linux-kernel - Re: [PATCH v2 13/25] fs: Add zero_user

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Message-ID: <20200218141634.zhhjgtv44ux23l3l@box>
Date:   Tue, 18 Feb 2020 17:16:34 +0300
From:   "Kirill A. Shutemov" <kirill@...temov.name>
To:     Matthew Wilcox <willy@...radead.org>
Cc:     linux-fsdevel@...r.kernel.org, linux-mm@...ck.org,
        linux-kernel@...r.kernel.org
Subject: Re: [PATCH v2 13/25] fs: Add zero_user_large

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@...radead.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