On 01/08/2014 11:42 PM, Mel Gorman wrote: > On Fri, Dec 20, 2013 at 10:05:02AM -0700, Matthew Wilcox wrote: >> >> I should like to discuss the current situation with Linux support for >> persistent memory. While I expect the current discussion to be long >> over by March, I am certain that there will be topics around persistent >> memory that have not been settled at that point. >> >> I believe this will mostly be of crossover interest between filesystem >> and MM people, and of lesser interest to storage people (since we're >> basically avoiding their code). >> >> Subtopics might include >> - Using persistent memory for FS metadata >> (The XIP code provides persistent memory to userspace. The filesystem >> still uses BIOs to fetch its metadata) >> - Supporting PMD/PGD mappings for userspace >> (Not only does the filesystem have to avoid fragmentation to make this >> happen, the VM code has to permit these giant mappings) > > The filesystem would also have to correctly align the data on disk. All > this implies that the underlying device is byte-addressible, similar access > speeds to RAM and directly accessible from userspace without the kernel > being involved. Without those conditions, I find it hard to believe that > TLB pressure dominates access cost. Then again I have no experience with > the devices or their intended use case so would not mind an education. > > However, if you really wanted the device to be accessible like this then > the shortest solutions (and I want to punch myself for even suggesting > this) is to extend hugetlbfs to directly access these devices. It's > almost certainly a bad direction to take though, there would need to be a > good justification for it. Anything in this direction is pushing usage of > persistent devices to userspace and the kernel just provides an interface, > maybe that is desirable maybe not. > >> - Persistent page cache >> (Another way to take advantage of persstent memory would be to place it >> in the page cache. But we don't have struct pages for it! What to do?) > I think one potential way is to use persistent memory as a second-level clean page cache through the cleancache API. -- Regards, -Bob -- To unsubscribe, send a message with 'unsubscribe linux-mm' in the body to majordomo@xxxxxxxxx. For more info on Linux MM, see: http://www.linux-mm.org/ . Don't email: <a href=mailto:"dont@xxxxxxxxx";> email@xxxxxxxxx </a>