Re: [PATCH 11/22] mm, compaction: Use free lists to quickly locate a migration target

[Vlastimil Babka]

On 2/1/19 3:51 PM, Mel Gorman wrote:
> On Thu, Jan 31, 2019 at 03:52:10PM +0100, Vlastimil Babka wrote:
>>> -/* Reorder the free list to reduce repeated future searches */
>>> +/*
>>> + * Used when scanning for a suitable migration target which scans freelists
>>> + * in reverse. Reorders the list such as the unscanned pages are scanned
>>> + * first on the next iteration of the free scanner
>>> + */
>>> +static void
>>> +move_freelist_head(struct list_head *freelist, struct page *freepage)
>>> +{
>>> +	LIST_HEAD(sublist);
>>> +
>>> +	if (!list_is_last(freelist, &freepage->lru)) {
>>
>> Shouldn't there be list_is_first() for symmetry?
>>
> 
> I don't think it would help. We're reverse traversing the list when this is
> called. If it's the last entry, it's moving just one page before breaking
> off the search and a shuffle has minimal impact. If it's the first page
> then list_cut_before moves the entire list to sublist before splicing it
> back so it's a pointless operation.

Yeah I thought the goal was to avoid the pointless operation, which is
why it was previously added as "if (!list_is_last())" in
move_freelist_head(). So in move_freelist_head() it would have to be as
"if (!list_is_first())" to achieve the same effect. Agree that it's
marginal but if that's so then I would just remove the checks completely
(from both functions) instead of having it subtly wrong in one of them?