On Mon, Jun 10, 2019 at 12:52 PM Dave Hansen <dave.hansen@xxxxxxxxx> wrote: > > On 6/10/19 12:38 PM, Yu-cheng Yu wrote: > >>> When an application starts, its highest stack address is determined. > >>> It uses that as the maximum the bitmap needs to cover. > >> Huh, I didn't think we ran code from the stack. ;) > >> > >> Especially given the way that we implemented the new 5-level-paging > >> address space, I don't think that expecting code to be below the stack > >> is a good universal expectation. > > Yes, you make a good point. However, allowing the application manage the bitmap > > is the most efficient and flexible. If the loader finds a legacy lib is beyond > > the bitmap can cover, it can deal with the problem by moving the lib to a lower > > address; or re-allocate the bitmap. > > How could the loader reallocate the bitmap and coordinate with other > users of the bitmap? > > > If the loader cannot allocate a big bitmap to cover all 5-level > > address space (the bitmap will be large), it can put all legacy lib's > > at lower address. We cannot do these easily in the kernel. > > This is actually an argument to do it in the kernel. The kernel can > always allocate the virtual space however it wants, no matter how large. > If we hide the bitmap behind a kernel API then we can put it at high > 5-level user addresses because we also don't have to worry about the > high bits confusing userspace. > That's a fairly compelling argument. The bitmap is one bit per page, right? So it's smaller than the address space by a factor of 8*2^12 == 2^15. This means that, if we ever get full 64-bit linear addresses reserved entirely for userspace (which could happen if my perennial request to Intel to split user and kernel addresses completely happens), then we'll need 2^48 bytes for the bitmap, which simply does not fit in the address space of a legacy application.
