Re: [PATCH v1 1/3] /proc/kpageflags: return KPF_BUDDY for "tail" buddy pages

[Naoya Horiguchi]

On Thu, Feb 04, 2016 at 07:42:26PM +0300, Vladimir Davydov wrote:
> On Thu, Feb 04, 2016 at 04:08:01PM +0900, Naoya Horiguchi wrote:
> > Currently /proc/kpageflags returns nothing for "tail" buddy pages, which
> > is inconvenient when grasping how free pages are distributed. This patch
> > sets KPF_BUDDY for such pages.
> 
> Looks reasonable to me,
> 
> Reviewed-by: Vladimir Davydov <vdavydov@xxxxxxxxxxxxx>

Thank you.

> > 
> > With this patch:
> > 
> >   $ grep MemFree /proc/meminfo ; tools/vm/page-types -b buddy
> >   MemFree:         3134992 kB
> >                flags      page-count       MB  symbolic-flags                     long-symbolic-flags
> >   0x0000000000000400          779272     3044  __________B_______________________________ buddy
> >   0x0000000000000c00            4385       17  __________BM______________________________ buddy,mmap
> >                total          783657     3061
> 
> Why are buddy pages reported as mmapped? That looks weird. Shouldn't we
> fix it? Something like this, may be?
> 
> --
> From: Vladimir Davydov <vdavydov@xxxxxxxxxxxxx>
> Subject: [PATCH] proc: kpageflags: do not report buddy and balloon pages as
>  mapped
> 
> PageBuddy and PageBalloon are not usual page flags - they are identified
> by a special negative (so as not to confuse with mapped pages) value of
> page->_mapcount. Since /proc/kpageflags uses page_mapcount helper to
> check if a page is mapped, it reports pages of these kinds as being
> mapped, which is confusing. Fix that by replacing page_mapcount with
> page_mapped.
> 
> Signed-off-by: Vladimir Davydov <vdavydov@xxxxxxxxxxxxx>
> 
> diff --git a/fs/proc/page.c b/fs/proc/page.c
> index b2855eea5405..332450d87ea4 100644
> --- a/fs/proc/page.c
> +++ b/fs/proc/page.c
> @@ -105,7 +105,7 @@ u64 stable_page_flags(struct page *page)
>  	 * Note that page->_mapcount is overloaded in SLOB/SLUB/SLQB, so the
>  	 * simple test in page_mapcount() is not enough.

We can do s/page_mapcount/page_mapped/ for this line too.

>  	 */
> -	if (!PageSlab(page) && page_mapcount(page))
> +	if (!PageSlab(page) && page_mapped(page))

Look good to me.

Acked-by: Naoya Horiguchi <n-horiguchi@xxxxxxxxxxxxx>

>  		u |= 1 << KPF_MMAP;
>  	if (PageAnon(page))
>  		u |= 1 << KPF_ANON;