[v5 PATCH] arm64: mm: force write fault for atomic RMW instructions

[David Hildenbrand]

On 02.07.24 12:26, Ryan Roberts wrote:
> On 01/07/2024 20:43, Catalin Marinas wrote:
>> On Fri, Jun 28, 2024 at 11:20:43AM -0700, Yang Shi wrote:
>>> On 6/28/24 10:24 AM, Catalin Marinas wrote:
>>>> This patch does feel a bit like working around a non-optimal user choice
>>>> in kernel space. Who knows, madvise() may even be quicker if you do a
>>>> single call for a larger VA vs touching each page.
>>>
>>> IMHO, I don't think so. I viewed this patch to solve or workaround some ISA
>>> inefficiency in kernel. Two faults are not necessary if we know we are
>>> definitely going to write the memory very soon, right?
>>
>> I agree the Arm architecture behaviour is not ideal here and any
>> timelines for fixing it in hardware, if they do happen, are far into the
>> future. Purely from a kernel perspective, what I want though is make
>> sure that longer term (a) we don't create additional maintenance burden
>> and (b) we don't keep dead code around.
>>
>> Point (a) could be mitigated if the architecture is changed so that any
>> new atomic instructions added to this range would also come with
>> additional syndrome information so that we don't have to update the
>> decoding patterns.
>>
>> Point (b), however, depends on the OpenJDK and the kernel versions in
>> distros. Nick Gasson kindly provided some information on the OpenJDK
>> changes. The atomic_add(0) change happened in early 2022, about 5-6
>> months after MADV_POPULATE_WRITE support was added to the kernel. What's
>> interesting is Ampere already contributed MADV_POPULATE_WRITE support to
>> OpenJDK a few months ago:
>>
>> https://github.com/openjdk/jdk/commit/a65a89522d2f24b1767e1c74f6689a22ea32ca6a
>>
>> The OpenJDK commit lacks explanation but what I gathered from the diff
>> is that this option is the preferred one in the presence of THP (which
>> most/all distros enable by default). If we merge your proposed kernel
>> patch, it will take time before it makes its way into distros. I'm
>> hoping that by that time, distros would have picked a new OpenJDK
>> version already that doesn't need the atomic_add(0) pattern. If that's
>> the case, we end up with some dead code in the kernel that's almost
>> never exercised.
>>
>> I don't follow OpenJDK development but I heard that updates are dragging
>> quite a lot. I can't tell whether people have picked up the
>> atomic_add(0) feature and whether, by the time a kernel patch would make
>> it into distros, they'd also move to the MADV_POPULATE_WRITE pattern.
>>
>> There's a point (c) as well on the overhead of reading the faulting
>> instruction. I hope that's negligible but I haven't measured it.
>>
> 
> Just to add to this, I note the existing kernel behaviour is that if a write
> fault happens in a region that has a (RO) huge zero page mapped at PMD level,
> then the PMD is shattered, the PTE of the fault address is populated with a
> writable page and the remaining PTEs are populated with order-0 zero pages
> (read-only).

That also recently popped up in [1]. CCing Jinjiang. Ever since I
replied there, I also thought some more about that handling in regard to the
huge zeropage.

> 
> This seems like odd behaviour to me. Surely it would be less effort and more
> aligned with the app's expectations to notice the huge zero page in the PMD,
> remove it, and install a THP, as would have been done if pmd_none() was true? I
> don't think there is a memory bloat argument here because, IIUC, with the
> current behaviour, khugepaged would eventually upgrade it to a THP anyway?

One detail: depending on the setting of khugepaged_max_ptes_none. zeropages
are treated like pte_none. But in the common case, that setting is left alone.

> 
> Changing to this new behaviour would only be a partial solution for your use
> case, since you would still have 2 faults. But it would remove the cost of the
> shattering and ensure you have a THP immediately after the write fault. But I
> can't think of a reason why this wouldn't be a generally useful change
> regardless? Any thoughts?

The "let's read before we write" as used by QEMU migration code is the desire
to not waste memory by populating the zeropages. Deferring consuming memory
until really required.

     /*
      * We read one byte of each page; this will preallocate page tables if
      * required and populate the shared zeropage on MAP_PRIVATE anonymous memory
      * where no page was populated yet. This might require adaption when
      * supporting other mappings, like shmem.
      */


Without THP this works as expected. With THP this currently also works as
expected, but of course with the price [1] of not getting anon THP
immediately, which usually we don't care about. As you note, khugepaged might
fix this up later.

If we disable the huge zeropage, we would get anon THPs when reading instead of
small zeropages.

As reply to [1], I suggested using preallcoation (using MADV_POPULATE_WRITE)
when we really care about that performance difference, which would also
avoid the huge zeropage completely, but it's also not quite optimal in some cases.


I don't really know what to do here: changing the handling for the huge zeropage
only unconditionally does not sound too wrong, but the change in behavior
might (or might not) be desired for some use cases.

Reading from unpopulated memory can be a clear sign that really the shared zeropage
is desired (as for QEMU), and concurrent memory preallcoation/population should
ideally use MADV_POPULATE_WRITE. Maybe there are some details buried in [2] regarding
the common use cases for the huge zeropage back than.

[1] https://lkml.kernel.org/r/740d7379-3e3d-4c8c-4350-6c496969db1f@huawei.com
[2] https://lwn.net/Articles/517465/

-- 
Cheers,

David / dhildenb