MAPPINGS	BIT(0)
-#define NO_CONT_MAPPINGS	BIT(1)
-#define NO_EXEC_MAPPINGS	BIT(2)	/* assumes FEAT_HPDS is not used */
+#define NO_PMD_BLOCK_MAPPINGS	BIT(0)
+#define NO_PUD_BLOCK_MAPPINGS	BIT(1)  /* Hotplug case: do not want block mapping for PUD */
+#define NO_BLOCK_MAPPINGS	(NO_PMD_BLOCK_MAPPINGS | NO_PUD_BLOCK_MAPPINGS)
+#define NO_PTE_CONT_MAPPINGS	BIT(2)
+#define NO_PMD_CONT_MAPPINGS	BIT(3)  /* Hotplug case: do not want cont mapping for PMD */
+#define NO_CONT_MAPPINGS	(NO_PTE_CONT_MAPPINGS | NO_PMD_CONT_MAPPINGS)
+#define NO_EXEC_MAPPINGS	BIT(4)	/* assumes FEAT_HPDS is not used */
 
 u64 kimage_voffset __ro_after_init;
 EXPORT_SYMBOL(kimage_voffset);
@@ -224,7 +228,7 @@ static void alloc_init_cont_pte(pmd_t *pmdp, unsigned long addr,
 
 		/* use a contiguous mapping if the range is suitably aligned */
 		if ((((addr | next | phys) & ~CONT_PTE_MASK) == 0) &&
-		    (flags & NO_CONT_MAPPINGS) == 0)
+		    (flags & NO_PTE_CONT_MAPPINGS) == 0)
 			__prot = __pgprot(pgprot_val(prot) | PTE_CONT);
 
 		init_pte(ptep, addr, next, phys, __prot);
@@ -254,7 +258,7 @@ static void init_pmd(pmd_t *pmdp, unsigned long addr, unsigned long end,
 
 		/* try section mapping first */
 		if (((addr | next | phys) & ~PMD_MASK) == 0 &&
-		    (flags & NO_BLOCK_MAPPINGS) == 0) {
+		    (flags & NO_PMD_BLOCK_MAPPINGS) == 0) {
 			pmd_set_huge(pmdp, phys, prot);
 
 			/*
@@ -311,7 +315,7 @@ static void alloc_init_cont_pmd(pud_t *pudp, unsigned long addr,
 
 		/* use a contiguous mapping if the range is suitably aligned */
 		if ((((addr | next | phys) & ~CONT_PMD_MASK) == 0) &&
-		    (flags & NO_CONT_MAPPINGS) == 0)
+		    (flags & NO_PMD_CONT_MAPPINGS) == 0)
 			__prot = __pgprot(pgprot_val(prot) | PTE_CONT);
 
 		init_pmd(pmdp, addr, next, phys, __prot, pgtable_alloc, flags);
@@ -358,8 +362,8 @@ static void alloc_init_pud(p4d_t *p4dp, unsigned long addr, unsigned long end,
 		 * For 4K granule only, attempt to put down a 1GB block
 		 */
 		if (pud_sect_supported() &&
-		   ((addr | next | phys) & ~PUD_MASK) == 0 &&
-		    (flags & NO_BLOCK_MAPPINGS) == 0) {
+		    ((addr | next | phys) & ~PUD_MASK) == 0 &&
+		    (flags & NO_PUD_BLOCK_MAPPINGS) == 0) {
 			pud_set_huge(pudp, phys, prot);
 
 			/*
@@ -1178,7 +1182,13 @@ int __meminit vmemmap_populate(unsigned long start, unsigned long end, int node,
 {
 	WARN_ON((start < VMEMMAP_START) || (end > VMEMMAP_END));
 
-	if (!IS_ENABLED(CONFIG_ARM64_4K_PAGES))
+	/*
+	 * Hotplugged section does not support hugepages as
+	 * PMD_SIZE (hence PUD_SIZE) section mapping covers
+	 * struct page range that exceeds a SUBSECTION_SIZE
+	 * i.e 2MB - for all available base page sizes.
+	 */
+	if (!IS_ENABLED(CONFIG_ARM64_4K_PAGES) || system_state != SYSTEM_BOOTING)
 		return vmemmap_populate_basepages(start, end, node, altmap);
 	else
 		return vmemmap_populate_hugepages(start, end, node, altmap);
@@ -1340,9 +1350,27 @@ int arch_add_memory(int nid, u64 start, u64 size,
 		    struct mhp_params *params)
 {
 	int ret, flags = NO_EXEC_MAPPINGS;
+	unsigned long start_pfn = PFN_DOWN(start);
+	struct mem_section *ms = __pfn_to_section(start_pfn);
 
 	VM_BUG_ON(!mhp_range_allowed(start, size, true));
 
+	/* should not be invoked by early section */
+	WARN_ON(early_section(ms));
+
+	/*
+	 * Disallow BLOCK/CONT mappings if the corresponding size exceeds
+	 * SUBSECTION_SIZE which now is 2MB.
+	 *
+	 * PUD_BLOCK or PMD_CONT should consistently exceed SUBSECTION_SIZE
+	 * across all variable page size configurations, so add them directly
+	 */
+	flags |= NO_PUD_BLOCK_MAPPINGS | NO_PMD_CONT_MAPPINGS;
+	if (SUBSECTION_SHIFT < PMD_SHIFT)
+		flags |= NO_PMD_BLOCK_MAPPINGS;
+	if (SUBSECTION_SHIFT < CONT_PTE_SHIFT)
+		flags |= NO_PTE_CONT_MAPPINGS;
+
 	if (can_set_direct_map())
 		flags |= NO_BLOCK_MAPPINGS | NO_CONT_MAPPINGS;
 
-- 
2.25.1

Re: [PATCH v6] arm64: mm: Populate vmemmap/linear at the page level for hotplugged sections

[David Hildenbrand]

On 13.02.25 08:57, Zhenhua Huang wrote:
> On the arm64 platform with 4K base page config, SECTION_SIZE_BITS is set
> to 27, making one section 128M. The related page struct which vmemmap
> points to is 2M then.
> Commit c1cc1552616d ("arm64: MMU initialisation") optimizes the
> vmemmap to populate at the PMD section level which was suitable
> initially since hot plug granule is always one section(128M). However,
> commit ba72b4c8cf60 ("mm/sparsemem: support sub-section hotplug")
> introduced a 2M(SUBSECTION_SIZE) hot plug granule, which disrupted the
> existing arm64 assumptions.
> 
> Considering the vmemmap_free -> unmap_hotplug_pmd_range path, when
> pmd_sect() is true, the entire PMD section is cleared, even if there is
> other effective subsection. For example page_struct_map1 and
> page_strcut_map2 are part of a single PMD entry and they are hot-added
> sequentially. Then page_struct_map1 is removed, vmemmap_free() will clear
> the entire PMD entry freeing the struct page map for the whole section,
> even though page_struct_map2 is still active. Similar problem exists
> with linear mapping as well, for 16K base page(PMD size = 32M) or 64K
> base page(PMD = 512M), their block mappings exceed SUBSECTION_SIZE.
> Tearing down the entire PMD mapping too will leave other subsections
> unmapped in the linear mapping.
> 
> To address the issue, we need to prevent PMD/PUD/CONT mappings for both
> linear and vmemmap for non-boot sections if corresponding size on the
> given base page exceeds SUBSECTION_SIZE(2MB now).
> 
> Cc: <stable@vger.kernel.org> # v5.4+
> Fixes: ba72b4c8cf60 ("mm/sparsemem: support sub-section hotplug")
> Reviewed-by: Catalin Marinas <catalin.marinas@arm.com>
> Signed-off-by: Zhenhua Huang <quic_zhenhuah@quicinc.com>

Just so I understand correctly: for ordinary memory-sections-size 
hotplug (NVDIMM, virtio-mem), we still get a large mapping where possible?


-- 
Cheers,

David / dhildenb

Re: [PATCH v6] arm64: mm: Populate vmemmap/linear at the page level for hotplugged sections

[Catalin Marinas]

On Thu, Feb 13, 2025 at 01:59:25PM +0100, David Hildenbrand wrote:
> On 13.02.25 08:57, Zhenhua Huang wrote:
> > On the arm64 platform with 4K base page config, SECTION_SIZE_BITS is set
> > to 27, making one section 128M. The related page struct which vmemmap
> > points to is 2M then.
> > Commit c1cc1552616d ("arm64: MMU initialisation") optimizes the
> > vmemmap to populate at the PMD section level which was suitable
> > initially since hot plug granule is always one section(128M). However,
> > commit ba72b4c8cf60 ("mm/sparsemem: support sub-section hotplug")
> > introduced a 2M(SUBSECTION_SIZE) hot plug granule, which disrupted the
> > existing arm64 assumptions.
> > 
> > Considering the vmemmap_free -> unmap_hotplug_pmd_range path, when
> > pmd_sect() is true, the entire PMD section is cleared, even if there is
> > other effective subsection. For example page_struct_map1 and
> > page_strcut_map2 are part of a single PMD entry and they are hot-added
> > sequentially. Then page_struct_map1 is removed, vmemmap_free() will clear
> > the entire PMD entry freeing the struct page map for the whole section,
> > even though page_struct_map2 is still active. Similar problem exists
> > with linear mapping as well, for 16K base page(PMD size = 32M) or 64K
> > base page(PMD = 512M), their block mappings exceed SUBSECTION_SIZE.
> > Tearing down the entire PMD mapping too will leave other subsections
> > unmapped in the linear mapping.
> > 
> > To address the issue, we need to prevent PMD/PUD/CONT mappings for both
> > linear and vmemmap for non-boot sections if corresponding size on the
> > given base page exceeds SUBSECTION_SIZE(2MB now).
> > 
> > Cc: <stable@vger.kernel.org> # v5.4+
> > Fixes: ba72b4c8cf60 ("mm/sparsemem: support sub-section hotplug")
> > Reviewed-by: Catalin Marinas <catalin.marinas@arm.com>
> > Signed-off-by: Zhenhua Huang <quic_zhenhuah@quicinc.com>
> 
> Just so I understand correctly: for ordinary memory-sections-size hotplug
> (NVDIMM, virtio-mem), we still get a large mapping where possible?

Up to 2MB blocks only since that's the SUBSECTION_SIZE value. The
vmemmap mapping is also limited to PAGE_SIZE mappings (we could use
contiguous mappings for vmemmap but it's not wired up; I don't think
it's worth the hassle).

-- 
Catalin

Re: [PATCH v6] arm64: mm: Populate vmemmap/linear at the page level for hotplugged sections

[David Hildenbrand]

On 13.02.25 16:49, Catalin Marinas wrote:
> On Thu, Feb 13, 2025 at 01:59:25PM +0100, David Hildenbrand wrote:
>> On 13.02.25 08:57, Zhenhua Huang wrote:
>>> On the arm64 platform with 4K base page config, SECTION_SIZE_BITS is set
>>> to 27, making one section 128M. The related page struct which vmemmap
>>> points to is 2M then.
>>> Commit c1cc1552616d ("arm64: MMU initialisation") optimizes the
>>> vmemmap to populate at the PMD section level which was suitable
>>> initially since hot plug granule is always one section(128M). However,
>>> commit ba72b4c8cf60 ("mm/sparsemem: support sub-section hotplug")
>>> introduced a 2M(SUBSECTION_SIZE) hot plug granule, which disrupted the
>>> existing arm64 assumptions.
>>>
>>> Considering the vmemmap_free -> unmap_hotplug_pmd_range path, when
>>> pmd_sect() is true, the entire PMD section is cleared, even if there is
>>> other effective subsection. For example page_struct_map1 and
>>> page_strcut_map2 are part of a single PMD entry and they are hot-added
>>> sequentially. Then page_struct_map1 is removed, vmemmap_free() will clear
>>> the entire PMD entry freeing the struct page map for the whole section,
>>> even though page_struct_map2 is still active. Similar problem exists
>>> with linear mapping as well, for 16K base page(PMD size = 32M) or 64K
>>> base page(PMD = 512M), their block mappings exceed SUBSECTION_SIZE.
>>> Tearing down the entire PMD mapping too will leave other subsections
>>> unmapped in the linear mapping.
>>>
>>> To address the issue, we need to prevent PMD/PUD/CONT mappings for both
>>> linear and vmemmap for non-boot sections if corresponding size on the
>>> given base page exceeds SUBSECTION_SIZE(2MB now).
>>>
>>> Cc: <stable@vger.kernel.org> # v5.4+
>>> Fixes: ba72b4c8cf60 ("mm/sparsemem: support sub-section hotplug")
>>> Reviewed-by: Catalin Marinas <catalin.marinas@arm.com>
>>> Signed-off-by: Zhenhua Huang <quic_zhenhuah@quicinc.com>
>>
>> Just so I understand correctly: for ordinary memory-sections-size hotplug
>> (NVDIMM, virtio-mem), we still get a large mapping where possible?
> 
> Up to 2MB blocks only since that's the SUBSECTION_SIZE value. The
> vmemmap mapping is also limited to PAGE_SIZE mappings (we could use
> contiguous mappings for vmemmap but it's not wired up; I don't think
> it's worth the hassle).

But that's messed up, no?

If someone hotplugs a memory section, they have to hotunplug a memory 
section, not parts of it.

That's why x86 does in vmemmap_populate():

if (end - start < PAGES_PER_SECTION * sizeof(struct page))
	err = vmemmap_populate_basepages(start, end, node, NULL);
else if (boot_cpu_has(X86_FEATURE_PSE))
	err = vmemmap_populate_hugepages(start, end, node, altmap);
...

Maybe I'm missing something. Most importantly, why the weird subsection 
stuff is supposed to degrade ordinary hotplug of dimms/virtio-mem etc.

-- 
Cheers,

David / dhildenb

Re: [PATCH v6] arm64: mm: Populate vmemmap/linear at the page level for hotplugged sections

[Catalin Marinas]

On Thu, Feb 13, 2025 at 05:16:37PM +0100, David Hildenbrand wrote:
> On 13.02.25 16:49, Catalin Marinas wrote:
> > On Thu, Feb 13, 2025 at 01:59:25PM +0100, David Hildenbrand wrote:
> > > On 13.02.25 08:57, Zhenhua Huang wrote:
> > > > On the arm64 platform with 4K base page config, SECTION_SIZE_BITS is set
> > > > to 27, making one section 128M. The related page struct which vmemmap
> > > > points to is 2M then.
> > > > Commit c1cc1552616d ("arm64: MMU initialisation") optimizes the
> > > > vmemmap to populate at the PMD section level which was suitable
> > > > initially since hot plug granule is always one section(128M). However,
> > > > commit ba72b4c8cf60 ("mm/sparsemem: support sub-section hotplug")
> > > > introduced a 2M(SUBSECTION_SIZE) hot plug granule, which disrupted the
> > > > existing arm64 assumptions.
> > > > 
> > > > Considering the vmemmap_free -> unmap_hotplug_pmd_range path, when
> > > > pmd_sect() is true, the entire PMD section is cleared, even if there is
> > > > other effective subsection. For example page_struct_map1 and
> > > > page_strcut_map2 are part of a single PMD entry and they are hot-added
> > > > sequentially. Then page_struct_map1 is removed, vmemmap_free() will clear
> > > > the entire PMD entry freeing the struct page map for the whole section,
> > > > even though page_struct_map2 is still active. Similar problem exists
> > > > with linear mapping as well, for 16K base page(PMD size = 32M) or 64K
> > > > base page(PMD = 512M), their block mappings exceed SUBSECTION_SIZE.
> > > > Tearing down the entire PMD mapping too will leave other subsections
> > > > unmapped in the linear mapping.
> > > > 
> > > > To address the issue, we need to prevent PMD/PUD/CONT mappings for both
> > > > linear and vmemmap for non-boot sections if corresponding size on the
> > > > given base page exceeds SUBSECTION_SIZE(2MB now).
> > > > 
> > > > Cc: <stable@vger.kernel.org> # v5.4+
> > > > Fixes: ba72b4c8cf60 ("mm/sparsemem: support sub-section hotplug")
> > > > Reviewed-by: Catalin Marinas <catalin.marinas@arm.com>
> > > > Signed-off-by: Zhenhua Huang <quic_zhenhuah@quicinc.com>
> > > 
> > > Just so I understand correctly: for ordinary memory-sections-size hotplug
> > > (NVDIMM, virtio-mem), we still get a large mapping where possible?
> > 
> > Up to 2MB blocks only since that's the SUBSECTION_SIZE value. The
> > vmemmap mapping is also limited to PAGE_SIZE mappings (we could use
> > contiguous mappings for vmemmap but it's not wired up; I don't think
> > it's worth the hassle).
> 
> But that's messed up, no?
> 
> If someone hotplugs a memory section, they have to hotunplug a memory
> section, not parts of it.
> 
> That's why x86 does in vmemmap_populate():
> 
> if (end - start < PAGES_PER_SECTION * sizeof(struct page))
> 	err = vmemmap_populate_basepages(start, end, node, NULL);
> else if (boot_cpu_has(X86_FEATURE_PSE))
> 	err = vmemmap_populate_hugepages(start, end, node, altmap);
> ...
> 
> Maybe I'm missing something. Most importantly, why the weird subsection
> stuff is supposed to degrade ordinary hotplug of dimms/virtio-mem etc.

I think that's based on the discussion for a previous version assuming
that the hotplug/unplug sizes are not guaranteed to be symmetric:

https://lore.kernel.org/lkml/a720aaa5-a75e-481e-b396-a5f2b50ed362@quicinc.com/

If that's not the case, we can indeed ignore the SUBSECTION_SIZE
altogether and just rely on the start/end of the hotplugged region.

-- 
Catalin

Re: [PATCH v6] arm64: mm: Populate vmemmap/linear at the page level for hotplugged sections

[David Hildenbrand]

On 13.02.25 18:56, Catalin Marinas wrote:
> On Thu, Feb 13, 2025 at 05:16:37PM +0100, David Hildenbrand wrote:
>> On 13.02.25 16:49, Catalin Marinas wrote:
>>> On Thu, Feb 13, 2025 at 01:59:25PM +0100, David Hildenbrand wrote:
>>>> On 13.02.25 08:57, Zhenhua Huang wrote:
>>>>> On the arm64 platform with 4K base page config, SECTION_SIZE_BITS is set
>>>>> to 27, making one section 128M. The related page struct which vmemmap
>>>>> points to is 2M then.
>>>>> Commit c1cc1552616d ("arm64: MMU initialisation") optimizes the
>>>>> vmemmap to populate at the PMD section level which was suitable
>>>>> initially since hot plug granule is always one section(128M). However,
>>>>> commit ba72b4c8cf60 ("mm/sparsemem: support sub-section hotplug")
>>>>> introduced a 2M(SUBSECTION_SIZE) hot plug granule, which disrupted the
>>>>> existing arm64 assumptions.
>>>>>
>>>>> Considering the vmemmap_free -> unmap_hotplug_pmd_range path, when
>>>>> pmd_sect() is true, the entire PMD section is cleared, even if there is
>>>>> other effective subsection. For example page_struct_map1 and
>>>>> page_strcut_map2 are part of a single PMD entry and they are hot-added
>>>>> sequentially. Then page_struct_map1 is removed, vmemmap_free() will clear
>>>>> the entire PMD entry freeing the struct page map for the whole section,
>>>>> even though page_struct_map2 is still active. Similar problem exists
>>>>> with linear mapping as well, for 16K base page(PMD size = 32M) or 64K
>>>>> base page(PMD = 512M), their block mappings exceed SUBSECTION_SIZE.
>>>>> Tearing down the entire PMD mapping too will leave other subsections
>>>>> unmapped in the linear mapping.
>>>>>
>>>>> To address the issue, we need to prevent PMD/PUD/CONT mappings for both
>>>>> linear and vmemmap for non-boot sections if corresponding size on the
>>>>> given base page exceeds SUBSECTION_SIZE(2MB now).
>>>>>
>>>>> Cc: <stable@vger.kernel.org> # v5.4+
>>>>> Fixes: ba72b4c8cf60 ("mm/sparsemem: support sub-section hotplug")
>>>>> Reviewed-by: Catalin Marinas <catalin.marinas@arm.com>
>>>>> Signed-off-by: Zhenhua Huang <quic_zhenhuah@quicinc.com>
>>>>
>>>> Just so I understand correctly: for ordinary memory-sections-size hotplug
>>>> (NVDIMM, virtio-mem), we still get a large mapping where possible?
>>>
>>> Up to 2MB blocks only since that's the SUBSECTION_SIZE value. The
>>> vmemmap mapping is also limited to PAGE_SIZE mappings (we could use
>>> contiguous mappings for vmemmap but it's not wired up; I don't think
>>> it's worth the hassle).
>>
>> But that's messed up, no?
>>
>> If someone hotplugs a memory section, they have to hotunplug a memory
>> section, not parts of it.
>>
>> That's why x86 does in vmemmap_populate():
>>
>> if (end - start < PAGES_PER_SECTION * sizeof(struct page))
>> 	err = vmemmap_populate_basepages(start, end, node, NULL);
>> else if (boot_cpu_has(X86_FEATURE_PSE))
>> 	err = vmemmap_populate_hugepages(start, end, node, altmap);
>> ...
>>
>> Maybe I'm missing something. Most importantly, why the weird subsection
>> stuff is supposed to degrade ordinary hotplug of dimms/virtio-mem etc.
> 
> I think that's based on the discussion for a previous version assuming
> that the hotplug/unplug sizes are not guaranteed to be symmetric:
> 
> https://lore.kernel.org/lkml/a720aaa5-a75e-481e-b396-a5f2b50ed362@quicinc.com/
> 
 > If that's not the case, we can indeed ignore the SUBSECTION_SIZE> 
altogether and just rely on the start/end of the hotplugged region.

All cases I know about hotunplug system RAM in the same granularity they 
hotplugged (virtio-mem, dax/kmem, dimm, dlpar), and if they wouldn't, 
they wouldn't operate on sub-section sizes either way.

Regarding dax/pmem, I also recall that it happens always in the same 
granularity. If not, it should be fixed: this weird subsection hotplug 
should not make all other hotplug users suffer (e.g., no vmemmap PMD).

What can likely happen (dax/pmem) is that we hotplug something that 
spans part of 128 MiB section (subsections), to then hotplug something 
that spans another part of a 128 MiB section (subsections). 
Hotunplugging either should not hotplug something part of the other 
device (e.g., rip out the vmemmap PMD).

I think this was expressed with:

"However, if start or end is not aligned to a section boundary, such as 
when a subsection is hot added, populating the entire section is 
wasteful." -- which is what we should focus on.

I thought x86-64 would handle that case; it would surprise me if 
handling between both archs would have to differ in that regard: with 4k 
arm64 we have the same section/subsection sizes as on x86-64.

-- 
Cheers,

David / dhildenb

Re: [PATCH v6] arm64: mm: Populate vmemmap/linear at the page level for hotplugged sections

[Zhenhua Huang]

On 2025/2/14 2:20, David Hildenbrand wrote:
> On 13.02.25 18:56, Catalin Marinas wrote:
>> On Thu, Feb 13, 2025 at 05:16:37PM +0100, David Hildenbrand wrote:
>>> On 13.02.25 16:49, Catalin Marinas wrote:
>>>> On Thu, Feb 13, 2025 at 01:59:25PM +0100, David Hildenbrand wrote:
>>>>> On 13.02.25 08:57, Zhenhua Huang wrote:
>>>>>> On the arm64 platform with 4K base page config, SECTION_SIZE_BITS 
>>>>>> is set
>>>>>> to 27, making one section 128M. The related page struct which vmemmap
>>>>>> points to is 2M then.
>>>>>> Commit c1cc1552616d ("arm64: MMU initialisation") optimizes the
>>>>>> vmemmap to populate at the PMD section level which was suitable
>>>>>> initially since hot plug granule is always one section(128M). 
>>>>>> However,
>>>>>> commit ba72b4c8cf60 ("mm/sparsemem: support sub-section hotplug")
>>>>>> introduced a 2M(SUBSECTION_SIZE) hot plug granule, which disrupted 
>>>>>> the
>>>>>> existing arm64 assumptions.
>>>>>>
>>>>>> Considering the vmemmap_free -> unmap_hotplug_pmd_range path, when
>>>>>> pmd_sect() is true, the entire PMD section is cleared, even if 
>>>>>> there is
>>>>>> other effective subsection. For example page_struct_map1 and
>>>>>> page_strcut_map2 are part of a single PMD entry and they are hot- 
>>>>>> added
>>>>>> sequentially. Then page_struct_map1 is removed, vmemmap_free() 
>>>>>> will clear
>>>>>> the entire PMD entry freeing the struct page map for the whole 
>>>>>> section,
>>>>>> even though page_struct_map2 is still active. Similar problem exists
>>>>>> with linear mapping as well, for 16K base page(PMD size = 32M) or 64K
>>>>>> base page(PMD = 512M), their block mappings exceed SUBSECTION_SIZE.
>>>>>> Tearing down the entire PMD mapping too will leave other subsections
>>>>>> unmapped in the linear mapping.
>>>>>>
>>>>>> To address the issue, we need to prevent PMD/PUD/CONT mappings for 
>>>>>> both
>>>>>> linear and vmemmap for non-boot sections if corresponding size on the
>>>>>> given base page exceeds SUBSECTION_SIZE(2MB now).
>>>>>>
>>>>>> Cc: <stable@vger.kernel.org> # v5.4+
>>>>>> Fixes: ba72b4c8cf60 ("mm/sparsemem: support sub-section hotplug")
>>>>>> Reviewed-by: Catalin Marinas <catalin.marinas@arm.com>
>>>>>> Signed-off-by: Zhenhua Huang <quic_zhenhuah@quicinc.com>
>>>>>
>>>>> Just so I understand correctly: for ordinary memory-sections-size 
>>>>> hotplug
>>>>> (NVDIMM, virtio-mem), we still get a large mapping where possible?
>>>>
>>>> Up to 2MB blocks only since that's the SUBSECTION_SIZE value. The
>>>> vmemmap mapping is also limited to PAGE_SIZE mappings (we could use
>>>> contiguous mappings for vmemmap but it's not wired up; I don't think
>>>> it's worth the hassle).
>>>
>>> But that's messed up, no?
>>>
>>> If someone hotplugs a memory section, they have to hotunplug a memory
>>> section, not parts of it.
>>>
>>> That's why x86 does in vmemmap_populate():
>>>
>>> if (end - start < PAGES_PER_SECTION * sizeof(struct page))
>>>     err = vmemmap_populate_basepages(start, end, node, NULL);
>>> else if (boot_cpu_has(X86_FEATURE_PSE))
>>>     err = vmemmap_populate_hugepages(start, end, node, altmap);
>>> ...
>>>
>>> Maybe I'm missing something. Most importantly, why the weird subsection
>>> stuff is supposed to degrade ordinary hotplug of dimms/virtio-mem etc.
>>
>> I think that's based on the discussion for a previous version assuming
>> that the hotplug/unplug sizes are not guaranteed to be symmetric:
>>
>> https://lore.kernel.org/lkml/a720aaa5-a75e-481e-b396- 
>> a5f2b50ed362@quicinc.com/
>>
>  > If that's not the case, we can indeed ignore the SUBSECTION_SIZE> 
> altogether and just rely on the start/end of the hotplugged region.
> 
> All cases I know about hotunplug system RAM in the same granularity they 
> hotplugged (virtio-mem, dax/kmem, dimm, dlpar), and if they wouldn't, 
> they wouldn't operate on sub-section sizes either way.
> 
> Regarding dax/pmem, I also recall that it happens always in the same 
> granularity. If not, it should be fixed: this weird subsection hotplug 
> should not make all other hotplug users suffer (e.g., no vmemmap PMD).
> 
> What can likely happen (dax/pmem) is that we hotplug something that 
> spans part of 128 MiB section (subsections), to then hotplug something 
> that spans another part of a 128 MiB section (subsections). 
> Hotunplugging either should not hotplug something part of the other 
> device (e.g., rip out the vmemmap PMD).
> 
> I think this was expressed with:
> 
> "However, if start or end is not aligned to a section boundary, such as 
> when a subsection is hot added, populating the entire section is 
> wasteful." -- which is what we should focus on.
> 
> I thought x86-64 would handle that case; it would surprise me if 
> handling between both archs would have to differ in that regard: with 4k 
> arm64 we have the same section/subsection sizes as on x86-64.
> 

Thanks David and Catalin. From your discussion, I understand that 
hotplug/unplug sizes are guaranteed to be symmetric ? Therefore, it 
should be straightforward to populate to base pages if (end - start < 
PAGES_PER_SECTION * sizeof(struct page)) ? I will write patch and verify.
Please correct me if my understanding is incorrect.