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Date: Mon, 19 Apr 2021 15:09:49 +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 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().
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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