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

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Date:   Thu, 9 Jun 2022 10:32:34 -0300
From:   Jason Gunthorpe <jgg@...dia.com>
To:     Robin Murphy <robin.murphy@....com>
Cc:     Lu Baolu <baolu.lu@...ux.intel.com>,
        Joerg Roedel <joro@...tes.org>,
        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 09, 2022 at 02:19:06PM +0100, Robin Murphy wrote:

> Is there a significant benefit to keeping both paths, or could we get away
> with just always using RCU? Realistically, pagetable pages aren't likely to
> be freed all that frequently, except perhaps at domain teardown, but that
> shouldn't really be performance-critical, and I guess we could stick an RCU
> sync point in iommu_domain_free() if we're really worried about releasing
> larger quantities of pages back to the allocator ASAP?

I think you are right, anything that uses the iommu_iotlb_gather may
as well use RCU too.

IIRC the allocators already know that RCU is often sitting on
freed-memory and have some contigency to flush it out before OOMing,
so nothing special should be needed.

Jason