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Message-ID: <048d4dba-153f-5d32-75fc-d7e7144d1e9c@linux.intel.com>
Date:   Mon, 17 Apr 2023 12:24:45 +0100
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Subject: Re: [PATCH v2 9/9] drm/i915: Use kmap_local_page() in
 gem/i915_gem_execbuffer.c
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To:     Zhao Liu <zhao1.liu@linux.intel.com>
Cc:     "Fabio M. De Francesco" <fmdefrancesco@gmail.com>,
        Ira Weiny <ira.weiny@intel.com>,
        Jani Nikula <jani.nikula@linux.intel.com>,
        Joonas Lahtinen <joonas.lahtinen@linux.intel.com>,
        Rodrigo Vivi <rodrigo.vivi@intel.com>,
        David Airlie <airlied@gmail.com>,
        Daniel Vetter <daniel@ffwll.ch>,
        Matthew Auld <matthew.auld@intel.com>,
        =?UTF-8?Q?Thomas_Hellstr=c3=b6m?= 
        <thomas.hellstrom@linux.intel.com>,
        Nirmoy Das <nirmoy.das@intel.com>,
        Maarten Lankhorst <maarten.lankhorst@linux.intel.com>,
        Chris Wilson <chris@chris-wilson.co.uk>,
        =?UTF-8?Q?Christian_K=c3=b6nig?= <christian.koenig@amd.com>,
        intel-gfx@lists.freedesktop.org, dri-devel@lists.freedesktop.org,
        linux-kernel@vger.kernel.org,
        Zhenyu Wang <zhenyu.z.wang@intel.com>,
        Zhao Liu <zhao1.liu@intel.com>
References: <20230329073220.3982460-1-zhao1.liu@linux.intel.com>
 <64265ef8725fe_375f7e294a@iweiny-mobl.notmuch>
 <fdc8a470-1e6b-815d-e367-a9df1b0b14dd@linux.intel.com>
 <2177327.1BCLMh4Saa@suse>
 <1b341218-f0e2-a613-2ac6-107064a813ca@linux.intel.com>
 <ZDku5SJhl2Ve51UC@liuzhao-OptiPlex-7080>
From:   Tvrtko Ursulin <tvrtko.ursulin@linux.intel.com>
Organization: Intel Corporation UK Plc
In-Reply-To: <ZDku5SJhl2Ve51UC@liuzhao-OptiPlex-7080>
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On 14/04/2023 11:45, Zhao Liu wrote:
> Hi Tvrtko,
> 
> On Wed, Apr 12, 2023 at 04:45:13PM +0100, Tvrtko Ursulin wrote:
> 
> [snip]
> 
>>>
>>> [snip]
>>>> However I am unsure if disabling pagefaulting is needed or not. Thomas,
>>>> Matt, being the last to touch this area, perhaps you could have a look?
>>>> Because I notice we have a fallback iomap path which still uses
>>>> io_mapping_map_atomic_wc. So if kmap_atomic to kmap_local conversion is
>>>> safe, does the iomap side also needs converting to
>>>> io_mapping_map_local_wc? Or they have separate requirements?
>>>
>>> AFAIK, the requirements for io_mapping_map_local_wc() are the same as for
>>> kmap_local_page(): the kernel virtual address is _only_ valid in the caller
>>> context, and map/unmap nesting must be done in stack-based ordering (LIFO).
>>>
>>> I think a follow up patch could safely switch to io_mapping_map_local_wc() /
>>> io_mapping_unmap_local_wc since the address is local to context.
>>>
>>> However, not being an expert, reading your note now I suspect that I'm missing
>>> something. Can I ask why you think that page-faults disabling might be
>>> necessary?
>>
>> I am not saying it is, was just unsure and wanted some people who worked on this code most recently to take a look and confirm.
>>
>> I guess it will work since the copying is done like this anyway:
>>
>> 		/*
>> 		 * This is the fast path and we cannot handle a pagefault
>> 		 * whilst holding the struct mutex lest the user pass in the
>> 		 * relocations contained within a mmaped bo. For in such a case
>> 		 * we, the page fault handler would call i915_gem_fault() and
>> 		 * we would try to acquire the struct mutex again. Obviously
>> 		 * this is bad and so lockdep complains vehemently.
>> 		 */
>> 		pagefault_disable();
>> 		copied = __copy_from_user_inatomic(r, urelocs, count * sizeof(r[0]));
>> 		pagefault_enable();
>> 		if (unlikely(copied)) {
>> 			remain = -EFAULT;
>> 			goto out;
>> 		}
>>
>> Comment is a bit outdated since we don't use that global "struct mutex" any longer, but in any case, if there is a page fault on the mapping where we need to recurse into i915 again to satisfy if, we seem to have code already to handle it. So kmap_local conversion I *think* can't regress anything.
> 
> Thanks for your explanation!
> 
>>
>> Patch to convert the io_mapping_map_atomic_wc can indeed come later.
> 
> Okay, I will also look at this.
> 
>>
>> In terms of logistics - if we landed this series to out branch it would be queued only for 6.5. Would that work for you?
> 
> Yeah, it's ok for me. But could I ask, did I miss the 6.4 merge time?

Yes, but just because we failed to review and merge in time, not because 
you did not provide patches in time.

Regards,

Tvrtko