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Date: Mon, 19 Apr 2021 15:35:09 +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 15:09, 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?
> 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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