On Tue, Dec 13, 2016 at 03:31:13PM -0500, Jerome Glisse wrote: > On Wed, Dec 14, 2016 at 07:15:15AM +1100, Dave Chinner wrote: > > On Tue, Dec 13, 2016 at 01:15:11PM -0500, Jerome Glisse wrote: > > > I would like to discuss un-addressable device memory in the context of > > > filesystem and block device. Specificaly how to handle write-back, read, > > > ... when a filesystem page is migrated to device memory that CPU can not > > > access. > > > > You mean pmem that is DAX-capable that suddenly, without warning, > > becomes non-DAX capable? > > > > If you are not talking about pmem and DAX, then exactly what does > > "when a filesystem page is migrated to device memory that CPU can > > not access" mean? What "filesystem page" are we talking about that > > can get migrated from main RAM to something the CPU can't access? > > I am talking about GPU, FPGA, ... any PCIE device that have fast on > board memory that can not be expose transparently to the CPU. I am > reusing ZONE_DEVICE for this, you can see HMM patchset on linux-mm > https://lwn.net/Articles/706856/ So ZONE_DEVICE memory that is a DMA target but not CPU addressable? > So in my case i am only considering non DAX/PMEM filesystem ie any > "regular" filesystem back by a "regular" block device. I want to be > able to migrate mmaped area of such filesystem to device memory while > the device is actively using that memory. "migrate mmapped area of such filesystem" means what, exactly? Are you talking about file data contents that have been copied into the page cache and mmapped into a user process address space? IOWs, migrating ZONE_NORMAL page cache page content and state to a new ZONE_DEVICE page, and then migrating back again somehow? > From kernel point of view such memory is almost like any other, it > has a struct page and most of the mm code is non the wiser, nor need > to be about it. CPU access trigger a migration back to regular CPU > accessible page. That sounds ... complex. Page migration on page cache access inside the filesytem IO path locking during read()/write() sounds like a great way to cause deadlocks.... > But for thing like writeback i want to be able to do writeback with- > out having to migrate page back first. So that data can stay on the > device while writeback is happening. Why can't you do writeback before migration, so only clean pages get moved? Cheers, Dave. -- Dave Chinner david@xxxxxxxxxxxxx -- To unsubscribe from this list: send the line "unsubscribe linux-block" in the body of a message to majordomo@xxxxxxxxxxxxxxx More majordomo info at http://vger.kernel.org/majordomo-info.html
