Re: [PATCH 1/4] rmap: add page_wrprotect() function,

Andrew Morton
2008-11-11T19:41:19Z

On Tue, 11 Nov 2008 15:21:38 +0200
Izik Eidus <ieidus@redhat.com> wrote:

> From: Izik Eidus <izike@qumranet.com>
>
> this function is useful for cases you want to compare page and know
> that its value wont change during you compare it.
>
> this function is working by walking over the whole rmap of a page
> and mark every pte related to the page as write_protect.
>
> the odirect_sync paramter is used to notify the caller of
> page_wrprotect() if one pte or more was not marked readonly
> in order to avoid race with odirect.

The grammar is a bit mangled. Missing apostrophes. Sentences start
with capital letters.

Yeah, it's a nit, but we don't actually gain anything from deliberately
mangling the language.

>
> ...
>
> --- a/mm/rmap.c
> +++ b/mm/rmap.c
> @@ -576,6 +576,103 @@ int page_mkclean(struct page *page)
> }
> EXPORT_SYMBOL_GPL(page_mkclean);
>
> +static int page_wrprotect_one(struct page *page, struct vm_area_struct *vma,
> + int *odirect_sync)
> +{
> + struct mm_struct *mm = vma->vm_mm;
> + unsigned long address;
> + pte_t *pte;
> + spinlock_t *ptl;
> + int ret = 0;
> +
> + address = vma_address(page, vma);
> + if (address == -EFAULT)
> + goto out;
> +
> + pte = page_check_address(page, mm, address, &ptl, 0);
> + if (!pte)
> + goto out;
> +
> + if (pte_write(*pte)) {
> + pte_t entry;
> +
> + if (page_mapcount(page) != page_count(page)) {
> + *odirect_sync = 0;
> + goto out_unlock;
> + }
> + flush_cache_page(vma, address, pte_pfn(*pte));
> + entry = ptep_clear_flush_notify(vma, address, pte);
> + entry = pte_wrprotect(entry);
> + set_pte_at(mm, address, pte, entry);
> + }
> + ret = 1;
> +
> +out_unlock:
> + pte_unmap_unlock(pte, ptl);
> +out:
> + return ret;
> +}

OK. I think. We need to find a way of provoking Hugh to look at it.

> +static int page_wrprotect_file(struct page *page, int *odirect_sync)
> +{
> + struct address_space *mapping;
> + struct prio_tree_iter iter;
> + struct vm_area_struct *vma;
> + pgoff_t pgoff = page->index << (PAGE_CACHE_SHIFT - PAGE_SHIFT);
> + int ret = 0;
> +
> + mapping = page_mapping(page);

What pins *mapping in memory? Usually this is done by requiring that
the caller has locked the page. But no such precondition is documented
here.

> + if (!mapping)
> + return ret;
> +
> + spin_lock(&mapping->i_mmap_lock);
> +
> + vma_prio_tree_foreach(vma, &iter, &mapping->i_mmap, pgoff, pgoff)
> + ret += page_wrprotect_one(page, vma, odirect_sync);
> +
> + spin_unlock(&mapping->i_mmap_lock);
> +
> + return ret;
> +}
> +
> +static int page_wrprotect_anon(struct page *page, int *odirect_sync)
> +{
> + struct vm_area_struct *vma;
> + struct anon_vma *anon_vma;
> + int ret = 0;
> +
> + anon_vma = page_lock_anon_vma(page);
> + if (!anon_vma)
> + return ret;
> +
> + list_for_each_entry(vma, &anon_vma->head, anon_vma_node)
> + ret += page_wrprotect_one(page, vma, odirect_sync);
> +
> + page_unlock_anon_vma(anon_vma);
> +
> + return ret;
> +}
> +
> +/**

This token means "this is a kerneldoc comment".

> + * set all the ptes pointed to a page as read only,
> + * odirect_sync is set to 0 in case we cannot protect against race with odirect
> + * return the number of ptes that were set as read only
> + * (ptes that were read only before this function was called are couned as well)
> + */

But it isn't.

I don't understand this odirect_sync thing. What race? Please expand
this comment to make the function of odirect_sync more understandable.

> +int page_wrprotect(struct page *page, int *odirect_sync)
> +{
> + int ret =0;
> +
> + *odirect_sync = 1;
> + if (PageAnon(page))
> + ret = page_wrprotect_anon(page, odirect_sync);
> + else
> + ret = page_wrprotect_file(page, odirect_sync);
> +
> + return ret;
> +}
> +EXPORT_SYMBOL(page_wrprotect);

What do you think about making all this new code dependent upon some
CONFIG_ switch which CONFIG_KVM can select?