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@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
