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It can be used by file > backends to report mapping order hints. > > By default, Linux assumed we will map in PAGE_SIZE chunks. With this hint, > the driver can report the possibility of mapping chunks that are larger > than PAGE_SIZE. Then, the VA allocator will try to use that as alignment > when allocating the VA ranges. > > This is useful because when chunks to be mapped are larger than PAGE_SIZE, > VA alignment matters and it needs to be aligned with the size of the chunk > to be mapped. > > Said that, no matter what is the alignment used for the VA allocation, the > driver can still decide which size to map the chunks. It is also not an > issue if it keeps mapping in PAGE_SIZE. > > get_mapping_order() is defined to take three parameters. Besides the 1st > parameter which will be the file object pointer, the 2nd + 3rd parameters > being the pgoff + size of the mmap() request. Its retval is defined as the > order, which must be non-negative to enable the alignment. When zero is > returned, it should behave like when the hint is not provided, IOW, > alignment will still be PAGE_SIZE. This should explain how it works when the incoming pgoff is not aligned.. I think for dpdk we want to support mapping around the MSI hole so something like pgoff 0 -> 2M skip 4k 2m + 4k -> 64M Should setup the last VMA to align to 2M + 4k so the first PMD is fragmented to 4k pages but the remaning part is 2M sized or better. We just noticed a bug very similer to this in qemu around it's manual alignment scheme where it would de-align things around the MSI window and spoil the PMDs. I guess ideally the file could return the order assuming an aligned-to-start pgoff and the core code could use that order to compute an adjustment for the actual pgoff so we maintain: va % order = pgoff % order Jason