[PATCH 1/3] mm: memcontrol: skip moving non-present pages that are mapped elsewhere
From: Johannes Weiner
Date: Tue Dec 06 2022 - 12:14:21 EST
During charge moving, the pte lock and the page lock cover nearly all
cases of stabilizing page_mapped(). The only exception is when we're
looking at a non-present pte and find a page in the page cache or in
the swapcache: if the page is mapped elsewhere, it can become unmapped
outside of our control. For this reason, rmap needs lock_page_memcg().
We don't like cgroup-specific locks in generic MM code - especially in
performance-critical MM code - and for a legacy feature that's
unlikely to have many users left - if any.
So remove the exception. Arguably that's better semantics anyway: the
page is shared, and another process seems to be the more active user.
Once we stop moving such pages, rmap doesn't need lock_page_memcg()
anymore. The next patch will remove it.
Suggested-by: Hugh Dickins <hughd@xxxxxxxxxx>
Signed-off-by: Johannes Weiner <hannes@xxxxxxxxxxx>
---
mm/memcontrol.c | 52 ++++++++++++++++++++++++++++++++++++-------------
1 file changed, 38 insertions(+), 14 deletions(-)
diff --git a/mm/memcontrol.c b/mm/memcontrol.c
index 48c44229cf47..b696354c1b21 100644
--- a/mm/memcontrol.c
+++ b/mm/memcontrol.c
@@ -5681,7 +5681,7 @@ static struct page *mc_handle_file_pte(struct vm_area_struct *vma,
* @from: mem_cgroup which the page is moved from.
* @to: mem_cgroup which the page is moved to. @from != @to.
*
- * The caller must make sure the page is not on LRU (isolate_page() is useful.)
+ * The page must be locked and not on the LRU.
*
* This function doesn't do "charge" to new cgroup and doesn't do "uncharge"
* from old cgroup.
@@ -5698,20 +5698,13 @@ static int mem_cgroup_move_account(struct page *page,
int nid, ret;
VM_BUG_ON(from == to);
+ VM_BUG_ON_FOLIO(!folio_test_locked(folio), folio);
VM_BUG_ON_FOLIO(folio_test_lru(folio), folio);
VM_BUG_ON(compound && !folio_test_large(folio));
- /*
- * Prevent mem_cgroup_migrate() from looking at
- * page's memory cgroup of its source page while we change it.
- */
- ret = -EBUSY;
- if (!folio_trylock(folio))
- goto out;
-
ret = -EINVAL;
if (folio_memcg(folio) != from)
- goto out_unlock;
+ goto out;
pgdat = folio_pgdat(folio);
from_vec = mem_cgroup_lruvec(from, pgdat);
@@ -5798,8 +5791,6 @@ static int mem_cgroup_move_account(struct page *page,
mem_cgroup_charge_statistics(from, -nr_pages);
memcg_check_events(from, nid);
local_irq_enable();
-out_unlock:
- folio_unlock(folio);
out:
return ret;
}
@@ -5848,6 +5839,29 @@ static enum mc_target_type get_mctgt_type(struct vm_area_struct *vma,
else if (is_swap_pte(ptent))
page = mc_handle_swap_pte(vma, ptent, &ent);
+ if (target && page) {
+ if (!trylock_page(page)) {
+ put_page(page);
+ return ret;
+ }
+ /*
+ * page_mapped() must be stable during the move. This
+ * pte is locked, so if it's present, the page cannot
+ * become unmapped. If it isn't, we have only partial
+ * control over the mapped state: the page lock will
+ * prevent new faults against pagecache and swapcache,
+ * so an unmapped page cannot become mapped. However,
+ * if the page is already mapped elsewhere, it can
+ * unmap, and there is nothing we can do about it.
+ * Alas, skip moving the page in this case.
+ */
+ if (!pte_present(ptent) && page_mapped(page)) {
+ unlock_page(page);
+ put_page(page);
+ return ret;
+ }
+ }
+
if (!page && !ent.val)
return ret;
if (page) {
@@ -5864,8 +5878,11 @@ static enum mc_target_type get_mctgt_type(struct vm_area_struct *vma,
if (target)
target->page = page;
}
- if (!ret || !target)
+ if (!ret || !target) {
+ if (target)
+ unlock_page(page);
put_page(page);
+ }
}
/*
* There is a swap entry and a page doesn't exist or isn't charged.
@@ -5905,6 +5922,10 @@ static enum mc_target_type get_mctgt_type_thp(struct vm_area_struct *vma,
ret = MC_TARGET_PAGE;
if (target) {
get_page(page);
+ if (!trylock_page(page)) {
+ put_page(page);
+ return MC_TARGET_NONE;
+ }
target->page = page;
}
}
@@ -6143,6 +6164,7 @@ static int mem_cgroup_move_charge_pte_range(pmd_t *pmd,
}
putback_lru_page(page);
}
+ unlock_page(page);
put_page(page);
} else if (target_type == MC_TARGET_DEVICE) {
page = target.page;
@@ -6151,6 +6173,7 @@ static int mem_cgroup_move_charge_pte_range(pmd_t *pmd,
mc.precharge -= HPAGE_PMD_NR;
mc.moved_charge += HPAGE_PMD_NR;
}
+ unlock_page(page);
put_page(page);
}
spin_unlock(ptl);
@@ -6193,7 +6216,8 @@ static int mem_cgroup_move_charge_pte_range(pmd_t *pmd,
}
if (!device)
putback_lru_page(page);
-put: /* get_mctgt_type() gets the page */
+put: /* get_mctgt_type() gets & locks the page */
+ unlock_page(page);
put_page(page);
break;
case MC_TARGET_SWAP:
--
2.38.1