mm/filemap: add AS_KERNEL_FILE

Patch series "introduce kernel file mapped folios", v4.

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.

This patch (of 3):

Add the flag AS_KERNEL_FILE to the address_space to indicate that this
mapping's memory is exempt from the usual memcg accounting.

Link: https://lore.kernel.org/linux-mm/b5fef5372ae454a7b6da4f2f75c427aeab6a07d6.1727498749.git.wqu@suse.com/
Link: https://lkml.kernel.org/r/f09c4e2c90351d4cb30a1969f7a863b9238bd291.1755812945.git.boris@bur.io
Signed-off-by: Boris Burkov <boris@bur.io>
Suggested-by: Qu Wenruo <wqu@suse.com>
Suggested-by: Shakeel Butt <shakeel.butt@linux.dev>
Acked-by: Shakeel Butt <shakeel.butt@linux.dev>
Cc: Johannes Weiner <hannes@cmpxchg.org>
Cc: Matthew Wilcox (Oracle) <willy@infradead.org>
Cc: Michal Hocko <mhocko@kernel.org>
Cc: Muchun Song <muchun.song@linux.dev>
Cc: Roman Gushchin <roman.gushchin@linux.dev>
Signed-off-by: Andrew Morton <akpm@linux-foundation.org>

diff --git a/include/linux/pagemap.h b/include/linux/pagemap.h
index 12a12dae727d01e1cf7d9171f26c12e5b24058ae..f0dfdfb13cd917b7e6138c152a07af978d4731a9 100644 (file)
--- 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 6e954156bb77f4206d310c15f3a1c99f1cd4c6dc..92ea20356f227d1e37510b0176ae5f7cd2206577 100644 (file)
--- 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;