Re: [PATCH v2 1/8] mm: migrate: remove PageTransHuge check in numamigrate_isolate_page()

[Kefeng Wang]

On 2023/8/21 20:38, Matthew Wilcox wrote:
> On Mon, Aug 21, 2023 at 07:56:17PM +0800, Kefeng Wang wrote:
> > Since we begin to convert the numa migration code to use folio, which
> > could let us to handle arbitrary sizes of folio, so drop assert that
> > we only support PageTransHuge page(PMD size) when order > 0.
>
> Have you looked at the implementation of PageTransHuge()?  Your
> description doesn't match what the code does.

How about change to the following description，

The assert VM_BUG_ON_PAGE(order && !PageTransHuge(page), page) is
not very usefull,

   1) for a tail/base page, order = 0, for a head page, the order > 0 &&
      PageTransHuge() is true
   2) there is a PageCompound() check and only base page is handled in
      do_numa_page(), and do_huge_pmd_numa_page() only handle PMD-mapped
      THP
   3) even though the page is a tail page, isolate_lru_page() will post
      a warning, and fail to isolate the page.
   4) and if folio migrate is supported in the future, it is probable to
      migrate the entire folio if numa fault on a tail page

so just remove the check.

Thanks