Re: [PATCH] mm: move buffer_heads_over_limit check up

[Hugh Dickins <hughd@google.com>]

On Fri, 17 Feb 2012, Andrew Morton wrote:
> 
> I don't think there's a lot of point in trying to micro-optimise the
> buffer_heads_over_limit==true case, either.  But I suppose that
> pointlessly locking 1000000 anon pages is indeed pointless.  Hopefully
> there _is_ a point in micro-optimising the actual test for
> buffer_heads_over_limit==true.  So...

That's fine, yes, and thanks for putting this in.  I just want to
exonerate Mel: it's not a fix to his useful work on buffer_heads_over_limit,
but I don't think he'll mind terribly that you've named it thus.

If it's a fix to anything, it's to my 3.3 free_hot_cold_page_list-ification
of shrink_active_list(), which was silly to leave the buffer_heads business
down there in move_active_pages_to_lru().

And the only reason I'm concerned to get it in, is that it's in an area
which I then trample over in the per-memcg per-zone locking series (as
is Hillf's rearrangement around update_isolated_counts()), so it's
convenient for me to have both of those in the base, instead of
having to put them in a prologue.

If I were to worry about the buffer_heads_over_limit situation itself,
I might worry about the unevictable pages which we never scan.

Hugh

> 
> --- a/mm/vmscan.c~mm-vmscan-forcibly-scan-highmem-if-there-are-too-many-buffer_heads-pinning-highmem-fix-fix
> +++ a/mm/vmscan.c
> @@ -1723,11 +1723,12 @@ static void shrink_active_list(unsigned 
>  			continue;
>  		}
>  
> -		if (buffer_heads_over_limit &&
> -		    page_has_private(page) && trylock_page(page)) {
> -			if (page_has_private(page))
> -				try_to_release_page(page, 0);
> -			unlock_page(page);
> +		if (unlikely(buffer_heads_over_limit)) {
> +			if (page_has_private(page) && trylock_page(page)) {
> +				if (page_has_private(page))
> +					try_to_release_page(page, 0);
> +				unlock_page(page);
> +			}
>  		}
>  
>  		if (page_referenced(page, 0, mz->mem_cgroup, &vm_flags)) {
> _

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