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b=I6esq+r6jOKQTWD243iyapiiUo/D7oSzgNZ/yPfvQVkJrw7NKpAw8Vs2cdvEkHLC6 9dr16Mkre69dNZ5teA7l4qKieFv5QxHvRUlaChOihWHvk+TjTiFCxT73bZDCyV7H7O a4j33KKBPY1HT3D+sbkMiwesFyl9IgoC2Buu2YN9u8ML0bwEge4XZZpl3XbF8HsJJK TJplOrd06l0Txhk1PWSyHI3cTUTlkmePckZOWoNWrfneWt+PyNXDSz0zI++19X5DVQ TkowH7a/aVf4enaTfpg4PiM0geRWXjRJgAltc7sC2tUl7C8tJ+UwM2PE9S7ke0bgwV ycYdjkzKl5tAg== Message-ID: <8d50f94b-6d95-4967-aa1f-2c14333ffd85@kernel.org> Date: Tue, 28 Jul 2026 14:50:46 +0200 MIME-Version: 1.0 User-Agent: Mozilla Thunderbird Subject: Re: [PATCH v4] mm/migrate_device: Clear stale mapping after freeing swapcache To: Arvind Yadav , linux-mm@kvack.org, linux-kernel@vger.kernel.org Cc: akpm@linux-foundation.org, matthew.brost@intel.com, joshua.hahnjy@gmail.com, ziy@nvidia.com, rakie.kim@sk.com, byungchul@sk.com, gourry@gourry.net, ying.huang@linux.alibaba.com, apopple@nvidia.com, balbirs@nvidia.com References: <20260728062832.1107127-1-arvind.yadav@intel.com> From: "David Hildenbrand (Arm)" Content-Language: en-US Autocrypt: addr=david@kernel.org; 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charset=UTF-8 Content-Transfer-Encoding: 7bit X-Rspamd-Server: rspam10 X-Rspamd-Queue-Id: 2CD4D180009 X-Stat-Signature: km5or6ab1nu6x1de178s9rgkxqh3xrpd X-Rspam-User: X-HE-Tag: 1785243054-797297 X-HE-Meta: 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 gkJ1Cs8/ 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 Sender: owner-linux-mm@kvack.org Precedence: bulk X-Loop: owner-majordomo@kvack.org List-ID: List-Subscribe: List-Unsubscribe: On 7/28/26 08:28, Arvind Yadav wrote: > __migrate_device_pages() reads the folio mapping before calling > folio_free_swap(). When folio_free_swap() succeeds, the folio is removed > from the swap cache, but the saved mapping still points to swap_space. > > Passing the stale mapping to folio_migrate_mapping() makes it use the > mapped-folio path for a folio that is no longer in swapcache. It can > then operate on swap_space.i_pages with invalid reference accounting, > eventually triggering a folio reference count BUG. > > After a successful split, nr still contains the number of pages in the > original large folio, although each resulting page is now a separate > order-0 folio. Reset nr to 1 so each split folio is processed separately, > including its own swapcache removal and mapping lookup. > > Refresh the saved mapping after folio_free_swap() so the current folio > state is used during migration. > > v2: > - Refresh the mapping using folio_mapping(), as suggested by Zi Yan. > > v3: > - Reset nr to 1 after a successful split so each resulting folio is > processed independently, as suggested by Zi Yan. > > v4: > - Re-read each source folio's mapping immediately before > folio_migrate_mapping(), as suggested by Balbir Singh. > - Add a warning to validate the post-split order-0 invariant, > as suggested by Balbir Singh. > > Fixes: df263d9a7dff ("mm/migrate_device: try to handle swapcache pages") > Cc: Andrew Morton > Cc: David Hildenbrand > Cc: Matthew Brost > Cc: Joshua Hahn > Cc: Rakie Kim > Cc: Byungchul Park > Cc: Gregory Price > Cc: Ying Huang > Cc: Alistair Popple > Reviewed-by: Zi Yan > Reviewed-by: Balbir Singh > Signed-off-by: Arvind Yadav > --- > mm/migrate_device.c | 13 +++++++++++++ > 1 file changed, 13 insertions(+) > > diff --git a/mm/migrate_device.c b/mm/migrate_device.c > index 554754eb26ff..0e6f00190fd8 100644 > --- a/mm/migrate_device.c > +++ b/mm/migrate_device.c > @@ -1182,6 +1182,13 @@ static void __migrate_device_pages(unsigned long *src_pfns, > MIGRATE_PFN_COMPOUND); > goto next; > } > + > + /* > + * reset nr so that only first after-split folio > + * is processed below > + */ > + VM_WARN_ON_ONCE(folio_test_large(folio)); > + nr = 1; This function is surely a beauty. (had to rephrase that sentence 3 times ;) ) migrate_vma_split_unmapped_folio() modifies the src_pfns() entries on success. (and somehow assumes that it's always a THP, what? After a MIGRATE_PFN_COMPOUND value is set? What? Why the "nr = 1 << folio_order(folio);" in the caller). If we ended up modifying the current entry in such a way, shouldn't we just have retry: label and restart at the very top of the function, where we just naturally re-read the entry/page/folio and do the right thing? > } else if ((src_pfns[i] & MIGRATE_PFN_MIGRATE) && > (dst_pfns[i] & MIGRATE_PFN_COMPOUND) && > !(src_pfns[i] & MIGRATE_PFN_COMPOUND)) { > @@ -1221,6 +1228,12 @@ static void __migrate_device_pages(unsigned long *src_pfns, > folio = page_folio(migrate_pfn_to_page(src_pfns[i+j])); > newfolio = page_folio(migrate_pfn_to_page(dst_pfns[i+j])); > > + /* > + * folio_free_swap() removed the folio from the swap > + * cache. Refresh the saved mapping before migration. > + */ > + mapping = folio_mapping(folio); > + > r = folio_migrate_mapping(mapping, newfolio, folio, extra_cnt); > if (r) > src_pfns[i+j] &= ~MIGRATE_PFN_MIGRATE; While this looks good, I do wonder why do we have to supply the mapping here at all? Is there a path where we call folio_migrate_mapping() and the old folio (folio) does no longer have the right mapping attached? It would be a lot less error prone if the function would just obtain the mapping from the old folio. -- Cheers, David