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Message-Id: <20230412142236.407d6d0e6d90232da004980e@linux-foundation.org>
Date: Wed, 12 Apr 2023 14:22:36 -0700
From: Andrew Morton <akpm@...ux-foundation.org>
To: "buddy.zhang" <buddy.zhang@...cuitos.cn>
Cc: linux-mm@...ck.org, linux-kernel@...r.kernel.org,
Venkatesh Pallipadi <venki@...gle.com>,
Suresh Siddha <sbsiddha@...il.com>,
Juergen Gross <jgross@...e.com>,
Dan Williams <dan.j.williams@...el.com>,
Konstantin Khlebnikov <khlebnikov@...nvz.org>
Subject: Re: [PATCH] mm: Keep memory type same on DEVMEM Page-Fault
On Sun, 19 Mar 2023 11:37:50 +0800 "buddy.zhang" <buddy.zhang@...cuitos.cn> wrote:
> 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.
Thanks. Nobody has worked on this code for a long time. I'll cc a few
folks who may be able to comment.
> 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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