linux-kernel - Re: [RFC PATCHES 1/2] iommu: Add RCU-protected page free support

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Message-ID: <6b8a4728-62c0-edb2-7b13-c4f01f5dc41a@linux.intel.com>
Date:   Thu, 16 Jun 2022 10:27:02 +0800
From:   Baolu Lu <baolu.lu@...ux.intel.com>
To:     Jason Gunthorpe <jgg@...dia.com>
Cc:     baolu.lu@...ux.intel.com, Joerg Roedel <joro@...tes.org>,
        Robin Murphy <robin.murphy@....com>,
        Kevin Tian <kevin.tian@...el.com>,
        Ashok Raj <ashok.raj@...el.com>,
        Christoph Hellwig <hch@...radead.org>,
        Will Deacon <will@...nel.org>,
        Joao Martins <joao.m.martins@...cle.com>,
        iommu@...ts.linux-foundation.org, linux-kernel@...r.kernel.org
Subject: Re: [RFC PATCHES 1/2] iommu: Add RCU-protected page free support

On 2022/6/15 23:40, Jason Gunthorpe wrote:
> On Fri, Jun 10, 2022 at 01:37:20PM +0800, Baolu Lu wrote:
>> On 2022/6/9 20:49, Jason Gunthorpe wrote:
>>>> +void iommu_free_pgtbl_pages(struct iommu_domain *domain,
>>>> +			    struct list_head *pages)
>>>> +{
>>>> +	struct page *page, *next;
>>>> +
>>>> +	if (!domain->concurrent_traversal) {
>>>> +		put_pages_list(pages);
>>>> +		return;
>>>> +	}
>>>> +
>>>> +	list_for_each_entry_safe(page, next, pages, lru) {
>>>> +		list_del(&page->lru);
>>>> +		call_rcu(&page->rcu_head, pgtble_page_free_rcu);
>>>> +	}
>>> It seems OK, but I wonder if there is benifit to using
>>> put_pages_list() from the rcu callback
>>
>> The price is that we need to allocate a "struct list_head" and free it
>> in the rcu callback as well. Currently the list_head is sitting in the
>> stack.
> 
> You'd have to use a different struct page layout so that the list_head
> was in the struct page and didn't overlap with the rcu_head

Okay, let me head this direction in the next version.

Best regards,
baolu