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

[Leon Romanovsky <leon@kernel.org>]

On Thu, Apr 24, 2025 at 03:50:52PM +0300, Leon Romanovsky wrote:
> On Thu, Apr 24, 2025 at 09:07:03AM -0300, Jason Gunthorpe wrote:
> > 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:
> 
> I can do it too, will change.

If you don't mind, I'll stick with my previous proposal.

GENMASK() alone is not enough and the best solution will include use
of FIELD_GET FIELD_PREP mocros. IMHO, that will make code unreadable.
The simple, clean and reliable bitfield OR operations much better fit
here.

Thanks

> 
> > 
> > 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
>