On Thu, Apr 12, 2018 at 8:59 PM, John Hubbard <jhubbard@xxxxxxxxxx> wrote: > On 04/12/2018 11:49 AM, Jann Horn wrote: >> On Thu, Apr 12, 2018 at 8:37 PM, Michael Kerrisk (man-pages) >> <mtk.manpages@xxxxxxxxx> wrote: >>> Hi John, >>> >>> On 12 April 2018 at 20:33, John Hubbard <jhubbard@xxxxxxxxxx> wrote: >>>> On 04/12/2018 08:39 AM, Jann Horn wrote: >>>>> Clarify that MAP_FIXED is appropriate if the specified address range has >>>>> been reserved using an existing mapping, but shouldn't be used otherwise. >>>>> >>>>> Signed-off-by: Jann Horn <jannh@xxxxxxxxxx> >>>>> --- >>>>> man2/mmap.2 | 19 +++++++++++-------- >>>>> 1 file changed, 11 insertions(+), 8 deletions(-) >>>>> >>>>> diff --git a/man2/mmap.2 b/man2/mmap.2 >> [...] >>>>> .IP >>>>> For example, suppose that thread A looks through >>>>> @@ -284,13 +285,15 @@ and the PAM libraries >>>>> .UR http://www.linux-pam.org >>>>> .UE . >>>>> .IP >>>>> -Newer kernels >>>>> -(Linux 4.17 and later) have a >>>>> +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 >>>>> .B MAP_FIXED_NOREPLACE >>>>> -option that avoids the corruption problem; if available, >>>>> -.B MAP_FIXED_NOREPLACE >>>>> -should be preferred over >>>>> -.BR MAP_FIXED . >>>>> +that should be used instead; older kernels require the caller to use >>>>> +.I addr >>>>> +as a hint (without >>>>> +.BR MAP_FIXED ) >>>> >>>> Here, I got lost: the sentence suddenly jumps into explaining non-MAP_FIXED >>>> behavior, in the MAP_FIXED section. Maybe if you break up the sentence, and >>>> possibly omit non-MAP_FIXED discussion, it will help. >>> >>> Hmmm -- true. That piece could be a little clearer. >> >> How about something like this? >> >> For cases in which MAP_FIXED can not be used because >> 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 and take appropriate action if >> the kernel places the new mapping at a different address. >> >> John, Michael, what do you think? > > > I'm still having difficulty with it, because this is in the MAP_FIXED section, > but I think you're documenting the behavior that you get if you do *not* > specify MAP_FIXED, right? Also, the hint behavior is true of both older and > new kernels... The manpage patch you and mhocko wrote mentioned MAP_FIXED_NOREPLACE in the MAP_FIXED section - I was trying to avoid undoing a change you had just explicitly made. > So, if that's your intent (you want to sort of document by contrast to what > would happen if this option were not used), then how about something like this: > > > Without the MAP_FIXED option, the kernel would treat addr as a hint, rather > than a requirement, and the caller would need to take appropriate action > if the kernel placed the mapping at a different address. (For example, > munmap and try again.) I'd be fine with removing the paragraph. As you rightly pointed out, it doesn't really describe MAP_FIXED.
