On 10/10/20 9:15 AM, yulei zhang wrote: > On Fri, Oct 9, 2020 at 7:53 PM Joao Martins <joao.m.martins@xxxxxxxxxx> wrote: >> 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? >> > sure, dmemfs uses bitmap to track the memory usage in the reserved > memory region in > a given page size granularity. And for each user the memory can be > discrete as well. > That same functionality of tracking reserved region usage across different users at any page granularity is covered the DAX series I mentioned below. The discrete part -- IIUC what you meant -- is then reduced using DAX ABI/tools to create a device file vs a filesystem. >>> On Fri, Oct 9, 2020 at 3:01 AM Joao Martins <joao.m.martins@xxxxxxxxxx> wrote: >>>> >>>> [adding a couple folks that directly or indirectly work on the subject] >>>> >>>> On 10/8/20 8:53 AM, yulei.kernel@xxxxxxxxx wrote: >>>>> From: Yulei Zhang <yuleixzhang@xxxxxxxxxxx> >>>>> >>>>> 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@xxxxxxxxxx/ >>>> >>>> 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@xxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxx/ >>>> >>>> (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@xxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxx/ >>>> >>>> >>>>> 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@xxxxxxxxxxxxx/ >>>> >>>> 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
