Re: [dpdk-dev] DDPK use of MAP_FIXED in mmap

["Xie, Huawei" <huawei.xie@intel.com>]

> -----Original Message-----
> From: dev [mailto:dev-bounces@dpdk.org] On Behalf Of Bruce Richardson
> Sent: Tuesday, December 09, 2014 2:55 AM
> To: Karmarkar Suyash
> Cc: dev@dpdk.org
> Subject: Re: [dpdk-dev] DDPK use of MAP_FIXED in mmap
> 
> On Mon, Dec 08, 2014 at 07:02:38PM +0000, Karmarkar Suyash wrote:
> > Hello,
> >
> > In DPDK when we use mmap why are we passing the MAP_FIXED flag when
> Linux man page itself says that the option is discouraged? Any specific reason
> for passing the MAP_FIXED flag?
> >
> >
> > http://linux.die.net/man/2/mmap
> >
> > MAP_FIXED
> > Don't interpret addr as a hint: place the mapping at exactly that address. addr
> must be a multiple of the page size. If the memory region specified by addr and
> len overlaps pages of any existing mapping(s), then the overlapped part of the
> existing mapping(s) will be discarded. If the specified address cannot be used,
> mmap() will fail. Because requiring a fixed address for a mapping is less portable,
> the use of this option is discouraged.
> >
> >
> > Regards
> > Suyash Karmarkar
> 
> I won't comment on every occurance of "MAP_FIXED" in DPDK, but it's main use
> is
> when mapping the hugepages into memory inside EAL init. In this case, we are
> ok to
> use it, as we take good care to ensure that our mapping space is free. What we
> do
> is, once we know how many contiguous hugepages we need to map, we request
> a mapping
> from /dev/zero for that particular size. We then record the address of the
> mapping
> we get, and then unmap /dev/zero again - thereby freeing up the entire address
> range. At this point, we then use MAP_FIXED to explicitly mmap in the
> hugepages
> into this region that we have just freed up - thereby guaranteeing contiguous
> hugepage mappings in the correct order. [The reason for doing things this way is
> that we found on some systems - particularly with 32-bit code, the regular
> mmaps
> of pages we being done in reverse order, meaning each page became it's own
> segment].
> 
> On the other hand, it's also good to note where we don't use MAP_FIXED. We
> don't
> use map fixed when initializing a secondary process and are mapping the
> hugepage
> memory into it. In this case, although we know where the memory has to be
> placed,
> we don't know if it is safe to use or not. Instead of using MAP_FIXED, we instead
> hint to the kernel our preferred address and check if the request was satisfied
> at that address.
> 
> Hope this clarifies things a bit,

Don't know if kernel always use the hinted address if the target region is free for mmap.
The safe way is if we are absolutely sure the region is free(like through checking maps file), use MMAP_FIXED(in secondary process).

> /Bruce