On 17 August 2016 at 13:12, Christopher Covington <c...@codeaurora.org> wrote: > > > On August 17, 2016 6:30:06 AM EDT, Catalin Marinas <catalin.mari...@arm.com> > wrote: >>On Tue, Aug 16, 2016 at 02:32:29PM -0400, Christopher Covington wrote: >>> Some userspace applications need to know the maximum virtual address >>they can >>> use (TASK_SIZE). >> >>Just curious, what are the cases needing TASK_SIZE in user space? > > Checkpoint/Restore In Userspace and the Mozilla Javascript Engine > https://bugzilla.mozilla.org/show_bug.cgi?id=1143022 are the specific cases > I've run into. I've heard LuaJIT might have a similar situation. In general I > think making allocations from the top down is a shortcut for finding a large > unused region of memory. >
One aspect of this that I would like to discuss is whether the current practice makes sense, of tying TASK_SIZE to whatever the size of the kernel VA space is. I could imagine simply limiting the user VA space to 39-bits (or even 36-bits, depending on how deeply we care about 16 KB pages), and implement an arch specific hook (prctl() perhaps?) to increase TASK_SIZE on demand. That would not only give us a reliable way to check whether this is supported (i.e., the prctl() would return error if it isn't), it also allows for some optimizations, since a 48-bit VA kernel can run all processes using 3 levels with relative ease (and switching between 4levels and 3levels processes would also be possible, but would either require a TLB flush, or would result in this optimization to be disabled globally, whichever is less costly in terms of performance) -- Ard.