Re: [PATCH 00/35] Enhance memory utilization with DMEMFS

[Joao Martins <joao.m.martins@oracle.com>]

On 10/9/20 12:39 PM, yulei zhang wrote:
> Joao, thanks a lot for the feedback. One more thing needs to mention
> is that dmemfs also support fine-grained
> memory management which makes it more flexible for tenants with
> different requirements.
> 
So as DAX when it allows to partition a region (starting 5.10). Meaning you have a region
which you dedicated to userspace. That region can then be partitioning into devices which
give you access to multiple (possibly discontinuous) extents with at a given page
granularity (selectable when you create the device), accessed through mmap().
You can then give that device to a cgroup. Or you can return that memory back to the
kernel (should you run into OOM situation), or you recreate the same mappings across
reboot/kexec.

I probably need to read your patches again, but can you extend on the 'dmemfs also support
fine-grained memory management' to understand what is the gap that you mention?

> On Fri, Oct 9, 2020 at 3:01 AM Joao Martins <joao.m.martins@oracle.com> wrote:
>>
>> [adding a couple folks that directly or indirectly work on the subject]
>>
>> On 10/8/20 8:53 AM, yulei.kernel@gmail.com wrote:
>>> From: Yulei Zhang <yuleixzhang@tencent.com>
>>>
>>> In current system each physical memory page is assocaited with
>>> a page structure which is used to track the usage of this page.
>>> But due to the memory usage rapidly growing in cloud environment,
>>> we find the resource consuming for page structure storage becomes
>>> highly remarkable. So is it an expense that we could spare?
>>>
>> Happy to see another person working to solve the same problem!
>>
>> I am really glad to see more folks being interested in solving
>> this problem and I hope we can join efforts?
>>
>> BTW, there is also a second benefit in removing struct page -
>> which is carving out memory from the direct map.
>>
>>> This patchset introduces an idea about how to save the extra
>>> memory through a new virtual filesystem -- dmemfs.
>>>
>>> Dmemfs (Direct Memory filesystem) is device memory or reserved
>>> memory based filesystem. This kind of memory is special as it
>>> is not managed by kernel and most important it is without 'struct page'.
>>> Therefore we can leverage the extra memory from the host system
>>> to support more tenants in our cloud service.
>>>
>> This is like a walk down the memory lane.
>>
>> About a year ago we followed the same exact idea/motivation to
>> have memory outside of the direct map (and removing struct page overhead)
>> and started with our own layer/thingie. However we realized that DAX
>> is one the subsystems which already gives you direct access to memory
>> for free (and is already upstream), plus a couple of things which we
>> found more handy.
>>
>> So we sent an RFC a couple months ago:
>>
>> https://lore.kernel.org/linux-mm/20200110190313.17144-1-joao.m.martins@oracle.com/
>>
>> Since then majority of the work has been in improving DAX[1].
>> But now that is done I am going to follow up with the above patchset.
>>
>> [1]
>> https://lore.kernel.org/linux-mm/159625229779.3040297.11363509688097221416.stgit@dwillia2-desk3.amr.corp.intel.com/
>>
>> (Give me a couple of days and I will send you the link to the latest
>> patches on a git-tree - would love feedback!)
>>
>> The struct page removal for DAX would then be small, and ticks the
>> same bells and whistles (MCE handling, reserving PAT memtypes, ptrace
>> support) that we both do, with a smaller diffstat and it doesn't
>> touch KVM (not at least fundamentally).
>>
>>         15 files changed, 401 insertions(+), 38 deletions(-)
>>
>> The things needed in core-mm is for handling PMD/PUD PAGE_SPECIAL much
>> like we both do. Furthermore there wouldn't be a need for a new vm type,
>> consuming an extra page bit (in addition to PAGE_SPECIAL) or new filesystem.
>>
>> [1]
>> https://lore.kernel.org/linux-mm/159625229779.3040297.11363509688097221416.stgit@dwillia2-desk3.amr.corp.intel.com/
>>
>>
>>> We uses a kernel boot parameter 'dmem=' to reserve the system
>>> memory when the host system boots up, the details can be checked
>>> in /Documentation/admin-guide/kernel-parameters.txt.
>>>
>>> Theoretically for each 4k physical page it can save 64 bytes if
>>> we drop the 'struct page', so for guest memory with 320G it can
>>> save about 5G physical memory totally.
>>>
>> Also worth mentioning that if you only care about 'struct page' cost, and not on the
>> security boundary, there's also some work on hugetlbfs preallocation of hugepages into
>> tricking vmemmap in reusing tail pages.
>>
>>   https://lore.kernel.org/linux-mm/20200915125947.26204-1-songmuchun@bytedance.com/
>>
>> Going forward that could also make sense for device-dax to avoid so many
>> struct pages allocated (which would require its transition to compound
>> struct pages like hugetlbfs which we are looking at too). In addition an
>> idea <handwaving> would be perhaps to have a stricter mode in DAX where
>> we initialize/use the metadata ('struct page') but remove the underlaying
>> PFNs (of the 'struct page') from the direct map having to bear the cost of
>> mapping/unmapping on gup/pup.
>>
>>         Joao