Hillf Danton wrote:
> On Thu, May 23, 2013 at 8:08 PM, Kirill A. Shutemov
> <kirill.shutemov@xxxxxxxxxxxxxxx> wrote:
> > Hillf Danton wrote:
> >> On Sun, May 12, 2013 at 9:23 AM, Kirill A. Shutemov
> >> <kirill.shutemov@xxxxxxxxxxxxxxx> wrote:
> >> > @@ -1120,7 +1119,7 @@ int do_huge_pmd_wp_page(struct mm_struct *mm, struct vm_area_struct *vma,
> >> >
> >> > page = pmd_page(orig_pmd);
> >> > VM_BUG_ON(!PageCompound(page) || !PageHead(page));
> >> > - if (page_mapcount(page) == 1) {
> >> > + if (PageAnon(page) && page_mapcount(page) == 1) {
> >>
> >> Could we avoid copying huge page if
> >> no-one else is using it, no matter anon?
> >
> > No. The page is still in page cache and can be later accessed later.
> > We could isolate the page from page cache, but I'm not sure whether it's
> > good idea.
> >
> Hugetlb tries to avoid copying pahe.
>
> /* If no-one else is actually using this page, avoid the copy
> * and just make the page writable */
> avoidcopy = (page_mapcount(old_page) == 1);
It makes sense for hugetlb, since it RAM-backed only.
Currently, the project supports only ramfs, but I hope we will bring
storage-backed filesystems later. For them it would be much cheaper to
copy the page then bring it back later from storage.
And one more point: we must not ever reuse dirty pages, since it will lead
to data lost. And ramfs pages are always dirty.
--
Kirill A. Shutemov
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