Re: [PATCH v9 15/24] mm: Introduce __vm_normal_page()

[Laurent Dufour]

On 03/04/2018 21:39, Jerome Glisse wrote:
> On Tue, Mar 13, 2018 at 06:59:45PM +0100, Laurent Dufour wrote:
>> When dealing with the speculative fault path we should use the VMA's field
>> cached value stored in the vm_fault structure.
>>
>> Currently vm_normal_page() is using the pointer to the VMA to fetch the
>> vm_flags value. This patch provides a new __vm_normal_page() which is
>> receiving the vm_flags flags value as parameter.
>>
>> Note: The speculative path is turned on for architecture providing support
>> for special PTE flag. So only the first block of vm_normal_page is used
>> during the speculative path.
> 
> Might be a good idea to explicitly have SPECULATIVE Kconfig option depends
> on ARCH_PTE_SPECIAL and a comment for !HAVE_PTE_SPECIAL in the function
> explaining that speculative page fault should never reach that point.

Unfortunately there is no ARCH_PTE_SPECIAL in the config file, it is defined in
the per architecture header files.
So I can't do anything in the Kconfig file

However, I can check that at build time, and doing such a check in
__vm_normal_page sounds to be a good place, like that:

@@ -869,6 +870,14 @@ struct page *__vm_normal_page(struct vm_area_struct *vma,
unsigned long addr,

        /* !HAVE_PTE_SPECIAL case follows: */

+#ifdef CONFIG_SPECULATIVE_PAGE_FAULT
+       /* This part should never get called when the speculative page fault
+        * handler is turned on. This is mainly because we can't rely on
+        * vm_start.
+        */
+#error CONFIG_SPECULATIVE_PAGE_FAULT requires HAVE_PTE_SPECIAL
+#endif
+
        if (unlikely(vma_flags & (VM_PFNMAP|VM_MIXEDMAP))) {
                if (vma_flags & VM_MIXEDMAP) {
                        if (!pfn_valid(pfn))

Thanks,
Laurent.