On Wed 29-11-17 19:16:39, John Hubbard wrote: [...] > Hi Michal, > > I've taken the liberty of mostly rewriting this part, in order to more > closely > match the existing paragraphs; to fix minor typos; and to attempt to slightly > clarify the paragraph. > > +.BR MAP_FIXED_SAFE " (since Linux 4.16)" > +Similar to MAP_FIXED with respect to the > +.I > +addr > +enforcement, but different in that MAP_FIXED_SAFE never clobbers a > pre-existing > +mapped range. If the requested range would collide with an existing > +mapping, then this call fails with > +.B EEXIST. > +This flag can therefore be used as a way to atomically (with respect to other > +threads) attempt to map an address range: one thread will succeed; all others > +will report failure. Please note that older kernels which do not recognize > this > +flag will typically (upon detecting a collision with a pre-existing mapping) > +fall back a "non-MAP_FIXED" type of behavior: they will return an address > that > +is different than the requested one. Therefore, backward-compatible software > +should check the returned address against the requested address. > +.TP
I have taken yours. Thanks a lot! > (I'm ignoring the naming, because there is another thread about that, > so please just the above as "MAP_FIXED_whatever-is-chosen".) > > > @@ -449,6 +461,12 @@ is not a valid file descriptor (and > > .B MAP_ANONYMOUS > > was not set). > > .TP > > +.B EEXIST > > +range covered by > > +.IR addr , > > nit: trailing space on the above line. fixed > > +.IR length > > +is clashing with an existing mapping. > > +.TP > > .B EINVAL > > We don't like > > .IR addr , > > > > One other thing: reading through mmap.2, I now want to add this as well: > > diff --git a/man2/mmap.2 b/man2/mmap.2 > index 622a7000d..780cad6d9 100644 > --- a/man2/mmap.2 > +++ b/man2/mmap.2 > @@ -222,20 +222,25 @@ part of the existing mapping(s) will be discarded. > If the specified address cannot be used, > .BR mmap () > will fail. > -Because requiring a fixed address for a mapping is less portable, > -the use of this option is discouraged. > +Software that aspires to be as portable as possible should use this option > with > +care, keeping in mind that different kernels and C libraries may set up quite > +different mapping ranges. > > > ...because that advice is just wrong (it presumes that "less portable" == > "must be discouraged"). > > Should I send out a separate patch for that, or is it better to glom it > together > with this one? yes please -- Michal Hocko SUSE Labs
