linux-kernel - Re: [PATCH v5 1/7] udmabuf: pre-fault when first page fault

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Message-ID: <55af756d-5002-4534-857b-f3ef3c8e34e6@vivo.com>
Date: Fri, 6 Sep 2024 16:37:18 +0800
From: Huan Yang <link@...o.com>
To: "Kasireddy, Vivek" <vivek.kasireddy@...el.com>,
 Sumit Semwal <sumit.semwal@...aro.org>,
 Christian König <christian.koenig@....com>,
 Gerd Hoffmann <kraxel@...hat.com>,
 "linux-media@...r.kernel.org" <linux-media@...r.kernel.org>,
 "dri-devel@...ts.freedesktop.org" <dri-devel@...ts.freedesktop.org>,
 "linaro-mm-sig@...ts.linaro.org" <linaro-mm-sig@...ts.linaro.org>,
 "linux-kernel@...r.kernel.org" <linux-kernel@...r.kernel.org>
Cc: "opensource.kernel@...o.com" <opensource.kernel@...o.com>
Subject: Re: [PATCH v5 1/7] udmabuf: pre-fault when first page fault


在 2024/9/6 16:12, Kasireddy, Vivek 写道:
> Hi Huan,
>
>> Subject: [PATCH v5 1/7] udmabuf: pre-fault when first page fault
>>
>> The current udmabuf mmap uses a page fault to populate the vma.
>>
>> However, the current udmabuf has already obtained and pinned the folio
>> upon completion of the creation.This means that the physical memory has
>> already been acquired, rather than being accessed dynamically.
>>
>> As a result, the page fault has lost its purpose as a demanding
>> page. Due to the fact that page fault requires trapping into kernel mode
>> and filling in when accessing the corresponding virtual address in mmap,
>> when creating a large size udmabuf, this represents a considerable
>> overhead.
>>
>> This patch first the pfn into page table, and then pre-fault each pfn
>> into vma, when first access. Should know, if anything wrong when
>> pre-fault, will not report it's error, else, report when task access it
>> at the first time.
>>
>> Suggested-by: Vivek Kasireddy <vivek.kasireddy@...el.com>
>> Signed-off-by: Huan Yang <link@...o.com>
>> ---
>>   drivers/dma-buf/udmabuf.c | 35 +++++++++++++++++++++++++++++++++--
>>   1 file changed, 33 insertions(+), 2 deletions(-)
>>
>> diff --git a/drivers/dma-buf/udmabuf.c b/drivers/dma-buf/udmabuf.c
>> index 047c3cd2ceff..0a8c231a36e1 100644
>> --- a/drivers/dma-buf/udmabuf.c
>> +++ b/drivers/dma-buf/udmabuf.c
>> @@ -43,7 +43,8 @@ static vm_fault_t udmabuf_vm_fault(struct vm_fault
>> *vmf)
>>   	struct vm_area_struct *vma = vmf->vma;
>>   	struct udmabuf *ubuf = vma->vm_private_data;
>>   	pgoff_t pgoff = vmf->pgoff;
>> -	unsigned long pfn;
>> +	unsigned long addr, end, pfn;
>> +	vm_fault_t ret;
>>
>>   	if (pgoff >= ubuf->pagecount)
>>   		return VM_FAULT_SIGBUS;
>> @@ -51,7 +52,37 @@ static vm_fault_t udmabuf_vm_fault(struct vm_fault
>> *vmf)
>>   	pfn = folio_pfn(ubuf->folios[pgoff]);
>>   	pfn += ubuf->offsets[pgoff] >> PAGE_SHIFT;
>>
>> -	return vmf_insert_pfn(vma, vmf->address, pfn);
>> +	ret = vmf_insert_pfn(vma, vmf->address, pfn);
>> +	if (ret & VM_FAULT_ERROR)
>> +		return ret;
>> +
>> +	/* pre fault */
>> +	pgoff = vma->vm_pgoff;
>> +	end = vma->vm_end;
> Nit: use vma->vm_end directly in the loop below, as end is used only once.
>
>> +	addr = vma->vm_start;
>> +
>> +	for (; addr < end; pgoff++, addr += PAGE_SIZE) {
>> +		if (addr == vmf->address)
>> +			continue;
>> +
>> +		if (WARN_ON(pgoff >= ubuf->pagecount))
>> +			break;
>> +
>> +		pfn = folio_pfn(ubuf->folios[pgoff]);
>> +
> Nit: no need for a blank line here.
>
>> +		pfn += ubuf->offsets[pgoff] >> PAGE_SHIFT;
>> +
>> +		/**
>> +		 * If something wrong, due to this vm fault success,
>> +		 * do not report in here, report only when true access
>> +		 * this addr.
>> +		 * So, don't update ret here, just break.
> Please rewrite the above comments as:
> * If the below vmf_insert_pfn() fails, we do not return an error here
> * during this pre-fault step. However, an error will be returned if the
> * failure occurs when the addr is truly accessed.
Thank you
>
> With that,

OK.

Thanks

> Acked-by: Vivek Kasireddy <vivek.kasireddy@...el.com>
>
>> +		 */
>> +		if (vmf_insert_pfn(vma, addr, pfn) & VM_FAULT_ERROR)
>> +			break;
>> +	}
>> +
>> +	return ret;
>>   }
>>
>>   static const struct vm_operations_struct udmabuf_vm_ops = {
>> --
>> 2.45.2