[Tarantool-patches] [PATCH luajit 2/2] Linux/ARM64: Make mremap() non-moving due to VA space woes.

[Sergey Kaplun]

Hi, Sergos!

Thanks for the review!

On 01.08.21, Sergey Ostanevich wrote:
> Hi! Thanks for the patch!
> 
> Now direct_resize() will fail if it doesn't fit into the current place
> with a new size and a direct_alloc() supposedly will be called. This one 
> doesn't help with 47-bit address AFAIU since it has no extra option -
> I doubt it exist at all - to ask kernel to fit.
>  
> So, how it helps?

For arm64 LJ_64 mode is enabled.
It means LJ_ALLOC_MMAP_PROBE is defined and for each huge allocation
`mmap_probe()` is called. This function tries several allocations
unless the limit (30) is reached, or any suitable address is found.

> 
> 
> Regards,
> Sergos
> 
> 
> > On 1 Aug 2021, at 13:36, Igor Munkin <imun at tarantool.org> wrote:
> > 
> > Sergey,
> > 
> > Thanks for the fixes! LGTM, except the single typo.
> > 
> > On 28.07.21, Sergey Kaplun wrote:
> > 
> > <snipped>
> > 
> >> 
> >> The new commit message is the following:
> >> 
> >> ===================================================================
> >> Linux/ARM64: Make mremap() non-moving due to VA space woes.
> >> 
> >> This reduces overall performance on ARM64, but we have no choice.
> >> Linux kernel default userspace VA is 48 bit, but we'd need 47 bit.
> >> mremap() ignores address hints due to a kernel API issue. The mapping
> >> may move to an undesired address which will cause an assert or crash.
> >> 
> >> Reported by Raymond W. Ko.
> >> 
> >> (cherry picked from commit 67dbec82f4f05a416a78a560a726553beaa7a223)
> >> 
> >> 47-bit VA space is required by LuaJIT for keeping a GC object pointer in
> >> TValue. In case of huge blobs that are mapped directly, `mremap()` may
> >> move the chunk out of 47-bit range of VA space on ARM64. `mremap()`
> >> accepts the fifth argument (new address hint) only with MREMAP_FIXED
> >> flag. In that case it unmaps any other mapping to specified address.
> >> 
> >> To avoid this behaviour this patch restricts `mremap()` to relocate
> >> the mapping to a new virtual address by set CALL_MREMAP_NOMOVE flag
> > 
> > Typo: s/set/setting/.
> > 
> >> instead of CALL_MREMAP_MAYMOVE for arm64 architecture.
> >> 
> >> Sergey Kaplun:
> >> * added the description and the test for the problem
> >> 
> >> Needed for tarantool/tarantool#6154
> > 
> > Minor: Why #5629 is not mentioned?
> > 
> >> ===================================================================
> >> 
> > 
> > <snipped>
> > 
> >> 
> >> -- 
> >> Best regards,
> >> Sergey Kaplun
> > 
> > -- 
> > Best regards,
> > IM
> 

-- 
Best regards,
Sergey Kaplun