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Howlett" , Vlastimil Babka , Suren Baghdasaryan , Michal Hocko , Nicholas Piggin , Christophe Leroy , aneesh.kumar@linux.ibm.com, joao.m.martins@oracle.com, linux-mm@kvack.org, linuxppc-dev@lists.ozlabs.org, linux-kernel@vger.kernel.org Subject: Re: [PATCH 01/49] mm/sparse: fix vmemmap accounting imbalance on memory hotplug error Message-ID: References: <20260405125240.2558577-1-songmuchun@bytedance.com> <20260405125240.2558577-2-songmuchun@bytedance.com> X-Mailing-List: linuxppc-dev@lists.ozlabs.org List-Id: List-Help: List-Owner: List-Post: List-Archive: , List-Subscribe: , , List-Unsubscribe: Precedence: list MIME-Version: 1.0 Content-Type: text/plain; charset=us-ascii Content-Disposition: inline In-Reply-To: <20260405125240.2558577-2-songmuchun@bytedance.com> On Sun, Apr 05, 2026 at 08:51:52PM +0800, Muchun Song wrote: > In section_activate(), if populate_section_memmap() fails, the error > handling path calls section_deactivate() to roll back the state. This > approach introduces an accounting imbalance. > > Since the commit c3576889d87b ("mm: fix accounting of memmap pages"), > memmap pages are accounted for only after populate_section_memmap() > succeeds. However, section_deactivate() unconditionally decrements the > vmemmap account. Consequently, a failure in populate_section_memmap() > leads to a negative offset (underflow) in the system's vmemmap tracking. > > We can fix this by ensuring that the vmemmap accounting is incremented > immediately before checking for the success of populate_section_memmap(). > If populate_section_memmap() fails, the subsequent call to > section_deactivate() will decrement the accounting, perfectly offsetting > the increment and maintaining balance. > > Fixes: c3576889d87b ("mm: fix accounting of memmap pages") > Signed-off-by: Muchun Song > --- > mm/sparse-vmemmap.c | 2 +- > 1 file changed, 1 insertion(+), 1 deletion(-) > > diff --git a/mm/sparse-vmemmap.c b/mm/sparse-vmemmap.c > index 6eadb9d116e4..ee27d0c0efe2 100644 > --- a/mm/sparse-vmemmap.c > +++ b/mm/sparse-vmemmap.c > @@ -822,11 +822,11 @@ static struct page * __meminit section_activate(int nid, unsigned long pfn, > return pfn_to_page(pfn); > > memmap = populate_section_memmap(pfn, nr_pages, nid, altmap, pgmap); > + memmap_pages_add(DIV_ROUND_UP(nr_pages * sizeof(struct page), PAGE_SIZE)); This logically belongs to success path in populate_section_memmap(). If we update the counter there, we won't need to temporarily increase it at all. > if (!memmap) { > section_deactivate(pfn, nr_pages, altmap); > return ERR_PTR(-ENOMEM); > } > - memmap_pages_add(DIV_ROUND_UP(nr_pages * sizeof(struct page), PAGE_SIZE)); > > return memmap; > } > -- > 2.20.1 > -- Sincerely yours, Mike.