linux-kernel - Re: [PATCH v3 2/5] mm: fix missing cache flush for all tail pages of compound page

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Date:   Tue, 1 Feb 2022 15:32:54 -0800
From:   Mike Kravetz <mike.kravetz@...cle.com>
To:     Muchun Song <songmuchun@...edance.com>, akpm@...ux-foundation.org,
        zi.yan@...rutgers.edu, kirill.shutemov@...ux.intel.com,
        rientjes@...gle.com, lars.persson@...s.com
Cc:     linux-mm@...ck.org, linux-kernel@...r.kernel.org,
        duanxiongchun@...edance.com, Zi Yan <ziy@...dia.com>
Subject: Re: [PATCH v3 2/5] mm: fix missing cache flush for all tail pages of
 compound page

On 1/31/22 08:02, Muchun Song wrote:
> The D-cache maintenance inside move_to_new_page() only consider one page,
> there is still D-cache maintenance issue for tail pages of compound page
> (e.g. THP or HugeTLB).  THP migration is only enabled on x86_64, ARM64
> and powerpc, while powerpc and arm64 need to maintain the consistency
> between I-Cache and D-Cache, which depends on flush_dcache_page() to
> maintain the consistency between I-Cache and D-Cache.  In theory, the
> issue can be found on arm64 and powerpc.  HugeTLB migration is enabled
> on arm, arm64, mips, parisc, powerpc, riscv, s390 and sh, while arm
> has handled the compound page cache flush in flush_dcache_page(), but
> most others do not.  In theory, the issue exists on many architectures.
> Fix this by not using flush_dcache_folio() since it is not backportable.
> 
> Fixes: 616b8371539a ("mm: thp: enable thp migration in generic path")
> Fixes: 290408d4a250 ("hugetlb: hugepage migration core")
> Signed-off-by: Muchun Song <songmuchun@...edance.com>
> Reviewed-by: Zi Yan <ziy@...dia.com>
> ---
>  mm/migrate.c | 7 +++++--
>  1 file changed, 5 insertions(+), 2 deletions(-)
> 
> diff --git a/mm/migrate.c b/mm/migrate.c
> index c9296d63878d..c418e8d92b9c 100644
> --- a/mm/migrate.c
> +++ b/mm/migrate.c
> @@ -933,9 +933,12 @@ static int move_to_new_page(struct page *newpage, struct page *page,
>  		if (!PageMappingFlags(page))
>  			page->mapping = NULL;
>  
> -		if (likely(!is_zone_device_page(newpage)))
> -			flush_dcache_page(newpage);
> +		if (likely(!is_zone_device_page(newpage))) {
> +			int i, nr = compound_nr(newpage);
>  
> +			for (i = 0; i < nr; i++)
> +				flush_dcache_page(newpage + i);
> +		}

As you have already noted elsewhere, the arm64 version of flush_dcache_page
seems to handle this and flush the entire compound_page.  Is this going to
flush the entire compound page compound_nr times?

-- 
Mike Kravetz

>  	}
>  out:
>  	return rc;