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

[Leon Romanovsky <leon@xxxxxxxxxx>]

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:

   38 enum hmm_pfn_flags {
   39         /* Output fields and flags */
   40         HMM_PFN_VALID = 1UL << (BITS_PER_LONG - 1),
   41         HMM_PFN_WRITE = 1UL << (BITS_PER_LONG - 2),
   42         HMM_PFN_ERROR = 1UL << (BITS_PER_LONG - 3),
   43         /*
   44          * Sticky flags, carried from input to output,
   45          * don't forget to update HMM_PFN_INOUT_FLAGS
   46          */
   47         HMM_PFN_DMA_MAPPED = 1UL << (BITS_PER_LONG - 4),
   48         HMM_PFN_P2PDMA     = 1UL << (BITS_PER_LONG - 5),
   49         HMM_PFN_P2PDMA_BUS = 1UL << (BITS_PER_LONG - 6),
   50
   51         HMM_PFN_ORDER_SHIFT = (BITS_PER_LONG - 11),
   52
   53         /* Input flags */
   54         HMM_PFN_REQ_FAULT = HMM_PFN_VALID,
   55         HMM_PFN_REQ_WRITE = HMM_PFN_WRITE,
   56
   57         HMM_PFN_FLAGS = ~((1UL << HMM_PFN_ORDER_SHIFT) - 1),
   58 };

So now, we just need to move HMM_PFN_ORDER_SHIFT if we add new flags
and HMM_PFN_FLAGS will be updated automatically.

Thanks