Re: [PATCH v5] mm: Proactive compaction

On Thu, May 28, 2020 at 2:50 AM Vlastimil Babka [off-list ref] wrote:

On 5/28/20 11:15 AM, Holger Hoffstätte wrote:

quoted

On 5/18/20 8:14 PM, Nitin Gupta wrote:
[patch v5 :)]

I've been successfully using this in my tree and it works great, but a friend
who also uses my tree just found a bug (actually an improvement ;) due to the
change from HUGETLB_PAGE_ORDER to HPAGE_PMD_ORDER in v5.

When building with CONFIG_TRANSPARENT_HUGEPAGE=n (for some reason it was off)
HPAGE_PMD_SHIFT expands to BUILD_BUG() and compilation fails like this:

Oops, I forgot about this. Still I believe HPAGE_PMD_ORDER is the best choice as
long as THP's are enabled. I guess fallback to HUGETLB_PAGE_ORDER would be
possible if THPS are not enabled, but AFAICS some architectures don't define
that. Such architectures perhaps won't benefit from proactive compaction anyway?

I am not sure about such architectures but in such cases, we would end
up calculating
"fragmentation score" based on a page size which does not match the
architecture's
view of the "default hugepage size" which is not a terrible thing in
itself as compaction
can still be done in the background, after all.

Since we always need a target order to calculate the fragmentation score, how
about this fallack scheme:

HPAGE_PMD_ORDER -> HUGETLB_PAGE_ORDER -> PMD_ORDER

Thanks,
Nitin