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Date: Mon, 19 Apr 2021 15:52:16 +0800
From: "Huang, Ying" <ying.huang@...el.com>
To: Miaohe Lin <linmiaohe@...wei.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
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.
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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