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X-OriginatorOrg: arm.com X-MS-Exchange-CrossTenant-OriginalArrivalTime: 04 Jul 2025 05:06:13.9432 (UTC) X-MS-Exchange-CrossTenant-Network-Message-Id: b4f286d4-4203-47e5-477d-08ddbab87ffb X-MS-Exchange-CrossTenant-Id: f34e5979-57d9-4aaa-ad4d-b122a662184d X-MS-Exchange-CrossTenant-OriginalAttributedTenantConnectingIp: TenantId=f34e5979-57d9-4aaa-ad4d-b122a662184d;Ip=[4.158.2.129];Helo=[outbound-uk1.az.dlp.m.darktrace.com] X-MS-Exchange-CrossTenant-AuthSource: DB5PEPF00014B9F.eurprd02.prod.outlook.com X-MS-Exchange-CrossTenant-AuthAs: Anonymous X-MS-Exchange-CrossTenant-FromEntityHeader: HybridOnPrem X-MS-Exchange-Transport-CrossTenantHeadersStamped: DB8PR08MB5420 On 26/06/25 10:55 am, Dev Jain wrote: > arm64 disables vmalloc-huge when kernel page table dumping is enabled, > because an intermediate table may be removed, potentially causing the > ptdump code to dereference an invalid address. We want to be able to > analyze block vs page mappings for kernel mappings with ptdump, so to > enable vmalloc-huge with ptdump, synchronize between page table removal in > pmd_free_pte_page()/pud_free_pmd_page() and ptdump pagetable walking. We > use mmap_read_lock and not write lock because we don't need to synchronize > between two different vm_structs; two vmalloc objects running this same > code path will point to different page tables, hence there is no race. > > For pud_free_pmd_page(), we isolate the PMD table to avoid taking the lock > 512 times again via pmd_free_pte_page(). > > We implement the locking mechanism using static keys, since the chance > of a race is very small. Observe that the synchronization is needed > to avoid the following race: > > CPU1 CPU2 > take reference of PMD table > pud_clear() > pte_free_kernel() > walk freed PMD table > > and similar race between pmd_free_pte_page and ptdump_walk_pgd. > > Therefore, there are two cases: if ptdump sees the cleared PUD, then > we are safe. If not, then the patched-in read and write locks help us > avoid the race. > > To implement the mechanism, we need the static key access from mmu.c and > ptdump.c. Note that in case !CONFIG_PTDUMP_DEBUGFS, ptdump.o won't be a > target in the Makefile, therefore we cannot initialize the key there, as > is being done, for example, in the static key implementation of > hugetlb-vmemmap. Therefore, include asm/cpufeature.h, which includes > the jump_label mechanism. Declare the key there and define the key to false > in mmu.c. > > No issues were observed with mm-selftests. No issues were observed while > parallelly running test_vmalloc.sh and dumping the kernel pagetable through > sysfs in a loop. > > Reviewed-by: Ryan Roberts > Signed-off-by: Dev Jain > --- Sorry, I observed that post v3 I forgot to add the change wherein I drop the PTDUMP condition from arch_vmap_pxd_supported, I'll add that back in the next version. Thanks Anshuman for the spot.