On 11/29/2017 06:45 AM, Michal Hocko wrote: > From: Michal Hocko <mhocko@xxxxxxxx> > > 4.16+ kernels offer a new MAP_FIXED_SAFE flag which allows to atomicaly "allows the caller to atomically" , if you care about polishing the commit message...see the real review, below. :) > probe for a given address range. > > Signed-off-by: Michal Hocko <mhocko@xxxxxxxx> > --- > man2/mmap.2 | 18 ++++++++++++++++++ > 1 file changed, 18 insertions(+) > > diff --git a/man2/mmap.2 b/man2/mmap.2 > index 385f3bfd5393..622a7000de83 100644 > --- a/man2/mmap.2 > +++ b/man2/mmap.2 > @@ -225,6 +225,18 @@ will fail. > Because requiring a fixed address for a mapping is less portable, > the use of this option is discouraged. > .TP > +.B MAP_FIXED_SAFE (since 4.16) > +Similar to MAP_FIXED wrt. to the > +.I > +addr > +enforcement except it never clobbers a colliding mapped range and rather fail with > +.B EEXIST > +in such a case. This flag can therefore be used as a safe and atomic probe for the > +the specific address range. Please note that older kernels which do not recognize > +this flag can fallback to the hint based implementation and map to a different > +location. Any backward compatible software should therefore check the returning > +address with the given one. > +.TP > .B MAP_GROWSDOWN > This flag is used for stacks. > It indicates to the kernel virtual memory system that the mapping 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'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. > +.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? thanks, John Hubbard NVIDIA -- 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
