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Message-ID: <84daf06a-84b4-0523-b278-123e546a92e2@huawei.com>
Date: Mon, 19 Apr 2021 16:20:15 +0800
From: Miaohe Lin <linmiaohe@...wei.com>
To: "Huang, Ying" <ying.huang@...el.com>
CC: <akpm@...ux-foundation.org>, <dennis@...nel.org>,
<tim.c.chen@...ux.intel.com>, <hughd@...gle.com>,
<hannes@...xchg.org>, <mhocko@...e.com>, <iamjoonsoo.kim@....com>,
<alexs@...nel.org>, <david@...hat.com>, <minchan@...nel.org>,
<richard.weiyang@...il.com>, <linux-kernel@...r.kernel.org>,
<linux-mm@...ck.org>
Subject: Re: [PATCH v2 1/5] mm/swapfile: add percpu_ref support for swap
On 2021/4/19 15:52, Huang, Ying wrote:
> Miaohe Lin <linmiaohe@...wei.com> writes:
>
>> On 2021/4/19 15:09, Huang, Ying wrote:
>>> Miaohe Lin <linmiaohe@...wei.com> writes:
>>>
>>>> On 2021/4/19 10:48, 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 swap.
>>>>>>
>>>>>> Signed-off-by: Miaohe Lin <linmiaohe@...wei.com>
>>>>>> ---
>>>>>> include/linux/swap.h | 3 +++
>>>>>> mm/swapfile.c | 33 +++++++++++++++++++++++++++++----
>>>>>> 2 files changed, 32 insertions(+), 4 deletions(-)
>>>>>>
>>>>>> diff --git a/include/linux/swap.h b/include/linux/swap.h
>>>>>> index 144727041e78..8be36eb58b7a 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 */
>>>>>
>>>>> The comments aren't general enough. We use this to check whether the
>>>>> swap device has been fully initialized, etc. May be something as below?
>>>>>
>>>>> /* indicate and keep swap device valid */
>>>>
>>>> Looks good.
>>>>
>>>>>
>>>>>> 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 +261,8 @@ 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 */
>>>>>> + bool ref_initialized; /* 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..66515a3a2824 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 +512,14 @@ 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);
>>>>>> +}
>>>>>> +
>>>>>> static void alloc_cluster(struct swap_info_struct *si, unsigned long idx)
>>>>>> {
>>>>>> struct swap_cluster_info *ci = si->cluster_info;
>>>>>> @@ -2500,7 +2509,7 @@ 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();
>>>>>
>>>>> You cannot remove this without changing get/put_swap_device(). It's
>>>>> better to squash at least PATCH 1-2.
>>>>
>>>> Will squash PATCH 1-2. Thanks.
>>>>
>>>>>
>>>>>> + percpu_ref_resurrect(&p->users);
>>>>>> spin_lock(&swap_lock);
>>>>>> spin_lock(&p->lock);
>>>>>> _enable_swap_info(p);
>>>>>> @@ -2621,11 +2630,18 @@ 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
>>>>>> + * We need synchronize_rcu() here to protect the accessing
>>>>>> + * to the swap cache data structure.
>>>>>> */
>>>>>> synchronize_rcu();
>>>>>> + /*
>>>>>> + * Wait for swap operations protected by get/put_swap_device()
>>>>>> + * to complete.
>>>>>> + */
>>>>>
>>>>> I think the comments (after some revision) can be moved before
>>>>> percpu_ref_kill(). The synchronize_rcu() comments can be merged.
>>>>>
>>>>
>>>> Ok.
>>>>
>>>>>> + wait_for_completion(&p->comp);
>>>>>>
>>>>>> flush_work(&p->discard_work);
>>>>>>
>>>>>> @@ -3132,7 +3148,7 @@ 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 +3179,15 @@ SYSCALL_DEFINE2(swapon, const char __user *, specialfile, int, swap_flags)
>>>>>>
>>>>>> INIT_WORK(&p->discard_work, swap_discard_work);
>>>>>>
>>>>>> + if (!p->ref_initialized) {
>>>>>
>>>>> I don't think it's necessary to add another flag p->ref_initialized. We
>>>>> can distinguish newly allocated and reused swap_info_struct in alloc_swap_info().
>>>>>
>>>>
>>>> If newly allocated swap_info_struct failed to init percpu_ref, it will be considered as
>>>> a reused one in alloc_swap_info() _but_ the field users of swap_info_struct is actually
>>>> uninitialized. Does this make sense for you?
>>>
>>> We can call percpu_ref_init() just after kvzalloc() in alloc_swap_info().
>>>
>>
>> Yes, we can do it this way. But using ref_initialized might make the code more straightforward
>> and simple?
>
> I think that it's simpler to call percpu_ref_init() in
> alloc_swap_info(). We can just call percpu_ref_init() for allocated
> swap_info_struct blindly, and call percpu_ref_exit() if we reuse.
>
Looks good. Will do. Many thanks again.
> Best Regards,
> Huang, Ying
>
>>> Best Regards,
>>> Huang, Ying
>>>
>>>> Many Thanks for quick review.
>>>>
>>>>> Best Regards,
>>>>> Huang, Ying
>>>>>
>>>>>> + error = percpu_ref_init(&p->users, swap_users_ref_free,
>>>>>> + PERCPU_REF_INIT_DEAD, GFP_KERNEL);
>>>>>> + if (unlikely(error))
>>>>>> + goto bad_swap;
>>>>>> + init_completion(&p->comp);
>>>>>> + p->ref_initialized = true;
>>>>>> + }
>>>>>> +
>>>>>> name = getname(specialfile);
>>>>>> if (IS_ERR(name)) {
>>>>>> error = PTR_ERR(name);
>>>>> .
>>>>>
>>> .
>>>
> .
>
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