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Date:   Fri, 14 Feb 2020 08:03:42 -0800
From:   Matthew Wilcox <willy@...radead.org>
To:     "Kirill A. Shutemov" <kirill@...temov.name>
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 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;
+                       }
+                       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().

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