Re: [PATCH v8 03/12] mm/bootmem_info: Introduce free_bootmem_page helper

From: Oscar Salvador <osalvador@suse.de>
To: Muchun Song <songmuchun@bytedance.com>
Cc: corbet@lwn.net, mike.kravetz@oracle.com, tglx@linutronix.de,
	mingo@redhat.com, bp@alien8.de, x86@kernel.org, hpa@zytor.com,
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	peterz@infradead.org, viro@zeniv.linux.org.uk,
	akpm@linux-foundation.org, paulmck@kernel.org,
	mchehab+huawei@kernel.org, pawan.kumar.gupta@linux.intel.com,
	rdunlap@infradead.org, oneukum@suse.com,
	anshuman.khandual@arm.com, jroedel@suse.de,
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	mhocko@suse.com, song.bao.hua@hisilicon.com, david@redhat.com,
	duanxiongchun@bytedance.com, linux-doc@vger.kernel.org,
	linux-kernel@vger.kernel.org, linux-mm@kvack.org,
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Subject: Re: [PATCH v8 03/12] mm/bootmem_info: Introduce free_bootmem_page helper
Date: Thu, 10 Dec 2020 15:15:47 +0100	[thread overview]
Message-ID: <20201210141547.GA8538@localhost.localdomain> (raw)
In-Reply-To: <20201210035526.38938-4-songmuchun@bytedance.com>

On Thu, Dec 10, 2020 at 11:55:17AM +0800, Muchun Song wrote:
> Any memory allocated via the memblock allocator and not via the buddy
> will be makred reserved already in the memmap. For those pages, we can
         marked
> call free_bootmem_page() to free it to buddy allocator.
> 
> Becasue we wan to free some vmemmap pages of the HugeTLB to the buddy
Because     want
> allocator, we can use this helper to do that in the later patchs.
                                                           patches

To be honest, I think if would be best to introduce this along with
patch#4, so we get to see where it gets used.

> Signed-off-by: Muchun Song <songmuchun@bytedance.com>
> ---
>  include/linux/bootmem_info.h | 19 +++++++++++++++++++
>  1 file changed, 19 insertions(+)
> 
> diff --git a/include/linux/bootmem_info.h b/include/linux/bootmem_info.h
> index 4ed6dee1adc9..20a8b0df0c39 100644
> --- a/include/linux/bootmem_info.h
> +++ b/include/linux/bootmem_info.h
> @@ -3,6 +3,7 @@
>  #define __LINUX_BOOTMEM_INFO_H
>  
>  #include <linux/mmzone.h>
> +#include <linux/mm.h>

<linux/mm.h> already includes <linux/mmzone.h>

> +static inline void free_bootmem_page(struct page *page)
> +{
> +	unsigned long magic = (unsigned long)page->freelist;
> +
> +	/* bootmem page has reserved flag in the reserve_bootmem_region */
reserve_bootmem_region sets the reserved flag on bootmem pages?

> +	VM_WARN_ON(!PageReserved(page) || page_ref_count(page) != 2);

We do check for PageReserved in patch#4 before calling in here.
Do we need yet another check here? IOW, do we need to be this paranoid?

> +	if (magic == SECTION_INFO || magic == MIX_SECTION_INFO)
> +		put_page_bootmem(page);
> +	else
> +		WARN_ON(1);

Lately, some people have been complaining about using WARN_ON as some
systems come with panic_on_warn set.

I would say that in this case it does not matter much as if the vmemmap
pages are not either SECTION_INFO or MIX_SECTION_INFO it means that a
larger corruption happened elsewhere.

But I think I would align the checks here.
It does not make sense to me to only scream under DEBUG_VM if page's
refcount differs from 2, and have a WARN_ON if the page we are trying
to free was not used for the memmap array.
Both things imply a corruption, so I would set the checks under the same
configurations.

-- 
Oscar Salvador
SUSE L3