Re: [PATCH] compaction: fix isolate_migratepages_block() for THP=n

From: Kirill A. Shutemov
Date: Tue Apr 28 2015 - 18:28:45 EST

On Tue, Apr 28, 2015 at 03:14:20PM -0700, Andrew Morton wrote:
> On Mon, 27 Apr 2015 14:26:46 +0300 "Kirill A. Shutemov" <kirill.shutemov@xxxxxxxxxxxxxxx> wrote:
> > PageTrans* helpers are always-false if THP is disabled compile-time.
> > It means the fucntion will fail to detect hugetlb pages in this case.
> >
> > Let's use PageCompound() instead. With small tweak to how we calculate
> > next low_pfn it will make function ready to see tail pages.
> <scratches head>
> So this patch has no runtime effects at present? It is preparation for
> something else?

I wrote this to fix bug I originally attributed to refcounting patchset,
but Sasha triggered the same bug on -next without the patchset applied:

Now I think it's related to changing of PageLRU() behaviour on tail page
by my page flags patchset. PageLRU() on tail pages now reports true if
head page is on LRU. It means no we can go futher insede
isolate_migratepages_block() with tail page.

Kirill A. Shutemov
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