Re: [dpdk-dev] [PATCH v2 2/5] mem: use address hint for mapping hugepages

[Alejandro Lucero <alejandro.lucero@netronome.com>]

On Thu, Oct 4, 2018 at 1:08 PM Burakov, Anatoly <anatoly.burakov@intel.com>
wrote:

> On 04-Oct-18 12:43 PM, Alejandro Lucero wrote:
> >
> >
> > On Wed, Oct 3, 2018 at 1:50 PM Burakov, Anatoly
> > <anatoly.burakov@intel.com <mailto:anatoly.burakov@intel.com>> wrote:
> >
> >     On 31-Aug-18 1:50 PM, Alejandro Lucero wrote:
> >      > Linux kernel uses a really high address as starting address for
> >      > serving mmaps calls. If there exist addressing limitations and
> >      > IOVA mode is VA, this starting address is likely too high for
> >      > those devices. However, it is possible to use a lower address in
> >      > the process virtual address space as with 64 bits there is a lot
> >      > of available space.
> >      >
> >      > This patch adds an address hint as starting address for 64 bits
> >      > systems.
> >      >
> >      > Signed-off-by: Alejandro Lucero <alejandro.lucero@netronome.com
> >     <mailto:alejandro.lucero@netronome.com>>
> >      > ---
> >
> >     <snip>
> >
> >      >
> >      >               mapped_addr = mmap(requested_addr, (size_t)map_sz,
> >     PROT_READ,
> >      >                               mmap_flags, -1, 0);
> >      > +
> >      >               if (mapped_addr == MAP_FAILED && allow_shrink)
> >
> >     Unintended whitespace change?
> >
> >
> > Yes. I'll fix it.
> >
> >      >                       *size -= page_sz;
> >      > -     } while (allow_shrink && mapped_addr == MAP_FAILED && *size
> >      > 0);
> >      > +
> >      > +             if (mapped_addr != MAP_FAILED && addr_is_hint &&
> >      > +                 mapped_addr != requested_addr) {
> >      > +                     /* hint was not used. Try with another
> >     offset */
> >      > +                     munmap(mapped_addr, map_sz);
> >      > +                     mapped_addr = MAP_FAILED;
> >      > +                     next_baseaddr = RTE_PTR_ADD(next_baseaddr,
> >     0x100000000);
> >
> >     Why not increment by page size? Sure, it could take some more time to
> >     allocate, but will result in less wasted memory.
> >
> >
> > I though the same or even using smaller increments than hugepage size.
> > Increment the address in such amount does not mean we are wasting memory
> > but just leaving space if some mmap fails. I think it is better to leave
> > as much as space as possible just in case the data allocated in the
> > conflicted area would need to grow in the future.
>
> Not sure i follow. Could you give an example of a scenario where leaving
> huge chunks of memory free would be preferable to just adding page size
> and starting from page-size-aligned address next time we allocate?
>
>
Usually there is nothing at 4GB address in 64 bit processes, usually the
text section being the first process region mapped and currently at far
higher than 4GB. If there is something mapped there before executing the
EAL hugepage/memory initialization code, not sure what it will be for, but
maybe it needs to grow using contiguous virtual addresses. As I say, no
idea what this could be used for, but the shorter the space when trying
again in this code, the less likely that flexibility could be there.

Maybe making the increment smaller just makes sense for virtual address
space randomization for security reasons.

Anyway, there is a lot of space with 64 bits, and, IMHO, this should not be
a problem while the increment is negligible against 64 bits address space
size, and 4GB are so negligible in this case as 4 bytes are to 4GB.


> >
> >     --
> >     Thanks,
> >     Anatoly
> >
>
>
> --
> Thanks,
> Anatoly
>