Btrfs currently tracks its metadata pages in the page cache, using a fake inode (fs_info->btree_inode) with offsets corresponding to where the metadata is stored in the filesystem's full logical address space. A consequence of this is that when btrfs uses filemap_add_folio(), this usage is charged to the cgroup of whichever task happens to be running at the time. These folios don't belong to any particular user cgroup, so I don't think it makes much sense for them to be charged in that way. Some negative consequences as a result: - A task can be holding some important btrfs locks, then need to lookup some metadata and go into reclaim, extending the duration it holds that lock for, and unfairly pushing its own reclaim pain onto other cgroups. - If that cgroup goes into reclaim, it might reclaim these folios a different non-reclaiming cgroup might need soon. This is naturally offset by LRU reclaim, but still. We have two options for how to manage such file pages: 1. charge them to the root cgroup. 2. don't charge them to any cgroup at all. 2. breaks the invariant that every mapped page has a cgroup. This is workable, but unnecessarily risky. Therefore, go with 1. A very similar proposal to use the root cgroup was previously made by Qu, where he eventually proposed the idea of setting it per address_space. This makes good sense for the btrfs use case, as the behavior should apply to all use of the address_space, not select allocations. I.e., if someone adds another filemap_add_folio() call using btrfs's btree_inode, we would almost certainly want to account that to the root cgroup as well. Link: https://lore.kernel.org/linux-mm/b5fef5372ae454a7b6da4f2f75c427aeab6a07d6.1727498749.git.wqu@xxxxxxxx/ Suggested-by: Qu Wenruo <wqu@xxxxxxxx> Suggested-by: Shakeel Butt <shakeel.butt@xxxxxxxxx> Tested-by: syzbot@xxxxxxxxxxxxxxxxxxxxxxxxx Signed-off-by: Boris Burkov <boris@xxxxxx> --- include/linux/pagemap.h | 2 ++ mm/filemap.c | 6 ++++++ 2 files changed, 8 insertions(+) diff --git a/include/linux/pagemap.h b/include/linux/pagemap.h index c9ba69e02e3e..a3e16d74792f 100644 --- a/include/linux/pagemap.h +++ b/include/linux/pagemap.h @@ -211,6 +211,8 @@ enum mapping_flags { folio contents */ AS_INACCESSIBLE = 8, /* Do not attempt direct R/W access to the mapping */ AS_WRITEBACK_MAY_DEADLOCK_ON_RECLAIM = 9, + AS_KERNEL_FILE = 10, /* mapping for a fake kernel file that shouldn't + account usage to user cgroups */ /* Bits 16-25 are used for FOLIO_ORDER */ AS_FOLIO_ORDER_BITS = 5, AS_FOLIO_ORDER_MIN = 16, diff --git a/mm/filemap.c b/mm/filemap.c index e4a5a46db89b..05c1384bd611 100644 --- a/mm/filemap.c +++ b/mm/filemap.c @@ -960,8 +960,14 @@ int filemap_add_folio(struct address_space *mapping, struct folio *folio, { void *shadow = NULL; int ret; + struct mem_cgroup *tmp; + bool kernel_file = test_bit(AS_KERNEL_FILE, &mapping->flags); + if (kernel_file) + tmp = set_active_memcg(root_mem_cgroup); ret = mem_cgroup_charge(folio, NULL, gfp); + if (kernel_file) + set_active_memcg(tmp); if (ret) return ret; -- 2.50.1
