On Wed, Sep 07, 2016 at 09:32:36PM -0700, Dan Williams wrote: > [ adding linux-fsdevel and linux-nvdimm ] > > On Wed, Sep 7, 2016 at 8:36 PM, Xiao Guangrong > <guangrong.xiao@xxxxxxxxxxxxxxx> wrote: > [..] > > However, it is not easy to handle the case that the new VMA overlays with > > the old VMA > > already got by userspace. I think we have some choices: > > 1: One way is completely skipping the new VMA region as current kernel code > > does but i > > do not think this is good as the later VMAs will be dropped. > > > > 2: show the un-overlayed portion of new VMA. In your case, we just show the > > region > > (0x2000 -> 0x3000), however, it can not work well if the VMA is a new > > created > > region with different attributions. > > > > 3: completely show the new VMA as this patch does. > > > > Which one do you prefer? > > > > I don't have a preference, but perhaps this breakage and uncertainty > is a good opportunity to propose a more reliable interface for NVML to > get the information it needs? > > My understanding is that it is looking for the VM_MIXEDMAP flag which > is already ambiguous for determining if DAX is enabled even if this > dynamic listing issue is fixed. XFS has arranged for DAX to be a > per-inode capability and has an XFS-specific inode flag. We can make > that a common inode flag, but it seems we should have a way to > interrogate the mapping itself in the case where the inode is unknown > or unavailable. I'm thinking extensions to mincore to have flags for > DAX and possibly whether the page is part of a pte, pmd, or pud > mapping. Just floating that idea before starting to look into the > implementation, comments or other ideas welcome... I think this goes back to our previous discussion about support for the PMEM programming model. Really I think what NVML needs isn't a way to tell if it is getting a DAX mapping, but whether it is getting a DAX mapping on a filesystem that fully supports the PMEM programming model. This of course is defined to be a filesystem where it can do all of its flushes from userspace safely and never call fsync/msync, and that allocations that happen in page faults will be synchronized to media before the page fault completes. IIUC this is what NVML needs - a way to decide "do I use fsync/msync for everything or can I rely fully on flushes from userspace?" For all existing implementations, I think the answer is "you need to use fsync/msync" because we don't yet have proper support for the PMEM programming model. My best idea of how to support this was a per-inode flag similar to the one supported by XFS that says "you have a PMEM capable DAX mapping", which NVML would then interpret to mean "you can do flushes from userspace and be fully safe". I think we really want this interface to be common over XFS and ext4. If we can figure out a better way of doing this interface, say via mincore, that's fine, but I don't think we can detangle this from the PMEM API discussion. -- To unsubscribe from this list: send the line "unsubscribe linux-fsdevel" in the body of a message to majordomo@xxxxxxxxxxxxxxx More majordomo info at http://vger.kernel.org/majordomo-info.html
