Re: [PATCH v2 13/25] fs: Add zero_user_large

Kirill A. Shutemov
2020-02-18T14:16:13Z

On Fri, Feb 14, 2020 at 08:03:42AM -0800, Matthew Wilcox wrote:
> On Fri, Feb 14, 2020 at 04:52:48PM +0300, Kirill A. Shutemov wrote:
> > On Tue, Feb 11, 2020 at 08:18:33PM -0800, Matthew Wilcox wrote:
> > > From: "Matthew Wilcox (Oracle)" <willy@infradead.org>
> > >
> > > We can't kmap() a THP, so add a wrapper around zero_user() for large
> > > pages.
> >
> > I would rather address it closer to the root: make zero_user_segments()
> > handle compound pages.
>
> Hah. I ended up doing that, but hadn't sent it out. I don't like
> how ugly it is:
>
> @@ -219,18 +219,57 @@ static inline void zero_user_segments(struct page *page,
> unsigned start1, unsigned end1,
> unsigned start2, unsigned end2)
> {
> - void *kaddr = kmap_atomic(page);
> -
> - BUG_ON(end1 > PAGE_SIZE || end2 > PAGE_SIZE);
> -
> - if (end1 > start1)
> - memset(kaddr + start1, 0, end1 - start1);
> -
> - if (end2 > start2)
> - memset(kaddr + start2, 0, end2 - start2);
> -
> - kunmap_atomic(kaddr);
> - flush_dcache_page(page);
> + unsigned int i;
> +
> + BUG_ON(end1 > thp_size(page) || end2 > thp_size(page));
> +
> + for (i = 0; i < hpage_nr_pages(page); i++) {
> + void *kaddr;
> + unsigned this_end;
> +
> + if (end1 == 0 && start2 >= PAGE_SIZE) {
> + start2 -= PAGE_SIZE;
> + end2 -= PAGE_SIZE;
> + continue;
> + }
> +
> + if (start1 >= PAGE_SIZE) {
> + start1 -= PAGE_SIZE;
> + end1 -= PAGE_SIZE;
> + if (start2) {
> + start2 -= PAGE_SIZE;
> + end2 -= PAGE_SIZE;
> + }

You assume start2/end2 is always after start1/end1 in the page.
Is it always true? If so, I would add BUG_ON() for it.

Otherwise, looks good.

> + continue;
> + }
> +
> + kaddr = kmap_atomic(page + i);
> +
> + this_end = min_t(unsigned, end1, PAGE_SIZE);
> + if (end1 > start1)
> + memset(kaddr + start1, 0, this_end - start1);
> + end1 -= this_end;
> + start1 = 0;
> +
> + if (start2 >= PAGE_SIZE) {
> + start2 -= PAGE_SIZE;
> + end2 -= PAGE_SIZE;
> + } else {
> + this_end = min_t(unsigned, end2, PAGE_SIZE);
> + if (end2 > start2)
> + memset(kaddr + start2, 0, this_end - start2);
> + end2 -= this_end;
> + start2 = 0;
> + }
> +
> + kunmap_atomic(kaddr);
> + flush_dcache_page(page + i);
> +
> + if (!end1 && !end2)
> + break;
> + }
> +
> + BUG_ON((start1 | start2 | end1 | end2) != 0);
> }
>
> I think at this point it has to move out-of-line too.
>
> > > +static inline void zero_user_large(struct page *page,
> > > + unsigned start, unsigned size)
> > > +{
> > > + unsigned int i;
> > > +
> > > + for (i = 0; i < thp_order(page); i++) {
> > > + if (start > PAGE_SIZE) {
> >
> > Off-by-one? >= ?
>
> Good catch; I'd also noticed that when I came to redo the zero_user_segments().
>

--
Kirill A. Shutemov