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

[Muchun Song]

On Mon, Jan 24, 2022 at 3:34 PM Christoph Hellwig  wrote:
>
> On Fri, Jan 21, 2022 at 03:55:11PM +0800, Muchun Song wrote:
> >  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);
>
> Do we need a flush_cache_folio here given that we must be dealing with
> what effectively is a folio here?

I think it is a future improvement. I suspect it will be easy if
someone wants to backport this patch. If we do not want
someone to do this, I think it is better to introduce
flush_cache_folio in this patch. What do you think?

>
> Also please avoid the overly long line.
>

OK.

Thanks.

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

[Christoph Hellwig]

On Fri, Jan 21, 2022 at 03:55:11PM +0800, Muchun Song wrote:
>  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);

Do we need a flush_cache_folio here given that we must be dealing with
what effectively is a folio here?

Also please avoid the overly long line.

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

[Yang Shi]

On Thu, Jan 20, 2022 at 11:56 PM Muchun Song  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


>
> Fixes: f27176cfc363 ("mm: convert page_mkclean_one() to use 
> page_vma_mapped_walk()")
> Signed-off-by: Muchun Song 
> ---
>  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
>