linux-kernel - Re: [PATCH v2 1/8] mm: migrate: remove PageTransHuge check in numamigrate_isolate

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Message-ID: <ff38c641-5b15-4b73-a4bb-9aad1a53d7c9@huawei.com>
Date:   Fri, 25 Aug 2023 11:51:34 +0800
From:   Kefeng Wang <wangkefeng.wang@...wei.com>
To:     Matthew Wilcox <willy@...radead.org>
CC:     Andrew Morton <akpm@...ux-foundation.org>, <linux-mm@...ck.org>,
        <linux-kernel@...r.kernel.org>, <ying.huang@...el.com>,
        <david@...hat.com>, Zi Yan <ziy@...dia.com>,
        Mike Kravetz <mike.kravetz@...cle.com>, <hughd@...gle.com>
Subject: Re: [PATCH v2 1/8] mm: migrate: remove PageTransHuge check in
 numamigrate_isolate_page()



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