linux-kernel - Re: [PATCH v3 2/2] mm: attempt to batch free swap entries for zap_pte

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Message-ID: <33f34a88-0130-5444-9b84-93198eeb50e7@google.com>
Date: Sun, 25 Aug 2024 13:09:46 -0700 (PDT)
From: Hugh Dickins <hughd@...gle.com>
To: Andrew Morton <akpm@...ux-foundation.org>, Barry Song <21cnbao@...il.com>
cc: chrisl@...nel.org, david@...hat.com, hughd@...gle.com, 
    justinjiang@...o.com, kaleshsingh@...gle.com, kasong@...cent.com, 
    linux-kernel@...r.kernel.org, linux-mm@...ck.org, ryan.roberts@....com, 
    v-songbaohua@...o.com, ying.huang@...el.com, yosryahmed@...gle.com
Subject: Re: [PATCH v3 2/2] mm: attempt to batch free swap entries for
 zap_pte_range()

On Fri, 16 Aug 2024, Barry Song wrote:
> Subject: [PATCH] mm: check all swaps belong to same swap_cgroup in
>  swap_pte_batch()
> 
> Right now, it is possible two folios are contiguous in swap slots
> but they don't belong to one memcg. In this case, even we return
> a large nr, we can't really batch free all slots.
> 
> Reported-by: Yosry Ahmed <yosryahmed@...gle.com>
> Reported-by: Chris Li <chrisl@...nel.org>
> Signed-off-by: Barry Song <v-songbaohua@...o.com>
> ---
>  mm/internal.h | 9 +++++++--
>  1 file changed, 7 insertions(+), 2 deletions(-)
> 
> diff --git a/mm/internal.h b/mm/internal.h
> index adbf8c88c9df..d1f1e221212d 100644
> --- a/mm/internal.h
> +++ b/mm/internal.h
> @@ -15,6 +15,7 @@
>  #include <linux/rmap.h>
>  #include <linux/swap.h>
>  #include <linux/swapops.h>
> +#include <linux/swap_cgroup.h>
>  #include <linux/tracepoint-defs.h>
>  
>  /* Internal core VMA manipulation functions. */
> @@ -275,18 +276,22 @@ static inline int swap_pte_batch(pte_t *start_ptep, int max_nr, pte_t pte)
>  {
>  	pte_t expected_pte = pte_next_swp_offset(pte);
>  	const pte_t *end_ptep = start_ptep + max_nr;
> +	swp_entry_t entry = pte_to_swp_entry(pte);
>  	pte_t *ptep = start_ptep + 1;
> +	unsigned short cgroup_id;
>  
>  	VM_WARN_ON(max_nr < 1);
>  	VM_WARN_ON(!is_swap_pte(pte));
> -	VM_WARN_ON(non_swap_entry(pte_to_swp_entry(pte)));
> +	VM_WARN_ON(non_swap_entry(entry));
>  
> +	cgroup_id = lookup_swap_cgroup_id(entry);
>  	while (ptep < end_ptep) {
>  		pte = ptep_get(ptep);
>  
>  		if (!pte_same(pte, expected_pte))
>  			break;
> -
> +		if (lookup_swap_cgroup_id(pte_to_swp_entry(pte)) != cgroup_id)
> +			break;
>  		expected_pte = pte_next_swp_offset(expected_pte);
>  		ptep++;
>  	}
> -- 

[PATCH] mm: check all swaps belong to same swap_cgroup in swap_pte_batch() fix

mm-unstable swap_pte_batch() adds a new usage of lookup_swap_cgroup_id(),
which crashes if CONFIG_MEMCG kernel booted with "cgroup_disable=memory":
it now needs a mem_cgroup_disabled() check.

Fixes: 92b50df44566 ("mm: check all swaps belong to same swap_cgroup in swap_pte_batch()")
Signed-off-by: Hugh Dickins <hughd@...gle.com>
---
 mm/swap_cgroup.c | 2 ++
 1 file changed, 2 insertions(+)

diff --git a/mm/swap_cgroup.c b/mm/swap_cgroup.c
index db6c4a26cf59..da1278f0563b 100644
--- a/mm/swap_cgroup.c
+++ b/mm/swap_cgroup.c
@@ -161,6 +161,8 @@ unsigned short swap_cgroup_record(swp_entry_t ent, unsigned short id,
  */
 unsigned short lookup_swap_cgroup_id(swp_entry_t ent)
 {
+	if (mem_cgroup_disabled())
+		return 0;
 	return lookup_swap_cgroup(ent, NULL)->id;
 }
 
-- 
2.35.3