linux-kernel - Re: [PATCH v2] mm: fix accounting of memmap pages for early sections

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Message-ID: <aJNOs0IB1d33GSSW@li-2b55cdcc-350b-11b2-a85c-a78bff51fc11.ibm.com>
Date: Wed, 6 Aug 2025 14:46:43 +0200
From: Sumanth Korikkar <sumanthk@...ux.ibm.com>
To: Wei Yang <richard.weiyang@...il.com>
Cc: Andrew Morton <akpm@...ux-foundation.org>, linux-mm <linux-mm@...ck.org>,
        LKML <linux-kernel@...r.kernel.org>,
        David Hildenbrand <david@...hat.com>,
        Gerald Schaefer <gerald.schaefer@...ux.ibm.com>,
        Heiko Carstens <hca@...ux.ibm.com>, Vasily Gorbik <gor@...ux.ibm.com>,
        Alexander Gordeev <agordeev@...ux.ibm.com>,
        linux-s390 <linux-s390@...r.kernel.org>
Subject: Re: [PATCH v2] mm: fix accounting of memmap pages for early sections

> The change here is reasonable. While maybe we still miss the counting at some
> other points.
> 
> For example:
> 
> a. 
> 
>   sparse_init_nid()
>     __populate_section_memmap()
> 
> If !CONFIG_SPARSEMEM_VMEMMAP, and sparse_buffer_alloc() return NULL, it
> allocate extra memory from bootmem, which looks not counted.

Currently, the accounting is done upfront in sparse_buffer_init(), where
memmap_boot_pages_add() is called for !CONFIG_SPARSEMEM_VMEMMAP.

The function sparse_buffer_alloc() can return NULL in two scenarios:

* During sparse_buffer_init(), if memmap_alloc() fails, sparsemap_buf will be NULL.
* Inside sparse_buffer_alloc(), if ptr + size exceeds sparsemap_buf_end,
  then ptr is set to NULL.

Considering this, perhaps memmap_boot_pages_add() could be moved into
__populate_section_memmap(), with the accounting done only if the
operation is successful. What do you think?

>   section_activate()
>     populate_section_memmap()
> 
> If !CONFIG_SPARSEMEM_VMEMMAP, it just call kvmalloc_node(), which looks not
> counted.

Sounds right. This means nr_memmap_pages adjustment is needed for
!CONFIG_SPARSEMEM_VMEMMAP here. I will recheck this.

Thank you