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From: Usama Arif In-Reply-To: Content-Type: text/plain; charset=UTF-8 Content-Transfer-Encoding: 7bit X-Migadu-Flow: FLOW_OUT X-Rspamd-Server: rspam02 X-Rspamd-Queue-Id: F1FE514000B X-Stat-Signature: 1b8rc7871tkcx6j4e1hsbpdt6syh5w43 X-Rspam-User: X-HE-Tag: 1773083487-328048 X-HE-Meta: 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 naGCaoE2 2aF88xJ5vGUyrIr6VMVx8Mv9oZSc+QuxjX7ai7rOeucC72+JZoOWOxZArY2eDvFi66vl73orhUp1qXfNR5ONwQJnhcs5u0nzotChvCBQh3aQhrAXZTIaF4zOSFj9oN5ZSJoQktgWFv4P/D+e1PTMspUpRYViXQGosDn/2ONqXc1G5aPMSZytTn5CAHxVY2Ulebhm4Trp4pFLDl1TF/8YJs02q6E0jYhm8FSL5qN5f/GA0EBRXE6aE224NtzpoT7t4cVBVCitEbTJaptFKxadpQHLG3Cv4iDt23+dz44i92OKshKwb6lEU/kB6r+v+R0lJuyq9Fip96MWNgwY7JUvTntlDaFXwyDaW9EFz7UZb52EkF5S2J5jj05FnhzNugBB8O3xmtHvkJcS4H7T1939J0HMgSDcFXSubsI9c Sender: owner-linux-mm@kvack.org Precedence: bulk X-Loop: owner-majordomo@kvack.org List-ID: List-Subscribe: List-Unsubscribe: On 09/03/2026 18:18, David Hildenbrand (Arm) wrote: > On 3/6/26 11:44, Usama Arif wrote: >> split_huge_pmd_address() with freeze=true splits a PMD migration entry >> into PTE migration entries, consuming one folio reference in the >> process. The folio_get() before it provides this reference. >> >> Add a comment explaining this relationship and a VM_WARN_ON_ONCE to >> catch an unexpected refcount != 1 entry state. >> >> Suggested-by: Zi Yan >> Signed-off-by: Usama Arif >> --- >> mm/migrate_device.c | 5 +++++ >> 1 file changed, 5 insertions(+) >> >> diff --git a/mm/migrate_device.c b/mm/migrate_device.c >> index 78c7acf024615..6fa2878848a7e 100644 >> --- a/mm/migrate_device.c >> +++ b/mm/migrate_device.c >> @@ -908,6 +908,11 @@ static int migrate_vma_split_unmapped_folio(struct migrate_vma *migrate, >> unsigned long flags; >> int ret = 0; >> >> + VM_WARN_ON_ONCE(folio_ref_count(folio) != 1); > > Can't we have speculative references here? In general, asserting that > the refcount has an exact value (besides 0) is often shaky. I hope not at this point in code. At this point, the folio is locked and unmapped (both done in migrate_vma_collect_huge_pmd()), and the present PMD was set to migration entry. It is isolated from LRU in migrate_device_unmap(). So the folio should not be visible to GUP or reclaim/compaction. Only anon, non-swapcache folios should reach here. So it won't run into any folio_try_get in page cache or swap cache. The folio_get() done in migrate_vma_split_unmapped_folio() is consumed by split_huge_pmd_address(), and folio_split_unmapped() expects a folio_reference of 1 after this [1]. If its not considered good to assert a non zero refcount value, I can change the warning to a comment, but I think refcount should be 1 at this point, otherwise folio_split_unmapped will fail. [1] https://elixir.bootlin.com/linux/v6.19.6/source/mm/huge_memory.c#L4137 > >> + /* >> + * take a reference, since split_huge_pmd_address() with freeze = true >> + * drops a reference at the end. >> + */ >> folio_get(folio); >> split_huge_pmd_address(migrate->vma, addr, true); >> ret = folio_split_unmapped(folio, 0); > >