On Tue, Jun 27, 2017 at 3:04 PM, Luck, Tony <tony.luck@xxxxxxxxx> wrote: >> > > > +if (set_memory_np(decoy_addr, 1)) >> > > > +pr_warn("Could not invalidate pfn=0x%lx from 1:1 map \n", >> >> Another concept to consider is mapping the page as UC rather than >> completely unmapping it. > > UC would also avoid the speculative prefetch issue. The Vol 3, Section 11.3 SDM says: > > Strong Uncacheable (UC) -System memory locations are not cached. All reads and writes > appear on the system bus and are executed in program order without reordering. No speculative > memory accesses, pagetable walks, or prefetches of speculated branch targets are made. > This type of cache-control is useful for memory-mapped I/O devices. When used with normal > RAM, it greatly reduces processor performance. > > But then I went and read the code for set_memory_uc() ... which calls "reserve_memtyep()" > which does all kinds of things to avoid issues with MTRRs and other stuff. Which all looks > really more complex that we need just here. > >> The uncorrectable error scope could be smaller than a page size, like: >> * memory ECC width (e.g., 8 bytes) >> * cache line size (e.g., 64 bytes) >> * block device logical block size (e.g., 512 bytes, for persistent memory) >> >> UC preserves the ability to access adjacent data within the page that >> hasn't gone bad, and is particularly useful for persistent memory. > > If you want to dig into the non-poisoned pieces of the page later it might be > better to set up a new scratch UC mapping to do that. > > My takeaway from Dan's comments on unpoisoning is that this isn't the context > that he wants to do that. He'd rather wait until he has somebody overwriting the > page with fresh data. > > So I think I'd like to keep the patch as-is. Yes, the persistent-memory poison interactions should be handled separately and not hold up this patch for the normal system-memory case. We might dove-tail support for this into stray write protection where we unmap all of pmem while nothing in the kernel is actively accessing it. -- 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>