On Thu, May 18, 2023 at 11:03:14 +0200, Alex Colomar wrote: > On 5/18/23 10:53, Alex Colomar wrote: > > Hi! > > > > I believe mmap(2) is confusing (see the emboldened parts): > > > > DESCRIPTION > > mmap() creates a new mapping in the virtual address space of > > the calling process. **The starting address for the new mapping > > is specified in addr.** The length argument specifies the length > > of the mapping (which must be greater than 0). > > > > If addr is NULL, then the kernel chooses the (page‐aligned) ad‐ > > dress at which to create the mapping; this is the most portable > > method of creating a new mapping. **If addr is not NULL, then > > the kernel takes it as a hint about where to place the mapping; > > on Linux, the kernel will pick a nearby page boundary** (but al‐ > > ways above or equal to the value specified by > > /proc/sys/vm/mmap_min_addr) and attempt to create the mapping > > there. If another mapping already exists there, the kernel > > picks a new address that may or may not depend on the hint. > > The address of the new mapping is returned as the result of the > > call. > > > > > > The first emboldened sentence contradicts the second. It's not the > > starting address, but just a hint to find a nearby (>=) starting > > I was a bit wrong there. It's >= mmap_min_addr, not >= hint. But the > suggestion below holds. > > > address. How about saying this?: > > > > A hint for the starting address for the new mapping is specified in > > addr. > > I think describing the default behavior first is a good idea. If a user needs an exact address they can read further for MAP_FIXED/_NOREPLACE (and the potential footguns involved). - Oskari
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