Re: [PATCH v2 4/7] mm/compaction: simplify pfn iteration in isolate_freepages_range

[Mel Gorman <mgorman@techsingularity.net>]

On Sat, Aug 26, 2023 at 11:36:14PM +0800, Kemeng Shi wrote:
> We call isolate_freepages_block in strict mode, continuous pages in
> pageblock will be isolated if isolate_freepages_block successed.
> Then pfn + isolated will point to start of next pageblock to scan
> no matter how many pageblocks are isolated in isolate_freepages_block.
> Use pfn + isolated as start of next pageblock to scan to simplify the
> iteration.
> 
> The pfn + isolated always points to start of next pageblock as:
> In case isolated buddy page has order higher than pageblock:
> 1. page in buddy page is aligned with it's order
> 2. order of page is higher than pageblock order
> Then page is aligned with pageblock order. So pfn of page and isolated
> pages count are both aligned pageblock order. So pfn + isolated is
> pageblock order aligned.
> 
> In case isolated buddy page has order lower than pageblock:
> Buddy page with order N contains two order N - 1 pages as following:
> |        order N        |
> |order N - 1|order N - 1|
> So buddy pages with order N - 1 will never cross boudary of order N.
> Similar, buddy pages with order N - 2 will never cross boudary of order
> N - 1 and so on. Then any pages with order less than pageblock order
> will never crosa boudary of pageblock.
> 
> Signed-off-by: Kemeng Shi <shikemeng@huaweicloud.com>
> Reviewed-by: Baolin Wang <baolin.wang@linux.alibaba.com>

While I don't think the patch is wrong, I also don't think it
meaningfully simplifies the code or optimises enough to be justified.
Even though a branch is eliminated, the whole path is not cheap.

-- 
Mel Gorman
SUSE Labs