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Message-ID: <87e0c968-f203-eaac-8993-0ae5d9abfd45@huawei.com>
Date: Mon, 12 Apr 2021 14:59:20 +0800
From: Miaohe Lin <linmiaohe@...wei.com>
To: "Huang, Ying" <ying.huang@...el.com>
CC: <akpm@...ux-foundation.org>, <hannes@...xchg.org>,
<mhocko@...e.com>, <iamjoonsoo.kim@....com>, <vbabka@...e.cz>,
<alex.shi@...ux.alibaba.com>, <willy@...radead.org>,
<minchan@...nel.org>, <richard.weiyang@...il.com>,
<hughd@...gle.com>, <tim.c.chen@...ux.intel.com>,
<linux-kernel@...r.kernel.org>, <linux-mm@...ck.org>
Subject: Re: [PATCH 1/5] mm/swapfile: add percpu_ref support for swap
On 2021/4/12 11:30, Huang, Ying wrote:
> Miaohe Lin <linmiaohe@...wei.com> writes:
>
>> We will use percpu-refcount to serialize against concurrent swapoff. This
>> patch adds the percpu_ref support for later fixup.
>>
>> Signed-off-by: Miaohe Lin <linmiaohe@...wei.com>
>> ---
>> include/linux/swap.h | 2 ++
>> mm/swapfile.c | 25 ++++++++++++++++++++++---
>> 2 files changed, 24 insertions(+), 3 deletions(-)
>>
>> diff --git a/include/linux/swap.h b/include/linux/swap.h
>> index 144727041e78..849ba5265c11 100644
>> --- a/include/linux/swap.h
>> +++ b/include/linux/swap.h
>> @@ -240,6 +240,7 @@ struct swap_cluster_list {
>> * The in-memory structure used to track swap areas.
>> */
>> struct swap_info_struct {
>> + struct percpu_ref users; /* serialization against concurrent swapoff */
>> unsigned long flags; /* SWP_USED etc: see above */
>> signed short prio; /* swap priority of this type */
>> struct plist_node list; /* entry in swap_active_head */
>> @@ -260,6 +261,7 @@ struct swap_info_struct {
>> struct block_device *bdev; /* swap device or bdev of swap file */
>> struct file *swap_file; /* seldom referenced */
>> unsigned int old_block_size; /* seldom referenced */
>> + struct completion comp; /* seldom referenced */
>> #ifdef CONFIG_FRONTSWAP
>> unsigned long *frontswap_map; /* frontswap in-use, one bit per page */
>> atomic_t frontswap_pages; /* frontswap pages in-use counter */
>> diff --git a/mm/swapfile.c b/mm/swapfile.c
>> index 149e77454e3c..724173cd7d0c 100644
>> --- a/mm/swapfile.c
>> +++ b/mm/swapfile.c
>> @@ -39,6 +39,7 @@
>> #include <linux/export.h>
>> #include <linux/swap_slots.h>
>> #include <linux/sort.h>
>> +#include <linux/completion.h>
>>
>> #include <asm/tlbflush.h>
>> #include <linux/swapops.h>
>> @@ -511,6 +512,15 @@ static void swap_discard_work(struct work_struct *work)
>> spin_unlock(&si->lock);
>> }
>>
>> +static void swap_users_ref_free(struct percpu_ref *ref)
>> +{
>> + struct swap_info_struct *si;
>> +
>> + si = container_of(ref, struct swap_info_struct, users);
>> + complete(&si->comp);
>> + percpu_ref_exit(&si->users);
>
> Because percpu_ref_exit() is used, we cannot use percpu_ref_tryget() in
> get_swap_device(), better to add comments there.
Will do.
>
>> +}
>> +
>> static void alloc_cluster(struct swap_info_struct *si, unsigned long idx)
>> {
>> struct swap_cluster_info *ci = si->cluster_info;
>> @@ -2500,7 +2510,7 @@ static void enable_swap_info(struct swap_info_struct *p, int prio,
>> * Guarantee swap_map, cluster_info, etc. fields are valid
>> * between get/put_swap_device() if SWP_VALID bit is set
>> */
>> - synchronize_rcu();
>> + percpu_ref_reinit(&p->users);
>
> Although the effect is same, I think it's better to use
> percpu_ref_resurrect() here to improve code readability.
>
Agree.
>> spin_lock(&swap_lock);
>> spin_lock(&p->lock);
>> _enable_swap_info(p);
>> @@ -2621,11 +2631,13 @@ SYSCALL_DEFINE1(swapoff, const char __user *, specialfile)
>> p->flags &= ~SWP_VALID; /* mark swap device as invalid */
>> spin_unlock(&p->lock);
>> spin_unlock(&swap_lock);
>> +
>> + percpu_ref_kill(&p->users);
>> /*
>> * wait for swap operations protected by get/put_swap_device()
>> * to complete
>> */
>> - synchronize_rcu();
>> + wait_for_completion(&p->comp);
>
> Better to move percpu_ref_kill() after the comments. And maybe revise
> the comments.
Will do.
>
>>
>> flush_work(&p->discard_work);
>>
>> @@ -3132,7 +3144,7 @@ static bool swap_discardable(struct swap_info_struct *si)
>> SYSCALL_DEFINE2(swapon, const char __user *, specialfile, int, swap_flags)
>> {
>> struct swap_info_struct *p;
>> - struct filename *name;
>> + struct filename *name = NULL;
>> struct file *swap_file = NULL;
>> struct address_space *mapping;
>> int prio;
>> @@ -3163,6 +3175,12 @@ SYSCALL_DEFINE2(swapon, const char __user *, specialfile, int, swap_flags)
>>
>> INIT_WORK(&p->discard_work, swap_discard_work);
>>
>> + init_completion(&p->comp);
>> + error = percpu_ref_init(&p->users, swap_users_ref_free,
>> + PERCPU_REF_INIT_DEAD, GFP_KERNEL);
>> + if (unlikely(error))
>> + goto bad_swap;
>> +
>> name = getname(specialfile);
>> if (IS_ERR(name)) {
>> error = PTR_ERR(name);
>> @@ -3356,6 +3374,7 @@ SYSCALL_DEFINE2(swapon, const char __user *, specialfile, int, swap_flags)
>> bad_swap_unlock_inode:
>> inode_unlock(inode);
>> bad_swap:
>> + percpu_ref_exit(&p->users);
>
> Usually the resource freeing order matches their allocating order
> reversely. So, if there's no special reason, please follow that rule.
>
My oversight. Will fix it in V2.
> Best Regards,
> Huang, Ying
>
>> free_percpu(p->percpu_cluster);
>> p->percpu_cluster = NULL;
>> free_percpu(p->cluster_next_cpu);
> .
>
Many thanks for review and nice suggestion! :)
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