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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