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

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Message-ID: <20250618062820.8477-1-lizhe.67@bytedance.com>
Date: Wed, 18 Jun 2025 14:28:20 +0800
From: lizhe.67@...edance.com
To: jgg@...pe.ca,
	david@...hat.com
Cc: akpm@...ux-foundation.org,
	alex.williamson@...hat.com,
	kvm@...r.kernel.org,
	linux-kernel@...r.kernel.org,
	linux-mm@...ck.org,
	lizhe.67@...edance.com,
	peterx@...hat.com
Subject: Re: [PATCH v4 2/3] gup: introduce unpin_user_folio_dirty_locked()

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.

Thanks,
Zhe