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

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Message-ID: <20250619123504.GA1643390@ziepe.ca>
Date: Thu, 19 Jun 2025 09:35:04 -0300
From: Jason Gunthorpe <jgg@...pe.ca>
To: lizhe.67@...edance.com
Cc: david@...hat.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 Thu, Jun 19, 2025 at 05:05:42PM +0800, lizhe.67@...edance.com wrote:

> As I understand it, there seem to be some issues with this
> implementation. How can we obtain the value of dma->has_reserved
> (acquiring it within vfio_pin_pages_remote() might be a good option)

Yes, you record it during vfio_pin_pages operations. If VFIO call
iommu_map on something that went down the non-GUP path then it sets
the flag.

> and ensure that this value remains unchanged from the time of
> assignment until we perform the unpin operation? 

Map/unmap are paired and not allowed to race so that isn't an issue.

> I've searched through the code and it appears that there are
> instances where SetPageReserved() is called outside of the
> initialization phase.  Please correct me if I am wrong.

It should not be relevant here, pages under use by VFIO are not
permitted to change it will break things.

Jason