On 05/20/2020 09:26 AM, Andrew Morton wrote:
> On Tue, 19 May 2020 18:03:28 +0800 Bibo Mao <maobibo@xxxxxxxxxxx> wrote:
>
>> If two threads concurrently fault at the same address, the thread that
>> won the race updates the PTE and its local TLB. For now, the other
>> thread gives up, simply does nothing, and continues.
>>
>> It could happen that this second thread triggers another fault, whereby
>> it only updates its local TLB while handling the fault. Instead of
>> triggering another fault, let's directly update the local TLB of the
>> second thread.
>>
>> It is only useful to architectures where software can update TLB, it may
>> bring out some negative effect if update_mmu_cache is used for other
>> purpose also. It seldom happens where multiple threads access the same
>> page at the same time, so the negative effect is limited on other arches.
>
> I'm still worried about the impact on other architectures. The
> additional update_mmu_cache() calls won't occur only when multiple
> threads are racing against the same page, I think? For example,
> insert_pfn() will do this when making a read-only page a writable one.
How about defining ptep_set_access_flags function like this on mips system?
which is the same on riscv platform.
static inline int ptep_set_access_flags(struct vm_area_struct *vma,
unsigned long address, pte_t *ptep,
pte_t entry, int dirty)
{
if (!pte_same(*ptep, entry))
set_pte_at(vma->vm_mm, address, ptep, entry);
/*
* update_mmu_cache will unconditionally execute, handling both
* the case that the PTE changed and the spurious fault case.
*/
return true;
}
And keep the following piece of code unchanged, the change will be smaller.
@@ -1770,8 +1770,8 @@ static vm_fault_t insert_pfn(struct vm_area_struct *vma, unsigned long addr,
}
entry = pte_mkyoung(*pte);
entry = maybe_mkwrite(pte_mkdirty(entry), vma);
- if (ptep_set_access_flags(vma, addr, pte, entry, 1))
- update_mmu_cache(vma, addr, pte);
+ ptep_set_access_flags(vma, addr, pte, entry, 1);
+ update_mmu_cache(vma, addr, pte);
}
@@ -2436,17 +2436,16 @@ static inline bool cow_user_page(struct page *dst, struct page *src,
entry = pte_mkyoung(vmf->orig_pte);
- if (ptep_set_access_flags(vma, addr, vmf->pte, entry, 0))
- update_mmu_cache(vma, addr, vmf->pte);
+ ptep_set_access_flags(vma, addr, vmf->pte, entry, 0);
+ update_mmu_cache(vma, addr, vmf->pte);
}
@@ -2618,8 +2618,8 @@ static inline void wp_page_reuse(struct vm_fault *vmf)
flush_cache_page(vma, vmf->address, pte_pfn(vmf->orig_pte));
entry = pte_mkyoung(vmf->orig_pte);
entry = maybe_mkwrite(pte_mkdirty(entry), vma);
- if (ptep_set_access_flags(vma, vmf->address, vmf->pte, entry, 1))
- update_mmu_cache(vma, vmf->address, vmf->pte);
+ ptep_set_access_flags(vma, vmf->address, vmf->pte, entry, 1);
+ update_mmu_cache(vma, vmf->address, vmf->pte);
pte_unmap_unlock(vmf->pte, vmf->ptl);
}
>
> Would you have time to add some instrumentation into update_mmu_cache()
> (maybe a tracepoint) and see what effect this change has upon the
> frequency at which update_mmu_cache() is called for a selection of
> workloads? And add this info to the changelog to set minds at ease?
OK, I will add some instrumentation data in the changelog.
