Date:   Sun, 19 Mar 2023 11:37:50 +0800
From:   "buddy.zhang" <buddy.zhang@...cuitos.cn>
To:     akpm@...ux-foundation.org
Cc:     linux-mm@...ck.org, linux-kernel@...r.kernel.org,
        "buddy.zhang" <buddy.zhang@...cuitos.cn>
Subject: [PATCH] mm: Keep memory type same on DEVMEM Page-Fault

On X86 architecture, supports memory type on Page-table, such as
PTE is PAT/PCD/PWD, which can setup up Memory Type as WC/WB/WT/UC etc.
Then, Virtual address from userspace or kernel space can map to
same physical page, if each page table has different memory type,
then it's confused to have more memory type for same physical page.

On DEVMEM, the 'remap_pfn_range()' keep memory type same on different
mapping. But if it happen on Page-Fault route, such as code:

 19 static vm_fault_t vm_fault(struct vm_fault *vmf)
 20 {
 21         struct vm_area_struct *vma = vmf->vma;
 22         unsigned long address = vmf->address;
 23         struct page *fault_page;
 24         unsigned long pfn;
 25         int r;
 26
 27         /* Allocate Page as DEVMEM */
 28         fault_page = alloc_page(GFP_KERNEL);
 29         if (!fault_page) {
 30                 printk("ERROR: NO Free Memory from DEVMEM.\n");
 31                 r = -ENOMEM;
 32                 goto err_alloc;
 33         }
 34         pfn = page_to_pfn(fault_page);
 35
 36         /* Clear PAT Attribute */
 37         pgprot_val(vma->vm_page_prot) &= ~(_PAGE_PCD | _PAGE_PWT | _PAGE_PAT);
 38
 39         /* Change Memory Type for Direct-Mapping Area */
 40         arch_io_reserve_memtype_wc(PFN_PHYS(pfn), PAGE_SIZE);
 41         pgprot_val(vma->vm_page_prot) |= cachemode2protval(_PAGE_CACHE_MODE_WT);
 42
 43         /* Establish pte and INC _mapcount for page */
 44         vm_flags_set(vma, VM_MIXEDMAP);
 45         if (vm_insert_page(vma, address, fault_page))
 46                 return -EAGAIN;
 47
 48         /* Add refcount for page */
 49         atomic_inc(&fault_page->_refcount);
 50         /* bind fault page */
 51         vmf->page = fault_page;
 52
 53         return 0;
 54
 55 err_alloc:
 56         return r;
 57 }
 58
 59 static const struct vm_operations_struct BiscuitOS_vm_ops = {
 60         .fault  = vm_fault,
 61 };
 62
 63 static int BiscuitOS_mmap(struct file *filp, struct vm_area_struct *vma)
 64 {
 65         /* setup vm_ops */
 66         vma->vm_ops = &BiscuitOS_vm_ops;
 67
 68         return 0;
 69 }

If invoke arch_io_reserve_memtype_wc() on Line-40, and modify memory type
as WC for Direct-Mapping area, and then setup meory type as WT on Line-41,
then invoke 'vm_insert_page()' to create mapping, so you can see:

    | <----- Usespace -----> | <- Kernel space -> |
----+------+---+-------------+---+---+------------+--
    |      |   |             |   |   |            |
----+------+---+-------------+---+---+------------+--
           WT|                     |WC
             o-------o    o--------o
                   WT|    |WC
                     V    V
-------------------+--------+------------------------
                   | DEVMEM |
-------------------+--------+------------------------
Physical Address Space

For this case, OS should check memory type before mapping on 'vm_insert_page()',
and keep memory type same, so add check on function:

07 int vm_insert_page(struct vm_area_struct *vma, unsigned long addr,
08                         struct page *page)
09 {
10         if (addr < vma->vm_start || addr >= vma->vm_end)
11                 return -EFAULT;
12         if (!page_count(page))
13                 return -EINVAL;
14         if (!(vma->vm_flags & VM_MIXEDMAP)) {
15                 BUG_ON(mmap_read_trylock(vma->vm_mm));
16                 BUG_ON(vma->vm_flags & VM_PFNMAP);
17                 vm_flags_set(vma, VM_MIXEDMAP);
18         }
19         if (track_pfn_remap(vma, &vma->vm_page_prot,
20                         page_to_pfn(page), addr, PAGE_SIZE))
21                 return -EINVAL;
22         return insert_page(vma, addr, page, vma->vm_page_prot);
23 }

And line 19 to 21, when mapping different memory type on this route, the
'track_pfn_remap()' will notify error and change request as current, e.g.

x86/PAT: APP:88 map pfn RAM range req write-through for [mem 0x025c1000-0x025c1fff], got write-combining

And then, we can keep memory type same on Page-fault route for DEVMEM, the end:

    | <----- Usespace -----> | <- Kernel space -> |
----+------+---+-------------+---+---+------------+--
    |      |   |             |   |   |            |
----+------+---+-------------+---+---+------------+--
           WT|                     |WC
             o---(X)----o----------o
                        |WC
                        V
-------------------+--------+------------------------
                   | DEVMEM |
-------------------+--------+------------------------

Signed-off-by: buddy.zhang@...cuitos.cn
---
 mm/memory.c | 3 +++
 1 file changed, 3 insertions(+)

diff --git a/mm/memory.c b/mm/memory.c
index f456f3b5049c..ed3d09f513f1 100644
--- a/mm/memory.c
+++ b/mm/memory.c
@@ -1989,6 +1989,9 @@ int vm_insert_page(struct vm_area_struct *vma, unsigned long addr,
 		BUG_ON(vma->vm_flags & VM_PFNMAP);
 		vm_flags_set(vma, VM_MIXEDMAP);
 	}
+	if (track_pfn_remap(vma, &vma->vm_page_prot,
+			page_to_pfn(page), addr, PAGE_SIZE))
+		return -EINVAL;
 	return insert_page(vma, addr, page, vma->vm_page_prot);
 }
 EXPORT_SYMBOL(vm_insert_page);
-- 
2.25.1

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