linux-kernel - Re: [Xen-devel] [PATCH 3/3] xen/swiotlb: remember having called xen_create_contiguous

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Date:   Thu, 25 Apr 2019 03:01:40 -0600
From:   "Jan Beulich" <JBeulich@...e.com>
To:     "Juergen Gross" <jgross@...e.com>
Cc:     "Stefano Stabellini" <sstabellini@...nel.org>,
        <iommu@...ts.linux-foundation.org>,
        "xen-devel" <xen-devel@...ts.xenproject.org>,
        "Boris Ostrovsky" <boris.ostrovsky@...cle.com>,
        "Konrad Rzeszutek Wilk" <konrad.wilk@...cle.com>,
        <linux-kernel@...r.kernel.org>
Subject: Re: [Xen-devel] [PATCH 3/3] xen/swiotlb: remember having
 called xen_create_contiguous_region()

>>> On 23.04.19 at 20:36, <jgross@...e.com> wrote:
> On 23/04/2019 19:05, Stefano Stabellini wrote:
>> On Tue, 23 Apr 2019, Juergen Gross wrote:
>>> Instead of always calling xen_destroy_contiguous_region() in case the
>>> memory is DMA-able for the used device, do so only in case it has been
>>> made DMA-able via xen_create_contiguous_region() before.
>>>
>>> This will avoid a lot of xen_destroy_contiguous_region() calls for
>>> 64-bit capable devices.
>>>
>>> As the memory in question is owned by swiotlb-xen the PG_owner_priv_1
>>> flag of the first allocated page can be used for remembering.
>> 
>> Although the patch looks OK, this sentence puzzles me. Why do you say
>> that the memory in question is owned by swiotlb-xen? Because it was
>> returned by xen_alloc_coherent_pages? Both the x86 and the Arm
>> implementation return fresh new memory, hence, it should be safe to set
>> the PageOwnerPriv1 flag?
>> 
>> My concern with this approach is with the semantics of PG_owner_priv_1.
>> Is a page marked with PG_owner_priv_1 only supposed to be used by the
>> owner?
> 
> The owner of the page is free to use the flag.
> 
> Like Grant pages are marked by the grant driver using this flag. And
> Xen page tables are using it in PV-guests for indicating a "Pinned"
> page table.

Considering the background of the series, isn't such multi-purpose use
of the flag a possible problem? You're already suspecting a wrong call
into here. The function finding the flag set (but for another reason)
might add to the confusion. But I realize there are only so many page
flags available.

Perhaps the freeing function should, first thing, check the handed
space actually matches the criteria (within dma_mask and contiguous)?

Jan