Re: [PATCH v5] mm: Proactive compaction

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

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?

...
./include/linux/huge_mm.h:284:28: note: in expansion of macro ‘BUILD_BUG’
   284 | #define HPAGE_PMD_SHIFT ({ BUILD_BUG(); 0; })
       |                            ^~~~~~~~~
./include/linux/huge_mm.h:78:26: note: in expansion of macro ‘HPAGE_PMD_SHIFT’
    78 | #define HPAGE_PMD_ORDER (HPAGE_PMD_SHIFT-PAGE_SHIFT)
       |                          ^~~~~~~~~~~~~~~
mm/compaction.c:1874:28: note: in expansion of macro ‘HPAGE_PMD_ORDER’
  1874 |    extfrag_for_order(zone, HPAGE_PMD_ORDER);
       |                            ^~~~~~~~~~~~~~~
...

It would be great if the whole thing would compile without THP; the only
occurrence is in fragmentation_score_zone(). Unfortunately I'm not familiar
enough with how to properly check for THP and properly calculate whatever
you're doing there, otherwise I would ifdef this away myself. ;)

Thanks for an otherwise great patch!

cheers,
Holger