linux-kernel - Re: [PATCHv4 20/39] thp, mm: naive support of thp in generic read/write routines

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Date:	Tue, 21 May 2013 14:28:13 -0700
From:	Dave Hansen <dave@...1.net>
To:	"Kirill A. Shutemov" <kirill.shutemov@...ux.intel.com>
CC:	Andrea Arcangeli <aarcange@...hat.com>,
	Andrew Morton <akpm@...ux-foundation.org>,
	Al Viro <viro@...iv.linux.org.uk>,
	Hugh Dickins <hughd@...gle.com>,
	Wu Fengguang <fengguang.wu@...el.com>, Jan Kara <jack@...e.cz>,
	Mel Gorman <mgorman@...e.de>, linux-mm@...ck.org,
	Andi Kleen <ak@...ux.intel.com>,
	Matthew Wilcox <matthew.r.wilcox@...el.com>,
	"Kirill A. Shutemov" <kirill@...temov.name>,
	Hillf Danton <dhillf@...il.com>, linux-fsdevel@...r.kernel.org,
	linux-kernel@...r.kernel.org
Subject: Re: [PATCHv4 20/39] thp, mm: naive support of thp in generic read/write
 routines

On 05/11/2013 06:23 PM, Kirill A. Shutemov wrote:
> +		if (PageTransHuge(page))
> +			offset = pos & ~HPAGE_PMD_MASK;
> +
>  		pagefault_disable();
> -		copied = iov_iter_copy_from_user_atomic(page, i, offset, bytes);
> +		copied = iov_iter_copy_from_user_atomic(
> +				page + (offset >> PAGE_CACHE_SHIFT),
> +				i, offset & ~PAGE_CACHE_MASK, bytes);
>  		pagefault_enable();
>  		flush_dcache_page(page);

I think there's enough voodoo in there to warrant a comment or adding
some temporary variables.  There are three things going on that you wan
to convey:

1. Offset is normally <PAGE_SIZE, but you make it <HPAGE_PMD_SIZE if
   you are dealing with a huge page
2. (offset >> PAGE_CACHE_SHIFT) is always 0 for small pages since
    offset < PAGE_SIZE
3. "offset & ~PAGE_CACHE_MASK" does nothing for small-page offsets, but
   it turns a large-page offset back in to a small-page-offset.

I think you can do it with something like this:

 	int subpage_nr = 0;
	off_t smallpage_offset = offset;
	if (PageTransHuge(page)) {
		// we transform 'offset' to be offset in to the huge
		// page instead of inside the PAGE_SIZE page
		offset = pos & ~HPAGE_PMD_MASK;
		subpage_nr = (offset >> PAGE_CACHE_SHIFT);
	}
	
> +		copied = iov_iter_copy_from_user_atomic(
> +				page + subpage_nr,
> +				i, smallpage_offset, bytes);


> @@ -2437,6 +2453,7 @@ again:
>  			 * because not all segments in the iov can be copied at
>  			 * once without a pagefault.
>  			 */
> +			offset = pos & ~PAGE_CACHE_MASK;

Urg, and now it's *BACK* in to a small-page offset?

This means that 'offset' has two _different_ meanings and it morphs
between them during the function a couple of times.  That seems very
error-prone to me.

>  			bytes = min_t(unsigned long, PAGE_CACHE_SIZE - offset,
>  						iov_iter_single_seg_count(i));
>  			goto again;
> 

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