linux-kernel - Re: [PATCH v2 1/1] swap: shmem: remove SWAP_MAP

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Message-ID: <87cyk79mes.fsf@yhuang6-desk2.ccr.corp.intel.com>
Date: Fri, 11 Oct 2024 14:35:55 +0800
From: "Huang, Ying" <ying.huang@...el.com>
To: Nhat Pham <nphamcs@...il.com>
Cc: akpm@...ux-foundation.org,  hannes@...xchg.org,  yosryahmed@...gle.com,
  hughd@...gle.com,  shakeel.butt@...ux.dev,  ryan.roberts@....com,
  chrisl@...nel.org,  david@...hat.com,  kasong@...cent.com,
  willy@...radead.org,  viro@...iv.linux.org.uk,  baohua@...nel.org,
  chengming.zhou@...ux.dev,  v-songbaohua@...o.com,  linux-mm@...ck.org,
  kernel-team@...a.com,  linux-kernel@...r.kernel.org
Subject: Re: [PATCH v2 1/1] swap: shmem: remove SWAP_MAP_SHMEM

Nhat Pham <nphamcs@...il.com> writes:

[snip]

> diff --git a/mm/swapfile.c b/mm/swapfile.c
> index 0cded32414a1..9bb94e618914 100644
> --- a/mm/swapfile.c
> +++ b/mm/swapfile.c
> @@ -1381,12 +1381,6 @@ static unsigned char __swap_entry_free_locked(struct swap_info_struct *si,
>  	if (usage == SWAP_HAS_CACHE) {
>  		VM_BUG_ON(!has_cache);
>  		has_cache = 0;
> -	} else if (count == SWAP_MAP_SHMEM) {
> -		/*
> -		 * Or we could insist on shmem.c using a special
> -		 * swap_shmem_free() and free_shmem_swap_and_cache()...
> -		 */
> -		count = 0;
>  	} else if ((count & ~COUNT_CONTINUED) <= SWAP_MAP_MAX) {
>  		if (count == COUNT_CONTINUED) {
>  			if (swap_count_continued(si, offset, count))
> @@ -3626,7 +3620,6 @@ static int __swap_duplicate(swp_entry_t entry, unsigned char usage, int nr)
>  
>  	offset = swp_offset(entry);
>  	VM_WARN_ON(nr > SWAPFILE_CLUSTER - offset % SWAPFILE_CLUSTER);
> -	VM_WARN_ON(usage == 1 && nr > 1);
>  	ci = lock_cluster_or_swap_info(si, offset);
>  
>  	err = 0;
> @@ -3652,6 +3645,13 @@ static int __swap_duplicate(swp_entry_t entry, unsigned char usage, int nr)
>  				err = -EEXIST;
>  		} else if ((count & ~COUNT_CONTINUED) > SWAP_MAP_MAX) {
>  			err = -EINVAL;
> +		} else {
> +			/*
> +			 * The only swap_duplicate_nr() caller that passes nr > 1 is shmem,
> +			 * who never re-duplicates any swap entry it owns. So this should
> +			 * not happen.
> +			 */
> +			VM_WARN_ON(nr > 1 && (count & ~COUNT_CONTINUED) == SWAP_MAP_MAX);

Why not

	VM_WARN_ON_ONCE(nr > 1 && count);

?

IIUC, count == 0 is always true for shmem swap entry allocation.  Then
developers who use __swap_duplicate() with nr > 1 without noticing the
unsupported feature can get warning during development immediately.
"(count & ~COUNT_CONTINUED) == SWAP_MAP_MAX" is hard to be triggered
during common swap test.

>  		}
>  
>  		if (err)
> @@ -3686,27 +3686,28 @@ static int __swap_duplicate(swp_entry_t entry, unsigned char usage, int nr)
>  	return err;
>  }
  
[snip]

--
Best Regards,
Huang, Ying