[Devel] [PATCH hci-8.1 v2 1/2] mm/memcontrol: allocate numa_migrate targets of the source folio order

[Konstantin Khorenko <khorenko@virtuozzo.com>]

memcg_numa_migrate_new_folio() treats every large folio as a PMD-sized
THP and allocates an HPAGE_PMD_ORDER destination for it. The branch is
the 4.18 pattern of new_page() in mm/mempolicy.c: a compound page on
the LRU could only be a THP then, so PageTransHuge() implied
HPAGE_PMD_ORDER.

Today the page cache holds large folios of any order (NFS sizes a folio
to the write, XFS and shmem have them too) and anonymous memory has
mTHP, while folio_test_large() is still just folio_test_head() and is
true for any order from 1 up. migrate_pages() never checks that the
destination order matches the source.

For an order-1 or order-4 file folio the destination is an order-9
folio that inherits the source index and page cache reference count,
so every later page cache operation that sizes the folio by its order
goes wrong: nrpages underflows, neighbouring xarray slots get wiped,
the folio can never be freed.

On a two-node host a VM migration to another NUMA node along with a
backup to NFS ends up with a negative nrpages, and closing the backup
file dies in nfs4_evict_inode():

  kernel BUG at fs/inode.c:737!
  RIP: 0010:clear_inode+0x75/0x80
  Call Trace:
   nfs4_evict_inode+0x21/0x80 [nfsv4]
   evict+0x115/0x2a0
   nfs4_delegreturn_release+0x55/0xa0 [nfsv4]
   rpc_free_task+0x39/0x60 [sunrpc]
   __rpc_execute+0x3d1/0x3e0 [sunrpc]

A debug build dies earlier, on the first migration of a large folio of
any file system, in mem_cgroup_migrate():

  VM_BUG_ON_FOLIO(folio_nr_pages(old) != folio_nr_pages(new))

Make memcg_numa_migrate_new_folio() handle large folios properly.
The thp_migration_supported() check goes: migrate_pages() splits PMD
THPs itself when unsupported, other large folios migrate whole anyway.

Fixes: 9f8a754ad6c2 ("mm: memcontrol: add memory.numa_migrate file")
https://virtuozzo.atlassian.net/browse/VSTOR-151607

Feature: mm: interface to migrate memory between NUMA nodes upon userspace request
Reviewed-by: Pavel Tikhomirov <ptikhomirov@virtuozzo.com>
Reviewed-by: Vasileios Almpanis <vasileios.almpanis@virtuozzo.com>
Signed-off-by: Konstantin Khorenko <khorenko@virtuozzo.com>
---
 mm/memcontrol.c | 11 ++++++-----
 1 file changed, 6 insertions(+), 5 deletions(-)

diff --git a/mm/memcontrol.c b/mm/memcontrol.c
index 0734c0fa21d05..d8a87913ace37 100644
--- a/mm/memcontrol.c
+++ b/mm/memcontrol.c
@@ -3480,6 +3480,7 @@ static struct folio *memcg_numa_migrate_new_folio(struct folio *old_folio,
 {
 	struct memcg_numa_migrate_struct *ms = (void *)private;
 	gfp_t gfp_mask = GFP_HIGHUSER_MOVABLE | __GFP_NORETRY | __GFP_NOWARN;
+	unsigned int order = 0;
 
 	ms->current_node = next_node(ms->current_node, *ms->target_nodes);
 	if (ms->current_node >= MAX_NUMNODES) {
@@ -3487,13 +3488,13 @@ static struct folio *memcg_numa_migrate_new_folio(struct folio *old_folio,
 		VM_BUG_ON(ms->current_node >= MAX_NUMNODES);
 	}
 
-	if (thp_migration_supported() && folio_test_large(old_folio)) {
-		return __folio_alloc(GFP_TRANSHUGE_LIGHT | __GFP_THISNODE,
-				HPAGE_PMD_ORDER, ms->current_node,
-				ms->target_nodes);
+	if (folio_test_large(old_folio)) {
+		gfp_mask = GFP_TRANSHUGE_LIGHT | __GFP_THISNODE;
+		order = folio_order(old_folio);
 	}
 
-	return __folio_alloc(gfp_mask, 0, ms->current_node, ms->target_nodes);
+	return __folio_alloc(gfp_mask, order, ms->current_node,
+			     ms->target_nodes);
 }
 
 /*

-- 
2.43.0

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