On Thu, Nov 9, 2017 at 8:37 AM, Mikulas Patocka <mpatocka@xxxxxxxxxx> wrote: > > > On Wed, 8 Nov 2017, Dan Williams wrote: > >> On Wed, Nov 8, 2017 at 12:26 PM, Mikulas Patocka <mpatocka@xxxxxxxxxx> wrote: >> > On Wed, 8 Nov 2017, Christoph Hellwig wrote: >> > >> >> Can you start by explaining what you actually need the vmap for? >> > >> > It is possible to use lvm on persistent memory. You can create linear or >> > striped logical volumes on persistent memory and these volumes still have >> > the direct_access method, so they can be mapped with the function >> > dax_direct_access(). >> > >> > If we create logical volumes on persistent memory, the method >> > dax_direct_access() won't return the whole device, it will return only a >> > part. When dax_direct_access() returns the whole device, my driver just >> > uses it without vmap. When dax_direct_access() return only a part of the >> > device, my driver calls it repeatedly to get all the parts and then >> > assembles the parts into a linear address space with vmap. >> >> I know I proposed "call dax_direct_access() once" as a strawman for an >> in-kernel driver user, but it's better to call it per access so you >> can better stay in sync with base driver events like new media errors >> and unplug / driver-unload. Either that, or at least have a plan how >> to handle those events. > > Calling it on every access would be inacceptable performance overkill. How > is it supposed to work anyway? - if something intends to move data on > persistent memory while some driver accesse it, then we need two functions > - dax_direct_access() and dax_relinquish_direct_access(). The current > kernel lacks a function dax_relinquish_direct_access() that would mark a > region of data as moveable, so we can't move the data anyway. We take a global reference on the hosting device while pages are registered, see the percpu_ref usage in kernel/memremap.c, and we hold the dax_read_lock() over calls to dax_direct_access() to temporarily hold the device alive for the duration of the call. > BTW. what happens if we create a write bio that has its pages pointing to > persistent memory and there is error when the storage controller attempts > to do DMA from persistent memory? Will the storage controller react to the > error in a sensible way and will the block layer report the error? While pages are pinned for DMA the devm_memremap_pages() mapping is pinned. Otherwise, an error reading persistent memory is identical to an error reading DRAM. -- dm-devel mailing list dm-devel@xxxxxxxxxx https://www.redhat.com/mailman/listinfo/dm-devel
