memcg: charge before adding to swapcache on swapin

Currently the kernel adds the page, allocated for swapin, to the
swapcache before charging the page.  This is fine but now we want a
per-memcg swapcache stat which is essential for folks who wants to
transparently migrate from cgroup v1's memsw to cgroup v2's memory and
swap counters.  In addition charging a page before exposing it to other
parts of the kernel is a step in the right direction.

To correctly maintain the per-memcg swapcache stat, this patch has
adopted to charge the page before adding it to swapcache.  One challenge
in this option is the failure case of add_to_swap_cache() on which we
need to undo the mem_cgroup_charge().  Specifically undoing
mem_cgroup_uncharge_swap() is not simple.

To resolve the issue, this patch decouples the charging for swapin pages
from mem_cgroup_charge().  Two new functions are introduced,
mem_cgroup_swapin_charge_page() for just charging the swapin page and
mem_cgroup_swapin_uncharge_swap() for uncharging the swap slot once the
page has been successfully added to the swapcache.

[[email protected]: set page->private before calling swap_readpage]
  Link: https://lkml.kernel.org/r/[email protected]

Link: https://lkml.kernel.org/r/[email protected]
Signed-off-by: Shakeel Butt <[email protected]>
Acked-by: Roman Gushchin <[email protected]>
Acked-by: Johannes Weiner <[email protected]>
Acked-by: Hugh Dickins <[email protected]>
Tested-by: Heiko Carstens <[email protected]>
Cc: Michal Hocko <[email protected]>
Cc: Stephen Rothwell <[email protected]>
Cc: Christian Borntraeger <[email protected]>
Signed-off-by: Andrew Morton <[email protected]>
Signed-off-by: Linus Torvalds <[email protected]>

0 files changed, 0 insertions, 0 deletions