Re: [PATCH] arm64/mm: don't WARN when alloc/free-ing device private pages

From: John Hubbard
Date: Fri May 12 2023 - 22:06:40 EST

On 5/12/23 07:42, Ard Biesheuvel wrote:
I'm still not sure of how to make room, but working on it.

The assumption that only the linear map needs to be covered by struct
pages is rather fundamental to the arm64 mm code, as far as I am

Given that these device memory regions have no direct correspondence
with the linear map at all, couldn't we simply vmalloc() a range of
memory for struct pages for such a region and wire that up in the
existing code? That seems far more maintainable to me than
reorganizing the entire kernel VA space, and only for some choices for
the dimensions.

The vmalloc approach does sound like it should Just Work, yes. I'll try it out.

And now I'm trying to remember why Jerome didn't use that approach for x86
originally. If this fixes HMM on arm64, I'll revisit that question too.

Really appreciate the help and advice here.

John Hubbard