linux-kernel - Re: [PATCH v8 03/12] mm/bootmem_info: Introduce free_bootmem

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Date:   Thu, 10 Dec 2020 15:15:47 +0100
From:   Oscar Salvador <osalvador@...e.de>
To:     Muchun Song <songmuchun@...edance.com>
Cc:     corbet@....net, mike.kravetz@...cle.com, tglx@...utronix.de,
        mingo@...hat.com, bp@...en8.de, x86@...nel.org, hpa@...or.com,
        dave.hansen@...ux.intel.com, luto@...nel.org, peterz@...radead.org,
        viro@...iv.linux.org.uk, akpm@...ux-foundation.org,
        paulmck@...nel.org, mchehab+huawei@...nel.org,
        pawan.kumar.gupta@...ux.intel.com, rdunlap@...radead.org,
        oneukum@...e.com, anshuman.khandual@....com, jroedel@...e.de,
        almasrymina@...gle.com, rientjes@...gle.com, willy@...radead.org,
        mhocko@...e.com, song.bao.hua@...ilicon.com, david@...hat.com,
        duanxiongchun@...edance.com, linux-doc@...r.kernel.org,
        linux-kernel@...r.kernel.org, linux-mm@...ck.org,
        linux-fsdevel@...r.kernel.org
Subject: Re: [PATCH v8 03/12] mm/bootmem_info: Introduce free_bootmem_page
 helper

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@...edance.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