On Sat, Dec 2, 2017 at 4:05 PM, Matthew Wilcox <willy@xxxxxxxxxxxxx> wrote: > On Fri, Dec 01, 2017 at 06:16:26PM -0800, john.hubbard@xxxxxxxxx wrote: >> MAP_FIXED has been widely used for a very long time, yet the man >> page still claims that "the use of this option is discouraged". > > I think we should continue to discourage the use of this option, but > I'm going to include some of your text in my replacement paragraph ... > > -Because requiring a fixed address for a mapping is less portable, > -the use of this option is discouraged. > +The use of this option is discouraged because it forcibly unmaps any > +existing mapping at that address. Programs which use this option need > +to be aware that their memory map may change significantly from one run to > +the next, depending on library versions, kernel versions and random numbers. How about adding something explicit about when it's okay to use MAP_FIXED? "This option should only be used to displace an existing mapping that is controlled by the caller, or part of such a mapping." or something like that? > +In a threaded process, checking the existing mappings can race against > +a new dynamic library being loaded malloc() and its various callers can also cause mmap() calls, which is probably more relevant than library loading.
