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. > > 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
