On Fri, Apr 13, 2018 at 6:05 PM, Jann Horn <jannh@xxxxxxxxxx> wrote: > On Fri, Apr 13, 2018 at 6:04 PM, Michal Hocko <mhocko@xxxxxxxxxx> wrote: >> On Fri 13-04-18 17:04:09, Jann Horn wrote: >>> On Fri, Apr 13, 2018 at 8:49 AM, Michal Hocko <mhocko@xxxxxxxxxx> wrote: >>> > On Fri 13-04-18 08:43:27, Michael Kerrisk wrote: >>> > [...] >>> >> So, you mean remove this entire paragraph: >>> >> >>> >> For cases in which the specified memory region has not been >>> >> reserved using an existing mapping, newer kernels (Linux >>> >> 4.17 and later) provide an option MAP_FIXED_NOREPLACE that >>> >> should be used instead; older kernels require the caller to >>> >> use addr as a hint (without MAP_FIXED) and take appropriate >>> >> action if the kernel places the new mapping at a different >>> >> address. >>> >> >>> >> It seems like some version of the first half of the paragraph is worth >>> >> keeping, though, so as to point the reader in the direction of a remedy. >>> >> How about replacing that text with the following: >>> >> >>> >> Since Linux 4.17, the MAP_FIXED_NOREPLACE flag can be used >>> >> in a multithreaded program to avoid the hazard described >>> >> above. >>> > >>> > Yes, that sounds reasonable to me. >>> >>> But that kind of sounds as if you can't avoid it before Linux 4.17, >>> when actually, you just have to call mmap() with the address as hint, >>> and if mmap() returns a different address, munmap() it and go on your >>> normal error path. >> >> This is still racy in multithreaded application which is the main point >> of the whole section, no? > > No, it isn't. mmap() with a hint (without MAP_FIXED) will always non-racily allocate a memory region for you or return an error code. If it does allocate a memory region, it belongs to you until you deallocate it. It might be at a different address than you requested - in that case you can emulate MAP_FIXED_NOREPLACE by calling munmap() and treating it as an error; or you can do something else with it. MAP_FIXED_NOREPLACE is just a performance optimization. -- To unsubscribe from this list: send the line "unsubscribe linux-api" in the body of a message to majordomo@xxxxxxxxxxxxxxx More majordomo info at http://vger.kernel.org/majordomo-info.html
