Re: [PATCH v2 14/20] mm: Provide speculative fault infrastructure

[Anshuman Khandual]

On 08/29/2017 07:15 PM, Peter Zijlstra wrote:
> On Tue, Aug 29, 2017 at 03:18:25PM +0200, Laurent Dufour wrote:
>> On 29/08/2017 14:04, Peter Zijlstra wrote:
>>> On Tue, Aug 29, 2017 at 09:59:30AM +0200, Laurent Dufour wrote:
>>>> On 27/08/2017 02:18, Kirill A. Shutemov wrote:
>>>>>> +
>>>>>> +	if (unlikely(!vma->anon_vma))
>>>>>> +		goto unlock;
>>>>>
>>>>> It deserves a comment.
>>>>
>>>> You're right I'll add it in the next version.
>>>> For the record, the root cause is that __anon_vma_prepare() requires the
>>>> mmap_sem to be held because vm_next and vm_prev must be safe.
>>>
>>> But should that test not be:
>>>
>>> 	if (unlikely(vma_is_anonymous(vma) && !vma->anon_vma))
>>> 		goto unlock;
>>>
>>> Because !anon vmas will never have ->anon_vma set and you don't want to
>>> exclude those.
>>
>> Yes in the case we later allow non anonymous vmas to be handled.
>> Currently only anonymous vmas are supported so the check is good enough,
>> isn't it ?
> 
> That wasn't at all clear from reading the code. This makes it clear
> ->anon_vma is only ever looked at for anonymous.
> 
> And like Kirill says, we _really_ should start allowing some (if not
> all) vm_ops. Large file based mappings aren't particularly rare.
> 
> I'm not sure we want to introduce a white-list or just bite the bullet
> and audit all ->fault() implementations. But either works and isn't
> terribly difficult, auditing all is more work though.

filemap_fault() is used as vma-vm_ops->fault() for most of the file
systems. Changing it can enable speculative fault support for all of
them. It will still exclude other driver based vma-vm_ops->fault()
implementation. AFAICS, __lock_page_or_retry() function can drop
mm->mmap_sem if the page could not be locked right away. As suggested
by Peterz, making it understand FAULT_FLAG_SPECULATIVE should be good
enough. The patch is lightly tested for file mappings on top of this
series.

diff --git a/mm/filemap.c b/mm/filemap.c
index a497024..08f3042 100644
--- a/mm/filemap.c
+++ b/mm/filemap.c
@@ -1181,6 +1181,18 @@ int __lock_page_killable(struct page *__page)
 int __lock_page_or_retry(struct page *page, struct mm_struct *mm,
                         unsigned int flags)
 {
+       if (flags & FAULT_FLAG_SPECULATIVE) {
+               if (flags & FAULT_FLAG_KILLABLE) {
+                       int ret;
+
+                       ret = __lock_page_killable(page);
+                       if (ret)
+                               return 0;
+               } else
+                       __lock_page(page);
+               return 1;
+       }
+
        if (flags & FAULT_FLAG_ALLOW_RETRY) {
                /*
                 * CAUTION! In this case, mmap_sem is not released
diff --git a/mm/memory.c b/mm/memory.c
index 549d235..02347f3 100644
--- a/mm/memory.c
+++ b/mm/memory.c
@@ -3836,8 +3836,6 @@ static int handle_pte_fault(struct vm_fault *vmf)
        if (!vmf->pte) {
                if (vma_is_anonymous(vmf->vma))
                        return do_anonymous_page(vmf);
-               else if (vmf->flags & FAULT_FLAG_SPECULATIVE)
-                       return VM_FAULT_RETRY;
                else
                        return do_fault(vmf);
        }
@@ -4012,17 +4010,7 @@ int handle_speculative_fault(struct mm_struct *mm, unsigned long address,
                goto unlock;
        }

-       /*
-        * Can't call vm_ops service has we don't know what they would do
-        * with the VMA.
-        * This include huge page from hugetlbfs.
-        */
-       if (vma->vm_ops) {
-               trace_spf_vma_notsup(_RET_IP_, vma, address);
-               goto unlock;
-       }
-
-       if (unlikely(!vma->anon_vma)) {
+       if (unlikely(vma_is_anonymous(vma) && !vma->anon_vma)) {
                trace_spf_vma_notsup(_RET_IP_, vma, address);
                goto unlock;
        }