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Subject: Re: [RFC PATCH 1/2] mm,drm/ttm: Block fast GUP to TTM huge pages
To:     "Williams, Dan J" <dan.j.williams@intel.com>,
        "dri-devel@lists.freedesktop.org" <dri-devel@lists.freedesktop.org>
Cc:     "daniel@ffwll.ch" <daniel@ffwll.ch>,
        "christian.koenig@amd.com" <christian.koenig@amd.com>,
        "jgg@nvidia.com" <jgg@nvidia.com>,
        "airlied@linux.ie" <airlied@linux.ie>,
        "linux-mm@kvack.org" <linux-mm@kvack.org>,
        "linux-kernel@vger.kernel.org" <linux-kernel@vger.kernel.org>,
        "akpm@linux-foundation.org" <akpm@linux-foundation.org>
References: <20210321184529.59006-1-thomas_os@shipmail.org>
 <20210321184529.59006-2-thomas_os@shipmail.org>
 <ec99146c7abc35d16b245816aba3e9d14862e624.camel@intel.com>
From:   =?UTF-8?Q?Thomas_Hellstr=c3=b6m_=28Intel=29?= 
        <thomas_os@shipmail.org>
Message-ID: <c2239da2-c514-2c88-c671-918909cdba6b@shipmail.org>
Date:   Tue, 23 Mar 2021 21:42:18 +0100
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On 3/23/21 8:52 PM, Williams, Dan J wrote:
> On Sun, 2021-03-21 at 19:45 +0100, Thomas Hellström (Intel) wrote:
>> TTM sets up huge page-table-entries both to system- and device
>> memory,
>> and we don't want gup to assume there are always valid backing struct
>> pages for these. For PTEs this is handled by setting the pte_special
>> bit,
>> but for the huge PUDs and PMDs, we have neither pmd_special nor
>> pud_special. Normally, huge TTM entries are identified by looking at
>> vma_is_special_huge(), but fast gup can't do that, so as an
>> alternative
>> define _devmap entries for which there are no backing dev_pagemap as
>> special, update documentation and make huge TTM entries _devmap,
>> after
>> verifying that there is no backing dev_pagemap.
> Please do not abuse p{m,u}d_devmap like this. I'm in the process of
> removing get_devpagemap() from the gup-fast path [1]. Instead there
> should be space for p{m,u}d_special in the page table entries (at least
> for x86-64). So the fix is to remove that old assumption that huge
> pages can never be special.
>
> [1]:
> http://lore.kernel.org/r/161604050866.1463742.7759521510383551055.stgit@dwillia2-desk3.amr.corp.intel.com
>
Hmm, yes with that patch it will obviously not work as intended.

Given that, I think we'll need to disable the TTM huge pages for now 
until we can sort out and agree on using a page table entry bit.

Thanks,

/Thomas