Re: Question about ptep_get_and_clear and TLB flush

[Jerome Glisse <jglisse@redhat.com>]

On Thu, Oct 18, 2018 at 11:04:02PM -0700, Joel Fernandes wrote:
> Hello friends,
> I was trying to understand the safety of this piece of code in
> move_ptes in mremap.c
> Here we have some code that does this in a loop:
> 
> for (; old_addr < old_end; old_pte++, old_addr += PAGE_SIZE,
>  new_pte++, new_addr += PAGE_SIZE) {
>   if (pte_none(*old_pte))
>        continue;
>     pte = ptep_get_and_clear(mm, old_addr, old_pte);
>     if (pte_present(pte) && pte_dirty(pte))
>          force_flush = true;
>     pte = move_pte(pte, new_vma->vm_page_prot, old_addr, new_addr);
>     pte = move_soft_dirty_pte(pte);
>     set_pte_at(mm, new_addr, new_pte, pte);
> }
> 
> If I understand correctly, the ptep_get_and_clear is needed to
> atomically get and clear the page table entry so that we do not miss
> any other bits in PTE that may get set but have not been read, before
> we clear it. Such as the dirty bit.
> 
> My question is, After the ptep_get_and_clear runs, what happens if
> another CPU has a valid TLB entry for this old_addr and does a
> memory-write *before* the TLBs are flushed. Would that not cause us to
> lose the dirty bit? Once set_pte_at runs, it would be using the PTE
> fetched earlier which did not have the dirty bit set. This seems wrong
> to me. What do you think?
> 

https://yarchive.net/comp/linux/x86_tlb.html