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

[Sergey Ostanevich]

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?


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