+mmap maintainers (maybe mm/mremap.c should be added to the file pattern for "MEMORY MAPPING" in "MAINTAINERS"? I'm not sure) On Fri, Dec 6, 2024 at 4:20 PM Brian Geffon <bgeffon@xxxxxxxxxx> wrote: > mmap(2) allows for a destination address to be specified without > MAP_FIXED and in this situation it's a hint to get_unmapped_area(). > This address need not be page aligned because get_unmapped_area() will > align the hint. > > In the case of mremap(2) with MREMAP_DONTUNMAP it shares a code path > with MREMAP_FIXED in mremap_to(), which means this function can be > called in 3 different scenarios: MREMAP_FIXED only, MREMAP_DONTUNMAP > only, or MREMAP_FIXED | MREMAP_DONTUNMAP. In the second case when only > MREMAP_DONTUNMAP is specified we don't need to do alignment or size > checks on newaddr because they will be passed to get_unmapped_area() and > dealt with appropriately. > > This patch corrects that behavior to match what non-MREMAP_DONTUNMAP > mremap(2) and mmap(2) do. This odd behavioral difference was reported by > Marco Vanotti. Additionally, I've included a self test to validate this > behavior. Marco pointed me to this; I had no idea mremap() had this undocumented behavior where it takes a hint address. The mremap() manpage is currently wrong about this, it sort of implies that the new_address argument is only used if MREMAP_FIXED is set. Marco also noticed that upstream glibc now assumes this behavior: https://sourceware.org/git/?p=glibc.git;a=commit;h=6c40cb0e9f893d49dc7caee580a055de53562206 Debian also has a test that explicitly checks for this behavior: https://sources.debian.org/src/glibc/2.40-4/debian/patches/git-updates.diff/?hl=22820#L22818 I guess it's too late to remove that behavior at this point, and the right thing to do is to update the manpage?
