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

Kirill A. Shutemov
2020-02-18T17:11:01Z

On Tue, Feb 18, 2020 at 08:13:49AM -0800, Matthew Wilcox wrote:
> On Tue, Feb 18, 2020 at 05:16:34PM +0300, Kirill A. Shutemov wrote:
> > > + 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.
>
> after or zero. Yes, I should add a BUG_ON to check for that.
>
> > Otherwise, looks good.
>
> Here's what I currently have (I'll add the BUG_ON later):
>
> commit 7fabe16755365cdc6e80343ef994843ecebde60a
> Author: Matthew Wilcox (Oracle) <willy@infradead.org>
> Date: Sat Feb 1 03:38:49 2020 -0500
>
> fs: Support THPs in zero_user_segments
>
> We can only kmap() one subpage of a THP at a time, so loop over all
> relevant subpages, skipping ones which don't need to be zeroed. This is
> too large to inline when THPs are enabled and we actually need highmem,
> so put it in highmem.c.
>
> Signed-off-by: Matthew Wilcox (Oracle) <willy@infradead.org>

Acked-by: Kirill A. Shutemov <kirill.shutemov@linux.intel.com>

>
> diff --git a/include/linux/highmem.h b/include/linux/highmem.h
> index ea5cdbd8c2c3..74614903619d 100644
> --- a/include/linux/highmem.h
> +++ b/include/linux/highmem.h
> @@ -215,13 +215,18 @@ static inline void clear_highpage(struct page *page)
> kunmap_atomic(kaddr);
> }
>
> +#if defined(CONFIG_HIGHMEM) && defined(CONFIG_TRANSPARENT_HUGEPAGE)
> +void zero_user_segments(struct page *page, unsigned start1, unsigned end1,
> + unsigned start2, unsigned end2);
> +#else /* !HIGHMEM || !TRANSPARENT_HUGEPAGE */

This is a neat trick. I like it.

Although, it means non-inlined version will never get tested :/

> static inline void zero_user_segments(struct page *page,
> - unsigned start1, unsigned end1,
> - unsigned start2, unsigned end2)
> + unsigned start1, unsigned end1,
> + unsigned start2, unsigned end2)
> {
> + unsigned long i;
> void *kaddr = kmap_atomic(page);
>
> - BUG_ON(end1 > PAGE_SIZE || end2 > PAGE_SIZE);
> + BUG_ON(end1 > thp_size(page) || end2 > thp_size(page));
>
> if (end1 > start1)
> memset(kaddr + start1, 0, end1 - start1);
> @@ -230,8 +235,10 @@ static inline void zero_user_segments(struct page *page,
> memset(kaddr + start2, 0, end2 - start2);
>
> kunmap_atomic(kaddr);
> - flush_dcache_page(page);
> + for (i = 0; i < hpage_nr_pages(page); i++)
> + flush_dcache_page(page + i);
> }
> +#endif /* !HIGHMEM || !TRANSPARENT_HUGEPAGE */
>
> static inline void zero_user_segment(struct page *page,
> unsigned start, unsigned end)
> diff --git a/mm/highmem.c b/mm/highmem.c
> index 64d8dea47dd1..3a85c66ef532 100644
> --- a/mm/highmem.c
> +++ b/mm/highmem.c
> @@ -367,9 +367,67 @@ void kunmap_high(struct page *page)
> if (need_wakeup)
> wake_up(pkmap_map_wait);
> }
> -
> EXPORT_SYMBOL(kunmap_high);
> -#endif
> +
> +#ifdef CONFIG_TRANSPARENT_HUGEPAGE
> +void zero_user_segments(struct page *page, unsigned start1, unsigned end1,
> + unsigned start2, unsigned end2)
> +{
> + 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;
> + }
> + 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);
> +}
> +EXPORT_SYMBOL(zero_user_segments);
> +#endif /* CONFIG_TRANSPARENT_HUGEPAGE */
> +#endif /* CONFIG_HIGHMEM */
>
> #if defined(HASHED_PAGE_VIRTUAL)
>
>
>
>
> > > + 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

--
Kirill A. Shutemov