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Message-ID: <753f414f-34a1-b16a-f826-7deb2dcd4af6@huawei.com>
Date: Mon, 19 Apr 2021 14:46:07 +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 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?
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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