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.

[shakeelb@google.com: set page->private before calling swap_readpage]
  Link: https://lkml.kernel.org/r/20210318015959.2986837-1-shakeelb@google.com

Link: https://lkml.kernel.org/r/20210305212639.775498-1-shakeelb@google.com
Signed-off-by: Shakeel Butt <shakeelb@google.com>
Acked-by: Roman Gushchin <guro@fb.com>
Acked-by: Johannes Weiner <hannes@cmpxchg.org>
Acked-by: Hugh Dickins <hughd@google.com>
Tested-by: Heiko Carstens <hca@linux.ibm.com>
Cc: Michal Hocko <mhocko@kernel.org>
Cc: Stephen Rothwell <sfr@canb.auug.org.au>
Cc: Christian Borntraeger <borntraeger@de.ibm.com>
Signed-off-by: Andrew Morton <akpm@linux-foundation.org>
Signed-off-by: Linus Torvalds <torvalds@linux-foundation.org>

1 files changed, 7 insertions, 9 deletions

diff --git a/mm/memory.c b/mm/memory.c
index 550405fc3b5e..3196fa5e7f7d 100644
--- a/mm/memory.c
+++ b/mm/memory.c
@@ -3309,28 +3309,26 @@ vm_fault_t do_swap_page(struct vm_fault *vmf)
 			page = alloc_page_vma(GFP_HIGHUSER_MOVABLE, vma,
 							vmf->address);
 			if (page) {
-				int err;
-
 				__SetPageLocked(page);
 				__SetPageSwapBacked(page);
-				set_page_private(page, entry.val);
 
-				/* Tell memcg to use swap ownership records */
-				SetPageSwapCache(page);
-				err = mem_cgroup_charge(page, vma->vm_mm,
-							GFP_KERNEL);
-				ClearPageSwapCache(page);
-				if (err) {
+				if (mem_cgroup_swapin_charge_page(page,
+					vma->vm_mm, GFP_KERNEL, entry)) {
 					ret = VM_FAULT_OOM;
 					goto out_page;
 				}
+				mem_cgroup_swapin_uncharge_swap(entry);
 
 				shadow = get_shadow_from_swap_cache(entry);
 				if (shadow)
 					workingset_refault(page, shadow);
 
 				lru_cache_add(page);
+
+				/* To provide entry to swap_readpage() */
+				set_page_private(page, entry.val);
 				swap_readpage(page, true);
+				set_page_private(page, 0);
 			}
 		} else {
 			page = swapin_readahead(entry, GFP_HIGHUSER_MOVABLE,