On Tue, Aug 22, 2023 at 12:11 PM Richard Henderson < richard.hender...@linaro.org> wrote:
> On 8/22/23 11:03, Warner Losh wrote: > > Hmm, bug with linux-user as well, because here we should > re-establish the reserved_va > > reservation. > > > > > > ... of the shared memory region we just detached? Right? > > Correct. > > On a related note, on FreeBSD is there any practical difference between > > PROT_NONE, MAP_ANON > and > PROT_NONE, MAP_GUARD > > for large memory regions? > They do different things. MAP_ANON maps the memory without a backing device. This means it allocates the VA space right away, but lazily allocates the backing pages as the pages are dirtied. MAP_GUARD creates the VA mapping, but never maps any pages to those pages (well, until it's remapped). Any read/write/exec access to MAP_GUARD pages results in a SIGSEGV. > I ask since FreeBSD doesn't have MAP_NORESERVE, which Linux uses to avoid > allocation of > gigabytes > Yea. It sounds like MAP_NORESERVE is what FreeBSD's default behavior is: We don't allocate backing store in the swap areas until there's memory pressure. You can safely allocate GB of space with MAP_ANON and get similar behavior to the MAP_NORESERVE. MAP_GUARD could be used if you wanted to reserve the VA space, but didn't want to assign anything to the VA space until later. As a practical matter, they both consume about the same resources until the MAP_ANON region starts to get populated with data... With PROT_NONE, I think they would have the same effect. If it is to be a backing store for something like malloc, then MAP_ANON would be best. If you are replacing it with a lot of things, like a mix of files, devices and/or anon memory, then MAP_GUARD and replace it with MAP_FIXED later. Most likely you'll want MAP_GUARD to reserve the area, and then MAP_FIXED to use it for mmap'd memory, shared memory, executable pages, etc. Does that tell you what you need to know? Warner > r~ >
