mm/compaction: Fix the incorrect hole in fast_isolate_freepages() 

Quoted 70 lines · Mike Rapoport

Hello Baoquan,

Quoted 66 lines · Baoquan He

Quoted 37 lines · Mike Rapoport

Hello Baoquan,

Quoted 7 lines · Baoquan He

Quoted 5 lines · Baoquan He

So let's add these unavailable ranges into memblock and reserve them in init_unavailable_range() instead. With this change, they will be added into appropriate node and zone in memmap_init(), and initialized in reserve_bootmem_region() just like any other memblock reserved regions.

Quoted 5 lines

Seems this is not right. They can't get nid in init_unavailable_range(). Adding e820 ranges may let them get nid. But the hole range won't be added to memblock, and still has the issue.

Nack this one for now, still considering.

Why won't we add the e820 reserved ranges to memblock.memory during early boot as I suggested?

arch/x86/kernel/e820.c +4 -1

index c5399e80c59c..b0940c618ed9 100644
@@ -1301,8 +1301,11 @@ void __init e820__memblock_setup(void)
 		if (end != (resource_size_t)end)
 			continue;
 
-		if (entry->type == E820_TYPE_SOFT_RESERVED)
+		if (entry->type == E820_TYPE_SOFT_RESERVED ||
+		    entry->type == E820_TYPE_RESERVED) {
+			memblock_add(entry->addr, entry->size);
 			memblock_reserve(entry->addr, entry->size);
+		}
 
 		if (entry->type != E820_TYPE_RAM && entry->type != E820_TYPE_RESERVED_KERN)
 			continue;

Quoted 2 lines

The setting of node later in numa_init() will assign the proper node for these regions as it does for the usable memory.

Yes, if it's only related to e820 reserved region, this truly works.

However, it also has ACPI table regions. That's why I changed to call the problematic area as firmware reserved ranges later.

Bisides, you can see below line, there's another reserved region which only occupies one page in one memory seciton. If adding to memblock.memory, we also will build struct mem_section and the relevant struct pages for the whole section. And then the holes around that page will be added and initialized in init_unavailable_mem(). numa_init() will assign proper node for memblock.memory and memblock.reserved, but won't assign proper node for the holes.

~~~

[    0.000000] BIOS-e820: [mem 0x00000000fed80000-0x00000000fed80fff] reserved

~~~

So I still think we should not add firmware reserved range into memblock for fixing this issue.

And, the fix in the original patch seems necessary. You can see in compaction code, the migration source is chosen from LRU pages or movable pages, the migration target has to be got from Buddy. However, only the min_pfn in fast_isolate_freepages(), it's calculated by distance between cc->free_pfn - cc->migrate_pfn, we can't guarantee it's safe, then use it as the target to handle.

Quoted 4 lines

I do not object to your original fix with careful check for pfn validity.

But I still think that the memory reserved by the firmware is still memory and it should be added to memblock.memory. This way the memory

If it's really memory that could be read/written, I think I agree.

Quoted 3 lines

map will be properly initialized from the very beginning and we won't need init_unavailable_mem() and alike workarounds and. Obviously, the patch

I remember init_unavailable_mem() is necessary for holes within sections, where we actually *don't* have memory, but we still have have a valid memmap (full section) that we have to initialize.

See the example from 4b094b7851bf ("mm/page_alloc.c: initialize memmap of unavailable memory directly"). Our main memory ends within a section, so we have to initialize the remaining parts because the whole section will be marked valid/online.

Any way to improve this handling is appreciated. In that patch I also spelled out that we might want to mark such holes via a new page type, e.g., PageHole(). Such a page is a memory hole, but has a valid memmap. Any content in the memmap (zone/node) should be ignored.

But it's all quite confusing, especially across architectures and ...

Quoted 4 lines

above is not enough, but it's a small step in this direction.

I believe that improving the early memory initialization would make many things simpler and more robust, but that's a different story :)

... I second that.

Signature · 3 lines

Thanks,

David / dhildenb

[email protected], [email protected], [email protected], [email protected],
Bcc: [email protected]
Subject: Re: [PATCH] mm/compaction: Fix the incorrect hole in
fast_isolate_freepages()
Reply-To:
In-Reply-To: <[email protected]>

Quoted 71 lines · Mike Rapoport

Quoted 70 lines

Hello Baoquan,

Quoted 66 lines · Baoquan He

Quoted 37 lines · Mike Rapoport

Hello Baoquan,

Quoted 7 lines · Baoquan He

Quoted 5 lines · Baoquan He

So let's add these unavailable ranges into memblock and reserve them in init_unavailable_range() instead. With this change, they will be added into appropriate node and zone in memmap_init(), and initialized in reserve_bootmem_region() just like any other memblock reserved regions.

Quoted 5 lines

Seems this is not right. They can't get nid in init_unavailable_range(). Adding e820 ranges may let them get nid. But the hole range won't be added to memblock, and still has the issue.

Nack this one for now, still considering.

Why won't we add the e820 reserved ranges to memblock.memory during early boot as I suggested?

arch/x86/kernel/e820.c +4 -1

index c5399e80c59c..b0940c618ed9 100644
@@ -1301,8 +1301,11 @@ void __init e820__memblock_setup(void)
 		if (end != (resource_size_t)end)
 			continue;
 
-		if (entry->type == E820_TYPE_SOFT_RESERVED)
+		if (entry->type == E820_TYPE_SOFT_RESERVED ||
+		    entry->type == E820_TYPE_RESERVED) {
+			memblock_add(entry->addr, entry->size);
 			memblock_reserve(entry->addr, entry->size);
+		}
 
 		if (entry->type != E820_TYPE_RAM && entry->type != E820_TYPE_RESERVED_KERN)
 			continue;

Quoted 2 lines

The setting of node later in numa_init() will assign the proper node for these regions as it does for the usable memory.

Yes, if it's only related to e820 reserved region, this truly works.

However, it also has ACPI table regions. That's why I changed to call the problematic area as firmware reserved ranges later.

Bisides, you can see below line, there's another reserved region which only occupies one page in one memory seciton. If adding to memblock.memory, we also will build struct mem_section and the relevant struct pages for the whole section. And then the holes around that page will be added and initialized in init_unavailable_mem(). numa_init() will assign proper node for memblock.memory and memblock.reserved, but won't assign proper node for the holes.

~~~

[    0.000000] BIOS-e820: [mem 0x00000000fed80000-0x00000000fed80fff] reserved

~~~

So I still think we should not add firmware reserved range into memblock for fixing this issue.

And, the fix in the original patch seems necessary. You can see in compaction code, the migration source is chosen from LRU pages or movable pages, the migration target has to be got from Buddy. However, only the min_pfn in fast_isolate_freepages(), it's calculated by distance between cc->free_pfn - cc->migrate_pfn, we can't guarantee it's safe, then use it as the target to handle.

Quoted 6 lines

Quoted 4 lines

I do not object to your original fix with careful check for pfn validity.

But I still think that the memory reserved by the firmware is still memory and it should be added to memblock.memory. This way the memory

If it's really memory that could be read/written, I think I agree.

I would say some of them may not be allowed to be read/written, if I understand it correctly. I roughly went through the x86 init code, there are some places where mem region is marked as E820_TYPE_RESERVED so that they are not touched after initialization. E.g:

1) pfn 0
In trim_bios_range(), we set the pfn 0 as E820_TYPE_RESERVED. You can see the code comment, this is a BIOS owned area, but not kernel RAM.

2)GART reserved region
In early_gart_iommu_check(), GART IOMMU firmware will reserve a region in an area, firmware designer won't map system RAM into that area.

And also intel_graphics_stolen(), arch_rmrr_sanity_check(), these regions are not system RAM backed area, reading from or writting into these area may cause error.

Futhermore, there's a KASLR bug found by HPE, its triggering and root cause are written into below commit log. You can see that accessing to firmware reserved region caused BIOS to halt system when cpu doing speculative.

commit 2aa85f246c181b1fa89f27e8e20c5636426be624

Author: Steve Wahl <[email protected]>
Date:   Tue Sep 24 16:03:55 2019 -0500

x86/boot/64: Make level2_kernel_pgt pages invalid outside kernel area

Our hardware (UV aka Superdome Flex) has address ranges marked reserved by the BIOS. Access to these ranges is caught as an error, causing the BIOS to halt the system.

Quoted 4 lines

Quoted 3 lines

map will be properly initialized from the very beginning and we won't need init_unavailable_mem() and alike workarounds and. Obviously, the patch

Quoted 8 lines

I remember init_unavailable_mem() is necessary for holes within sections, where we actually *don't* have memory, but we still have have a valid memmap (full section) that we have to initialize.

See the example from 4b094b7851bf ("mm/page_alloc.c: initialize memmap of unavailable memory directly"). Our main memory ends within a section, so we have to initialize the remaining parts because the whole section will be marked valid/online.

Yes, memory hole need be handled in init_unavailable_mem(). Since we have created struct page for them, need initialize them. We can't discard init_unavailable_mem() for now.

Quoted 5 lines

Any way to improve this handling is appreciated. In that patch I also spelled out that we might want to mark such holes via a new page type, e.g., PageHole(). Such a page is a memory hole, but has a valid memmap. Any content in the memmap (zone/node) should be ignored.

As I said at above, I am a little conservative to add all those regions of E820_TYPE_RESERVED into memblock.memory and memblock.reserved, because most of them are firmware reserved region, they may be not backed by normal RAM.

I was thinking to step back to use mm_zero_struct_page() inside init_unavailable_range() as below. But it doesn't differ much from __init_single_page(), except of the _refcount and mapcount. Zeroing struct page equals to putting them into node 0, zero 0.

mm/page_alloc.c +1 -1

index 3973b5fdfe3f..4e4b72cf5283 100644
@@ -6901,7 +6901,7 @@ static u64 __init init_unavailable_range(unsigned long spfn, unsigned long epfn)
 		 * (in memblock.reserved but not in memblock.memory) will
 		 * get re-initialized via reserve_bootmem_region() later.
 		 */
-		__init_single_page(pfn_to_page(pfn), pfn, 0, 0);
+		mm_zero_struct_page(pfn_to_page(pfn));
 		__SetPageReserved(pfn_to_page(pfn));
 		pgcnt++;
 	}

About adding these unavailable ranges into node/zone, in the old code, it just happened to add them into expected node/zone. You can see in early_pfn_in_nid(), if no nid found from memblock, the returned '-1' will make it true ironically. But that is not saying the bad thing always got good result. If the last zone of node 0 is DMA32 zone, the deferred init will skip the only chance to add some of unavailable rnages into expected node/zone. Means they were not always added into appropriate node/zone before, the change of iterating memblock.memory in memmap_init() dones't introduce regression.

static inline bool __meminit early_pfn_in_nid(unsigned long pfn, int node)

{
        int nid;

        nid = __early_pfn_to_nid(pfn, &early_pfnnid_cache);
        if (nid >= 0 && nid != node)
                return false;
        return true;
}

So if no anybody need access them after boot, not adding them into any node/zone sounds better. Otherwise, better add them in the appropriate node/zone.

Quoted 14 lines

But it's all quite confusing, especially across architectures and ...

Quoted 4 lines

above is not enough, but it's a small step in this direction.

I believe that improving the early memory initialization would make many things simpler and more robust, but that's a different story :)

... I second that.

Signature · 3 lines

Thanks,

David / dhildenb