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Howlett" , Vlastimil Babka , Suren Baghdasaryan , Michal Hocko , linux-kernel@vger.kernel.org Subject: Re: [PATCH v2 1/5] mm/sparse-vmemmap: provide generic vmemmap_set_pmd() and vmemmap_check_pmd() Message-ID: References: <20260404122105.3989557-1-songmuchun@bytedance.com> <20260404122105.3989557-2-songmuchun@bytedance.com> MIME-Version: 1.0 Content-Type: text/plain; charset=us-ascii Content-Disposition: inline In-Reply-To: X-Rspam-User: X-Stat-Signature: rcmphh14m18h9yzt1bf6zzrikhza6cmp X-Rspamd-Queue-Id: 1BF8518000D X-Rspamd-Server: rspam09 X-HE-Tag: 1775731457-983482 X-HE-Meta: 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 ILx/8aGU wdiuu Sender: owner-linux-mm@kvack.org Precedence: bulk X-Loop: owner-majordomo@kvack.org List-ID: List-Subscribe: List-Unsubscribe: On Sun, Apr 05, 2026 at 10:07:36PM +0800, Muchun Song wrote: > > On Apr 5, 2026, at 15:07, Mike Rapoport wrote: > > On Sat, Apr 04, 2026 at 08:20:54PM +0800, Muchun Song wrote: > >> The two weak functions are currently no-ops on every architecture, > >> forcing each platform that needs them to duplicate the same handful > >> of lines. Provide a generic implementation: > >> > >> - vmemmap_set_pmd() simply sets a huge PMD with PAGE_KERNEL protection. > >> > >> - vmemmap_check_pmd() verifies that the PMD is present and leaf, > >> then calls the existing vmemmap_verify() helper. > >> > >> Architectures that need special handling can continue to override the > >> weak symbols; everyone else gets the standard version for free. > >> > >> Signed-off-by: Muchun Song > >> --- > >> mm/sparse-vmemmap.c | 7 ++++++- > >> 1 file changed, 6 insertions(+), 1 deletion(-) > >> > >> diff --git a/mm/sparse-vmemmap.c b/mm/sparse-vmemmap.c > >> index 6eadb9d116e4..1eb990610d50 100644 > >> --- a/mm/sparse-vmemmap.c > >> +++ b/mm/sparse-vmemmap.c > >> @@ -391,12 +391,17 @@ int __meminit vmemmap_populate_hvo(unsigned long addr, unsigned long end, > >> void __weak __meminit vmemmap_set_pmd(pmd_t *pmd, void *p, int node, > >> unsigned long addr, unsigned long next) > >> { > >> + BUG_ON(!pmd_set_huge(pmd, virt_to_phys(p), PAGE_KERNEL)); > > > > Do we have to crash the kernel here? > > Wouldn't be better to make vmemmap_set_pmd() return error and make > > vmemmap_populate_hugepages() fall back to base pages in case > > vmemmap_set_pmd() errored? > > Hi Mike, > > Thanks for the review. Let me explain my original thought process here. > > My assumption was that pmd_set_huge() for the kernel virtual address space > should rarely, if ever, fail in this context. Furthermore, if we look at the > architectures this patch replaces (e.g., arm64 and riscv), they are either > ignoring the return value of pmd_set_huge() entirely or lacking any graceful > fallback mechanism anyway. > > So, to keep the initial generic implementation as simple as possible, I used > BUG_ON() as a strict assertion. Fair enough, let's just make it VM_BUG_ON(). -- Sincerely yours, Mike.