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

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Message-ID: <20250618114629.GL1376515@ziepe.ca>
Date: Wed, 18 Jun 2025 08:46:29 -0300
From: Jason Gunthorpe <jgg@...pe.ca>
To: David Hildenbrand <david@...hat.com>
Cc: lizhe.67@...edance.com, 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 Wed, Jun 18, 2025 at 01:42:09PM +0200, David Hildenbrand wrote:
> 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 ...

You stop batching when you hit any invalid_reserved_pfn and flush it.

It still has to check read back and check every PFN to make sure it is
contiguous, checking reserved too is not a problemm.

Jason