The patch titled Subject: mm: migrate: fix getting incorrect page mapping during page migration has been added to the -mm mm-unstable branch. Its filename is mm-migrate-fix-getting-incorrect-page-mapping-during-page-migration.patch

This patch will shortly appear at https://git.kernel.org/pub/scm/linux/kernel/git/akpm/25-new.git/tree/patches...

This patch will later appear in the mm-unstable branch at git://git.kernel.org/pub/scm/linux/kernel/git/akpm/mm

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------------------------------------------------------ From: Baolin Wang baolin.wang@linux.alibaba.com Subject: mm: migrate: fix getting incorrect page mapping during page migration Date: Fri, 15 Dec 2023 20:07:52 +0800

When running stress-ng testing, we found below kernel crash after a few hours:

Unable to handle kernel NULL pointer dereference at virtual address 0000000000000000 pc : dentry_name+0xd8/0x224 lr : pointer+0x22c/0x370 sp : ffff800025f134c0 ...... Call trace: dentry_name+0xd8/0x224 pointer+0x22c/0x370 vsnprintf+0x1ec/0x730 vscnprintf+0x2c/0x60 vprintk_store+0x70/0x234 vprintk_emit+0xe0/0x24c vprintk_default+0x3c/0x44 vprintk_func+0x84/0x2d0 printk+0x64/0x88 __dump_page+0x52c/0x530 dump_page+0x14/0x20 set_migratetype_isolate+0x110/0x224 start_isolate_page_range+0xc4/0x20c offline_pages+0x124/0x474 memory_block_offline+0x44/0xf4 memory_subsys_offline+0x3c/0x70 device_offline+0xf0/0x120 ......

After analyzing the vmcore, I found this issue is caused by page migration. The scenario is that, one thread is doing page migration, and we will use the target page's ->mapping field to save 'anon_vma' pointer between page unmap and page move, and now the target page is locked and refcount is 1.

Currently, there is another stress-ng thread performing memory hotplug, attempting to offline the target page that is being migrated. It discovers that the refcount of this target page is 1, preventing the offline operation, thus proceeding to dump the page. However, page_mapping() of the target page may return an incorrect file mapping to crash the system in dump_mapping(), since the target page->mapping only saves 'anon_vma' pointer without setting PAGE_MAPPING_ANON flag.

There are seveval ways to fix this issue: (1) Setting the PAGE_MAPPING_ANON flag for target page's ->mapping when saving 'anon_vma', but this can confuse PageAnon() for PFN walkers, since the target page has not built mappings yet. (2) Getting the page lock to call page_mapping() in __dump_page() to avoid crashing the system, however, there are still some PFN walkers that call page_mapping() without holding the page lock, such as compaction. (3) Using target page->private field to save the 'anon_vma' pointer and 2 bits page state, just as page->mapping records an anonymous page, which can remove the page_mapping() impact for PFN walkers and also seems a simple way.

So I choose option 3 to fix this issue, and this can also fix other potential issues for PFN walkers, such as compaction.

Link: https://lkml.kernel.org/r/e60b17a88afc38cb32f84c3e30837ec70b343d2b.170264170... Fixes: 64c8902ed441 ("migrate_pages: split unmap_and_move() to _unmap() and _move()") Signed-off-by: Baolin Wang baolin.wang@linux.alibaba.com Cc: Matthew Wilcox willy@infradead.org Cc: David Hildenbrand david@redhat.com Cc: "Huang, Ying" ying.huang@intel.com Cc: Xu Yu xuyu@linux.alibaba.com Cc: Zi Yan ziy@nvidia.com Cc: stable@vger.kernel.org Signed-off-by: Andrew Morton akpm@linux-foundation.org ---

mm/migrate.c | 27 ++++++++++----------------- 1 file changed, 10 insertions(+), 17 deletions(-)

--- a/mm/migrate.c~mm-migrate-fix-getting-incorrect-page-mapping-during-page-migration +++ a/mm/migrate.c @@ -1025,38 +1025,31 @@ out: }

/* - * To record some information during migration, we use some unused - * fields (mapping and private) of struct folio of the newly allocated - * destination folio. This is safe because nobody is using them - * except us. + * To record some information during migration, we use unused private + * field of struct folio of the newly allocated destination folio. + * This is safe because nobody is using it except us. */ -union migration_ptr { - struct anon_vma *anon_vma; - struct address_space *mapping; -}; - enum { PAGE_WAS_MAPPED = BIT(0), PAGE_WAS_MLOCKED = BIT(1), + PAGE_OLD_STATES = PAGE_WAS_MAPPED | PAGE_WAS_MLOCKED, };

static void __migrate_folio_record(struct folio *dst, - unsigned long old_page_state, + int old_page_state, struct anon_vma *anon_vma) { - union migration_ptr ptr = { .anon_vma = anon_vma }; - dst->mapping = ptr.mapping; - dst->private = (void *)old_page_state; + dst->private = (void *)anon_vma + old_page_state; }

static void __migrate_folio_extract(struct folio *dst, int *old_page_state, struct anon_vma **anon_vmap) { - union migration_ptr ptr = { .mapping = dst->mapping }; - *anon_vmap = ptr.anon_vma; - *old_page_state = (unsigned long)dst->private; - dst->mapping = NULL; + unsigned long private = (unsigned long)dst->private; + + *anon_vmap = (struct anon_vma *)(private & ~PAGE_OLD_STATES); + *old_page_state = private & PAGE_OLD_STATES; dst->private = NULL; }

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Patches currently in -mm which might be from baolin.wang@linux.alibaba.com are

mm-migrate-fix-getting-incorrect-page-mapping-during-page-migration.patch