Re: [PATCH v9 10/24] mm/hmm: let users to tag specific PFN with DMA mapped bit

[Jason Gunthorpe]

On Thu, Apr 24, 2025 at 11:46:26AM +0300, Leon Romanovsky wrote:
> On Thu, Apr 24, 2025 at 10:11:01AM +0200, Christoph Hellwig wrote:
> > On Thu, Apr 24, 2025 at 11:07:44AM +0300, Leon Romanovsky wrote:
> > > > I see, so yes order occupies 5 bits [-4,-5,-6,-7,-8] and the
> > > > DMA_MAPPED overlaps, it should be 9 not 7 because of the backwardness.
> > > 
> > > Thanks for the fix.
> > 
> > Maybe we can use the chance to make the scheme less fragile?  i.e.
> > put flags in the high bits and derive the first valid bit from the
> > pfn order?
>
> It can be done too. This is what I got:

Use genmask:

enum hmm_pfn_flags {
	HMM_FLAGS_START = BITS_PER_LONG - PAGE_SHIFT,
	HMM_PFN_FLAGS = GENMASK(BITS_PER_LONG - 1, HMM_FLAGS_START),

	/* Output fields and flags */
	HMM_PFN_VALID = 1UL << HMM_FLAGS_START + 0,
	HMM_PFN_WRITE = 1UL << HMM_FLAGS_START + 1,
	HMM_PFN_ERROR = 1UL << HMM_FLAGS_START + 2,
	HMM_PFN_ORDER_MASK = GENMASK(HMM_FLAGS_START + 7, HMM_FLAGS_START + 3),

	/* Input flags */
	HMM_PFN_REQ_FAULT = HMM_PFN_VALID,
	HMM_PFN_REQ_WRITE = HMM_PFN_WRITE,
};

Jason