linux-kernel - Re: [PATCH 1/5] mm: rmap: fix cache flush on THP pages

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Date:   Fri, 21 Jan 2022 10:05:51 -0800
From:   Yang Shi <shy828301@...il.com>
To:     Muchun Song <songmuchun@...edance.com>
Cc:     Dan Williams <dan.j.williams@...el.com>,
        Matthew Wilcox <willy@...radead.org>, Jan Kara <jack@...e.cz>,
        Alexander Viro <viro@...iv.linux.org.uk>,
        Andrew Morton <akpm@...ux-foundation.org>,
        Alistair Popple <apopple@...dia.com>,
        Ralph Campbell <rcampbell@...dia.com>,
        Hugh Dickins <hughd@...gle.com>, xiyuyang19@...an.edu.cn,
        "Kirill A. Shutemov" <kirill.shutemov@...ux.intel.com>,
        zwisler@...nel.org,
        Linux FS-devel Mailing List <linux-fsdevel@...r.kernel.org>,
        nvdimm@...ts.linux.dev,
        Linux Kernel Mailing List <linux-kernel@...r.kernel.org>,
        Linux MM <linux-mm@...ck.org>
Subject: Re: [PATCH 1/5] mm: rmap: fix cache flush on THP pages

On Thu, Jan 20, 2022 at 11:56 PM Muchun Song <songmuchun@...edance.com> wrote:
>
> The flush_cache_page() only remove a PAGE_SIZE sized range from the cache.
> However, it does not cover the full pages in a THP except a head page.
> Replace it with flush_cache_range() to fix this issue. At least, no
> problems were found due to this. Maybe because the architectures that
> have virtual indexed caches is less.

Yeah, actually flush_cache_page()/flush_cache_range() are no-op for
the most architectures which have THP supported, i.e. x86, aarch64,
powerpc, etc.

And currently just tmpfs and read-only files support PMD-mapped THP,
but both don't have to do writeback. And it seems DAX doesn't have
writeback either, which uses __set_page_dirty_no_writeback() for
set_page_dirty. So this code should never be called IIUC.

But anyway your fix looks correct to me. Reviewed-by: Yang Shi
<shy828301@...il.com>

>
> Fixes: f27176cfc363 ("mm: convert page_mkclean_one() to use page_vma_mapped_walk()")
> Signed-off-by: Muchun Song <songmuchun@...edance.com>
> ---
>  mm/rmap.c | 2 +-
>  1 file changed, 1 insertion(+), 1 deletion(-)
>
> diff --git a/mm/rmap.c b/mm/rmap.c
> index b0fd9dc19eba..65670cb805d6 100644
> --- a/mm/rmap.c
> +++ b/mm/rmap.c
> @@ -974,7 +974,7 @@ static bool page_mkclean_one(struct page *page, struct vm_area_struct *vma,
>                         if (!pmd_dirty(*pmd) && !pmd_write(*pmd))
>                                 continue;
>
> -                       flush_cache_page(vma, address, page_to_pfn(page));
> +                       flush_cache_range(vma, address, address + HPAGE_PMD_SIZE);
>                         entry = pmdp_invalidate(vma, address, pmd);
>                         entry = pmd_wrprotect(entry);
>                         entry = pmd_mkclean(entry);
> --
> 2.11.0
>