Re: [PATCH v4 3/3] x86/vmemmap: Handle unpopulated sub-pmd ranges

From: Oscar Salvador <osalvador@suse.de>
To: Dave Hansen <dave.hansen@intel.com>
Cc: Andrew Morton <akpm@linux-foundation.org>,
	David Hildenbrand <david@redhat.com>,
	Dave Hansen <dave.hansen@linux.intel.com>,
	Andy Lutomirski <luto@kernel.org>,
	Peter Zijlstra <peterz@infradead.org>,
	Thomas Gleixner <tglx@linutronix.de>,
	Ingo Molnar <mingo@redhat.com>, Borislav Petkov <bp@alien8.de>,
	x86@kernel.org, "H . Peter Anvin" <hpa@zytor.com>,
	Michal Hocko <mhocko@kernel.org>,
	linux-mm@kvack.org, linux-kernel@vger.kernel.org
Subject: Re: [PATCH v4 3/3] x86/vmemmap: Handle unpopulated sub-pmd ranges
Date: Tue, 9 Mar 2021 09:25:35 +0100	[thread overview]
Message-ID: <YEcw/wXCQPHnUdOS@localhost.localdomain> (raw)
In-Reply-To: <20210308184330.GB25767@linux>

On Mon, Mar 08, 2021 at 07:43:30PM +0100, Oscar Salvador wrote:
> On Thu, Mar 04, 2021 at 09:02:36AM -0800, Dave Hansen wrote:
> > Also, logically, it would make a lot of sense if you can move the actual
> > PMD freeing logic in here.  That way, the caller is just saying, "unuse
> > this PMD region", and then this takes care of the rest.  As it stands,
> > it's a bit weird that the caller takes care of the freeing.
> 
> You mean to move the 
> 
>  free_hugepage_table(pmd_page(*pmd), altmap);
>  spin_lock(&init_mm.page_table_lock);
>  pmd_clear(pmd);
>  spin_unlock(&init_mm.page_table_lock);
> 
> block in there?
> 
> Well, from where I see it, it is more like:
> 
>  if (is_the_range_unused())
>   : if so, free everything
> 
> But I agree with you what it might make some sense to move it there.
> Since I do not feel strong about this, I will move it.

hi Dave,

So, after splitting this patch and re-shape it to address all the
feedback, I am still not sure about this one.
Honestly, I think the freeing logic would be better off kept in
remove_pmd_table.

The reason for me is that vmemmap_unuse_sub_pmd only 1) marks the range
to be removed as unused and 2) checks whether after that, the whole
PMD range is unused.

I think the confusion comes from the name.
"vmemmap_pmd_is_unused" might be a better fit?

What do you think? Do you feel strong about moving the log in there
regardless of the name?

-- 
Oscar Salvador
SUSE L3