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

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Date:   Mon, 20 Jun 2022 01:04:36 -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 Thu, Jun 16, 2022 at 10:27:02AM +0800, Baolu Lu wrote:
> 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.

I'm not sure it is worth it performance wise, would be interesting to
know perhaps. But see my prior email about how slab has a custom
struct page.

Jason