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mm: memcontrol: skip moving non-present pages that are mapped elsewhere

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Patch series "mm: push down lock_page_memcg()", v2.


This patch (of 3):

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.

Link: https://lkml.kernel.org/r/20221206171340.139790-1-hannes@cmpxchg.org
Link: https://lkml.kernel.org/r/20221206171340.139790-2-hannes@cmpxchg.org
Signed-off-by: Johannes Weiner <hannes@cmpxchg.org>
Suggested-by: Hugh Dickins <hughd@google.com>
Acked-by: Hugh Dickins <hughd@google.com>
Acked-by: Shakeel Butt <shakeelb@google.com>
Acked-by: Michal Hocko <mhocko@suse.com>
Cc: Roman Gushchin <roman.gushchin@linux.dev>
Cc: Muchun Song <songmuchun@bytedance.com>
Signed-off-by: Andrew Morton <akpm@linux-foundation.org>
This commit is contained in:
Johannes Weiner 2022-12-06 18:13:39 +01:00 committed by Andrew Morton
parent c5094ec79c
commit 4e0cf05f60
1 changed files with 38 additions and 14 deletions
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52
mm/memcontrol.c
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@ -5692,7 +5692,7 @@ 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.
@ -5709,20 +5709,13 @@ 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);
@ -5809,8 +5802,6 @@ 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;
}
@ -5859,6 +5850,29 @@ 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) {
@ -5875,8 +5889,11 @@ 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.
@ -5916,6 +5933,10 @@ 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;
}
}
@ -6154,6 +6175,7 @@ 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;
@ -6162,6 +6184,7 @@ 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);
@ -6204,7 +6227,8 @@ retry:
}
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: