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

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Message-ID: <20220615154040.GA4927@nvidia.com>
Date:   Wed, 15 Jun 2022 12:40:40 -0300
From:   Jason Gunthorpe <jgg@...dia.com>
To:     Baolu Lu <baolu.lu@...ux.intel.com>
Cc:     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 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

Jason