Re: [PATCH] btrfs: use helper to zero end of last page

[David Sterba <dsterba@suse.cz>]

On Tue, Dec 17, 2019 at 05:31:14PM +0800, Anand Jain wrote:
> > -		if (page->index == end_index) {
> > -			char *userpage;
> > -			size_t zero_offset = offset_in_page(isize);
> > -
> > -			if (zero_offset) {
> > -				int zeros;
> > -				zeros = PAGE_SIZE - zero_offset;
> > -				userpage = kmap_atomic(page);
> > -				memset(userpage + zero_offset, 0, zeros);
> > -				flush_dcache_page(page);
> > -				kunmap_atomic(userpage);
> > -			}
> > -		}
> > +		if (page->index == end_index)
> > +			zero_user_segment(page, offset_in_page(isize), PAGE_SIZE);
> 
>   Before we zero-ed only when the page offset is not starting at 0.
>   Can you confirm and update the change log if that is ok.

If the page offset is 0 then this would mean the whole page will be
zeroed, but we can have entire page within i_size so that would
mistakenly clear the last page. So the check is still needed.

While reading the code around the index calculations for the last page I
also found some oddities and potential bugs so I'll drop this patch for
now so I can look at the bugs first.