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

[Jason Gunthorpe]

On Wed, Apr 23, 2025 at 09:37:24PM +0300, Mika Penttilä wrote:
> 
> On 4/23/25 21:17, Jason Gunthorpe wrote:
> > On Wed, Apr 23, 2025 at 08:54:05PM +0300, Mika Penttilä wrote:
> >>> @@ -36,6 +38,13 @@ enum hmm_pfn_flags {
> >>>  	HMM_PFN_VALID = 1UL << (BITS_PER_LONG - 1),
> >>>  	HMM_PFN_WRITE = 1UL << (BITS_PER_LONG - 2),
> >>>  	HMM_PFN_ERROR = 1UL << (BITS_PER_LONG - 3),
> >>> +
> >>> +	/*
> >>> +	 * Sticky flags, carried from input to output,
> >>> +	 * don't forget to update HMM_PFN_INOUT_FLAGS
> >>> +	 */
> >>> +	HMM_PFN_DMA_MAPPED = 1UL << (BITS_PER_LONG - 7),
> >>> +
> >> How is this playing together with the mapped order usage?
> > Order shift starts at bit 8, DMA_MAPPED is at bit 7
> 
> hmm bits are the high bits, and order is 5 bits starting from
> (BITS_PER_LONG - 8)

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.

Probably testing didn't hit this because the usual 2M order of 9 only
sets bits -4 and -8 .. The way the order works it doesn't clear the
0 bits, which I wonder if is a little bug..

Jason