linux-kernel - Re: [PATCH v6 08/12] x86/sgx: Use a list to track to-be-reclaimed pages

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Date:   Wed, 15 Nov 2023 22:59:40 +0200
From:   "Jarkko Sakkinen" <jarkko@...nel.org>
To:     "Haitao Huang" <haitao.huang@...ux.intel.com>,
        <dave.hansen@...ux.intel.com>, <tj@...nel.org>, <mkoutny@...e.com>,
        <linux-kernel@...r.kernel.org>, <linux-sgx@...r.kernel.org>,
        <x86@...nel.org>, <cgroups@...r.kernel.org>, <tglx@...utronix.de>,
        <mingo@...hat.com>, <bp@...en8.de>, <hpa@...or.com>,
        <sohil.mehta@...el.com>
Cc:     <zhiquan1.li@...el.com>, <kristen@...ux.intel.com>,
        <seanjc@...gle.com>, <zhanb@...rosoft.com>,
        <anakrish@...rosoft.com>, <mikko.ylinen@...ux.intel.com>,
        <yangjie@...rosoft.com>,
        "Sean Christopherson" <sean.j.christopherson@...el.com>
Subject: Re: [PATCH v6 08/12] x86/sgx: Use a list to track to-be-reclaimed
 pages

On Mon Oct 30, 2023 at 8:20 PM EET, Haitao Huang wrote:
> From: Sean Christopherson <sean.j.christopherson@...el.com>
>
> Change sgx_reclaim_pages() to use a list rather than an array for
> storing the epc_pages which will be reclaimed. This change is needed
> to transition to the LRU implementation for EPC cgroup support.
>
> When the EPC cgroup is implemented, the reclaiming process will do a
> pre-order tree walk for the subtree starting from the limit-violating
> cgroup.  When each node is visited, candidate pages are selected from
> its "reclaimable" LRU list and moved into this temporary list. Passing a
> list from node to node for temporary storage in this walk is more
> straightforward than using an array.
>
> Signed-off-by: Sean Christopherson <sean.j.christopherson@...el.com>
> Co-developed-by: Kristen Carlson Accardi <kristen@...ux.intel.com>
> Signed-off-by: Kristen Carlson Accardi <kristen@...ux.intel.com>
> Co-developed-by: Haitao Huang<haitao.huang@...ux.intel.com>
> Signed-off-by: Haitao Huang<haitao.huang@...ux.intel.com>
> Cc: Sean Christopherson <seanjc@...gle.com>
> ---
> V6:
> - Remove extra list_del_init and style fix (Kai)
>
> V4:
> - Changes needed for patch reordering
> - Revised commit message
>
> V3:
> - Removed list wrappers
> ---
>  arch/x86/kernel/cpu/sgx/main.c | 35 +++++++++++++++-------------------
>  1 file changed, 15 insertions(+), 20 deletions(-)
>
> diff --git a/arch/x86/kernel/cpu/sgx/main.c b/arch/x86/kernel/cpu/sgx/main.c
> index e27ac73d8843..33bcba313d40 100644
> --- a/arch/x86/kernel/cpu/sgx/main.c
> +++ b/arch/x86/kernel/cpu/sgx/main.c
> @@ -296,12 +296,11 @@ static void sgx_reclaimer_write(struct sgx_epc_page *epc_page,
>   */
>  static void sgx_reclaim_pages(void)
>  {
> -	struct sgx_epc_page *chunk[SGX_NR_TO_SCAN];
>  	struct sgx_backing backing[SGX_NR_TO_SCAN];
> +	struct sgx_epc_page *epc_page, *tmp;
>  	struct sgx_encl_page *encl_page;
> -	struct sgx_epc_page *epc_page;
>  	pgoff_t page_index;
> -	int cnt = 0;
> +	LIST_HEAD(iso);
>  	int ret;
>  	int i;
>  
> @@ -317,7 +316,7 @@ static void sgx_reclaim_pages(void)
>  
>  		if (kref_get_unless_zero(&encl_page->encl->refcount) != 0) {
>  			sgx_epc_page_set_state(epc_page, SGX_EPC_PAGE_RECLAIM_IN_PROGRESS);
> -			chunk[cnt++] = epc_page;
> +			list_move_tail(&epc_page->list, &iso);
>  		} else
>  			/* The owner is freeing the page. No need to add the
>  			 * page back to the list of reclaimable pages.
> @@ -326,8 +325,11 @@ static void sgx_reclaim_pages(void)
>  	}
>  	spin_unlock(&sgx_global_lru.lock);
>  
> -	for (i = 0; i < cnt; i++) {
> -		epc_page = chunk[i];
> +	if (list_empty(&iso))
> +		return;
> +
> +	i = 0;
> +	list_for_each_entry_safe(epc_page, tmp, &iso, list) {
>  		encl_page = epc_page->owner;
>  
>  		if (!sgx_reclaimer_age(epc_page))
> @@ -342,6 +344,7 @@ static void sgx_reclaim_pages(void)
>  			goto skip;
>  		}
>  
> +		i++;
>  		encl_page->desc |= SGX_ENCL_PAGE_BEING_RECLAIMED;
>  		mutex_unlock(&encl_page->encl->lock);
>  		continue;
> @@ -349,27 +352,19 @@ static void sgx_reclaim_pages(void)
>  skip:
>  		spin_lock(&sgx_global_lru.lock);
>  		sgx_epc_page_set_state(epc_page, SGX_EPC_PAGE_RECLAIMABLE);
> -		list_add_tail(&epc_page->list, &sgx_global_lru.reclaimable);
> +		list_move_tail(&epc_page->list, &sgx_global_lru.reclaimable);
>  		spin_unlock(&sgx_global_lru.lock);
>  
>  		kref_put(&encl_page->encl->refcount, sgx_encl_release);
> -
> -		chunk[i] = NULL;
> -	}
> -
> -	for (i = 0; i < cnt; i++) {
> -		epc_page = chunk[i];
> -		if (epc_page)
> -			sgx_reclaimer_block(epc_page);
>  	}
>  
> -	for (i = 0; i < cnt; i++) {
> -		epc_page = chunk[i];
> -		if (!epc_page)
> -			continue;
> +	list_for_each_entry(epc_page, &iso, list)
> +		sgx_reclaimer_block(epc_page);
>  
> +	i = 0;
> +	list_for_each_entry_safe(epc_page, tmp, &iso, list) {
>  		encl_page = epc_page->owner;
> -		sgx_reclaimer_write(epc_page, &backing[i]);
> +		sgx_reclaimer_write(epc_page, &backing[i++]);

Couldn't you alternatively "&backing[--i]" and not reset i to zero
before the loop?

>  
>  		kref_put(&encl_page->encl->refcount, sgx_encl_release);
>  		sgx_epc_page_reset_state(epc_page);

BR, Jarkko