linux-kernel - Re: [PATCH v4 2/3] gup: introduce unpin_user_folio_dirty

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Message-ID: <a1d62bf1-59e5-4dd5-926a-d6cdddf3deb5@redhat.com>
Date: Wed, 18 Jun 2025 13:42:09 +0200
From: David Hildenbrand <david@...hat.com>
To: Jason Gunthorpe <jgg@...pe.ca>, lizhe.67@...edance.com
Cc: akpm@...ux-foundation.org, alex.williamson@...hat.com,
 kvm@...r.kernel.org, linux-kernel@...r.kernel.org, linux-mm@...ck.org,
 peterx@...hat.com
Subject: Re: [PATCH v4 2/3] gup: introduce unpin_user_folio_dirty_locked()

On 18.06.25 13:40, David Hildenbrand wrote:
> On 18.06.25 13:36, Jason Gunthorpe wrote:
>> On Wed, Jun 18, 2025 at 02:28:20PM +0800, lizhe.67@...edance.com wrote:
>>> On Tue, 17 Jun 2025 12:22:10 -0300, jgg@...pe.ca wrote:
>>>> +	while (npage) {
>>>> +		long nr_pages = 1;
>>>> +
>>>> +		if (!is_invalid_reserved_pfn(pfn)) {
>>>> +			struct page *page = pfn_to_page(pfn);
>>>> +			struct folio *folio = page_folio(page);
>>>> +			long folio_pages_num = folio_nr_pages(folio);
>>>> +
>>>> +			/*
>>>> +			 * For a folio, it represents a physically
>>>> +			 * contiguous set of bytes, and all of its pages
>>>> +			 * share the same invalid/reserved state.
>>>> +			 *
>>>> +			 * Here, our PFNs are contiguous. Therefore, if we
>>>> +			 * detect that the current PFN belongs to a large
>>>> +			 * folio, we can batch the operations for the next
>>>> +			 * nr_pages PFNs.
>>>> +			 */
>>>> +			if (folio_pages_num > 1)
>>>> +				nr_pages = min_t(long, npage,
>>>> +					folio_pages_num -
>>>> +					folio_page_idx(folio, page));
>>>> +
>>>> +			unpin_user_folio_dirty_locked(folio, nr_pages,
>>>> +					dma->prot & IOMMU_WRITE);
>>>
>>> Are you suggesting that we should directly call
>>> unpin_user_page_range_dirty_lock() here (patch 3/3) instead?
>>
>> I'm saying you should not have the word 'folio' inside the VFIO. You
>> accumulate a contiguous range of pfns, by only checking the pfn, and
>> then call
>>
>> unpin_user_page_range_dirty_lock(pfn_to_page(first_pfn)...);
>>
>> No need for any of this. vfio should never look at the struct page
>> except as the last moment to pass the range.
> 
> Hah, agreed, that's actually simpler and there is no need to factor
> anything out.

Ah, no, wait, the problem is that we don't know how many pages we can 
supply, because there might be is_invalid_reserved_pfn() in the range ...

-- 
Cheers,

David / dhildenb