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

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Message-ID: <c485543e-8450-448e-9db3-d459f2096496@redhat.com>
Date: Wed, 18 Jun 2025 10:20:46 +0200
From: David Hildenbrand <david@...hat.com>
To: lizhe.67@...edance.com, jgg@...pe.ca
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 08:28, lizhe.67@...edance.com wrote:
> On Tue, 17 Jun 2025 12:22:10 -0300, jgg@...pe.ca wrote:
>   
>> Weird, but I would not expect this as a general rule, not sure we
>> should rely on it.
>>
>> I would say exported function should not get automatically
>> inlined. That throws all the kprobes into chaos :\
>>
>> BTW, why can't the other patches in this series just use
>> unpin_user_page_range_dirty_lock? The way this stuff is supposed to
>> work is to combine adjacent physical addresses and then invoke
>> unpin_user_page_range_dirty_lock() on the start page of the physical
>> range. This is why we have the gup_folio_range_next() which does the
>> segmentation in an efficient way.
>>
>> Combining adjacent physical is basically free math.
>>
>> Segmenting to folios in the vfio side doesn't make a lot of sense,
>> IMHO.
>>
>>   drivers/vfio/vfio_iommu_type1.c | 35 +++++++++++++++++++++++++++++----
>>   1 file changed, 31 insertions(+), 4 deletions(-)
>>
>> diff --git a/drivers/vfio/vfio_iommu_type1.c b/drivers/vfio/vfio_iommu_type1.c
>> index e952bf8bdfab..159ba80082a8 100644
>> --- a/drivers/vfio/vfio_iommu_type1.c
>> +++ b/drivers/vfio/vfio_iommu_type1.c
>> @@ -806,11 +806,38 @@ static long vfio_unpin_pages_remote(struct vfio_dma *dma, dma_addr_t iova,
>>   				    bool do_accounting)
>>   {
>>   	long unlocked = 0, locked = vpfn_pages(dma, iova, npage);
>> -	long i;
>>   
>> -	for (i = 0; i < npage; i++)
>> -		if (put_pfn(pfn++, dma->prot))
>> -			unlocked++;
>> +	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?
> 
> BTW, it appears that implementing unpin_user_folio_dirty_locked()
> as an inline function may not be viable for vfio, given that
> gup_put_folio() is not exported.

The compiler seems to properly inline like before, so I think we can 
keep that. @Jason correct me if I am wrong.

-- 
Cheers,

David / dhildenb