On Sat, Sep 21, 2019 at 09:50:54PM +0800, Jia He wrote:
> @@ -2151,21 +2163,53 @@ static inline void cow_user_page(struct page *dst,
> struct page *src, unsigned lo
> * fails, we just zero-fill it. Live with it.
> */
> if (unlikely(!src)) {
> - void *kaddr = kmap_atomic(dst);
> - void __user *uaddr = (void __user *)(va & PAGE_MASK);
> + void *kaddr;
> + pte_t entry;
> + void __user *uaddr = (void __user *)(addr & PAGE_MASK);
>
> + /* On architectures with software "accessed" bits, we would
> + * take a double page fault, so mark it accessed here.
> + */
Nitpick: please follow the kernel coding style for multi-line comments
(above and the for the rest of the patch):
/*
* Your multi-line comment.
*/
> + if (arch_faults_on_old_pte() && !pte_young(vmf->orig_pte)) {
> + vmf->pte = pte_offset_map_lock(mm, vmf->pmd, addr,
> + &vmf->ptl);
> + if (likely(pte_same(*vmf->pte, vmf->orig_pte))) {
> + entry = pte_mkyoung(vmf->orig_pte);
> + if (ptep_set_access_flags(vma, addr,
> + vmf->pte, entry, 0))
> + update_mmu_cache(vma, addr, vmf->pte);
> + } else {
> + /* Other thread has already handled the fault
> + * and we don't need to do anything. If it's
> + * not the case, the fault will be triggered
> + * again on the same address.
> + */
> + pte_unmap_unlock(vmf->pte, vmf->ptl);
> + return false;
> + }
> + pte_unmap_unlock(vmf->pte, vmf->ptl);
> + }
Another nit, you could rewrite this block slightly to avoid too much
indentation. Something like (untested):
if (arch_faults_on_old_pte() && !pte_young(vmf->orig_pte)) {
vmf->pte = pte_offset_map_lock(mm, vmf->pmd, addr,
&vmf->ptl);
if (unlikely(!pte_same(*vmf->pte, vmf->orig_pte))) {
/*
* Other thread has already handled the fault
* and we don't need to do anything. If it's
* not the case, the fault will be triggered
* again on the same address.
*/
pte_unmap_unlock(vmf->pte, vmf->ptl);
return false;
}
entry = pte_mkyoung(vmf->orig_pte);
if (ptep_set_access_flags(vma, addr,
vmf->pte, entry, 0))
update_mmu_cache(vma, addr, vmf->pte);
pte_unmap_unlock(vmf->pte, vmf->ptl);
}
> +
> + kaddr = kmap_atomic(dst);
Since you moved the kmap_atomic() here, could the above
arch_faults_on_old_pte() run in a preemptible context? I suggested to
add a WARN_ON in patch 2 to be sure.
> /*
> * This really shouldn't fail, because the page is there
> * in the page tables. But it might just be unreadable,
> * in which case we just give up and fill the result with
> * zeroes.
> */
> - if (__copy_from_user_inatomic(kaddr, uaddr, PAGE_SIZE))
> + if (__copy_from_user_inatomic(kaddr, uaddr, PAGE_SIZE)) {
> + /* Give a warn in case there can be some obscure
> + * use-case
> + */
> + WARN_ON_ONCE(1);
That's more of a question for the mm guys: at this point we do the
copying with the ptl released; is there anything else that could have
made the pte old in the meantime? I think unuse_pte() is only called on
anonymous vmas, so it shouldn't be the case here.
> clear_page(kaddr);
> + }
> kunmap_atomic(kaddr);
> flush_dcache_page(dst);
> } else
> - copy_user_highpage(dst, src, va, vma);
> + copy_user_highpage(dst, src, addr, vma);
> +
> + return true;
> }
--
Catalin
