Re: [PATCH]swap-zero-page-bug

[Hugh Dickins]

On Wed, 29 Oct 2008, Li Xiaodong wrote:

> According to Documentation/filesystems/proc.txt, kernel should swap one page
> as vm.page-cluster is 0, while it seems not like so in mm/swapfile.c:
>      ...
> 	int our_page_cluster = page_cluster;
> 	...
> 	if (!our_page_cluster)  /* no readahead */
>                 return 0;
> 	...
> 
> It is better to remove that checkpoint.

No, page_cluster 0 allows swapin one page at a time, but disallows
swapin readahead (better named readaround) i.e. disallows reading
more than the one page actually requested.

Compare with how valid_swaphandles() is used from swapin_readahead():
there's actually one extra call to read_swap_cache_async() - because
we must ignore failure on the readaround but not on the page requested.

That "return 0" avoids the overhead of a pointlessly doubled
read_swap_cache_async() when there's only one page to be read.

Your patch might be needed if you'd found that swapin of one page
was not working at all - but you'd be making a stronger case if
you'd found that, I think!

Hugh
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