linux-kernel - Re: [PATCH] zsmalloc: fix a race with deferred

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Date:   Wed, 1 Feb 2023 10:16:04 +0900
From:   Sergey Senozhatsky <senozhatsky@...omium.org>
To:     Nhat Pham <nphamcs@...il.com>
Cc:     akpm@...ux-foundation.org, hannes@...xchg.org, linux-mm@...ck.org,
        linux-kernel@...r.kernel.org, minchan@...nel.org,
        ngupta@...are.org, senozhatsky@...omium.org, sjenning@...hat.com,
        ddstreet@...e.org, vitaly.wool@...sulko.com, kernel-team@...a.com
Subject: Re: [PATCH] zsmalloc: fix a race with deferred_handles storing

On (23/01/10 15:17), Nhat Pham wrote:
> +#ifdef CONFIG_ZPOOL
> +/*
> + * The second least-significant bit in the object's header identifies if the
> + * value stored at the header is a deferred handle from the last reclaim
> + * attempt.
> + *
> + * As noted above, this is valid because we have room for two bits.
> + */
> +#define OBJ_DEFERRED_HANDLE_TAG	2
> +#define OBJ_TAG_BITS	2
> +#define OBJ_TAG_MASK	(OBJ_ALLOCATED_TAG | OBJ_DEFERRED_HANDLE_TAG)
> +#else
> +#define OBJ_TAG_BITS	1
> +#define OBJ_TAG_MASK	OBJ_ALLOCATED_TAG
> +#endif /* CONFIG_ZPOOL */
> +
>  #define OBJ_INDEX_BITS	(BITS_PER_LONG - _PFN_BITS - OBJ_TAG_BITS)
>  #define OBJ_INDEX_MASK	((_AC(1, UL) << OBJ_INDEX_BITS) - 1)
>  
> @@ -222,6 +238,12 @@ struct link_free {
>  		 * Handle of allocated object.
>  		 */
>  		unsigned long handle;
> +#ifdef CONFIG_ZPOOL
> +		/*
> +		 * Deferred handle of a reclaimed object.
> +		 */
> +		unsigned long deferred_handle;
> +#endif

A nit:
Do we really need to have that #ifdef and add a member to anon uion?
I see that we use ->deferred_handle only in one place, so I'm not sure
if it makes code any simpler.