On Tue, Jan 26, 2016 at 2:11 PM, Andrew Morton <akpm@xxxxxxxxxxxxxxxxxxxx> wrote: > On Mon, 25 Jan 2016 16:06:40 -0800 Dan Williams <dan.j.williams@xxxxxxxxx> wrote: > >> It appears devices requiring ZONE_DMA are still prevalent (see link >> below). For this reason the proposal to require turning off ZONE_DMA to >> enable ZONE_DEVICE is untenable in the short term. > > More than "short term". When can we ever nuke ZONE_DMA? I'm assuming at some point these legacy devices will die off or move to something attached over a more capable bus like USB? > This was a pretty big goof - the removal of ZONE_DMA whizzed straight > past my attention, alas. In fact I never noticed the patch at all > until I got some conflicts in -next a few weeks later (wasn't cc'ed). > And then I didn't read the changelog closely enough. I endeavor to never surprise you again... To be clear the patch did not disable ZONE_DMA by default, but it was indeed a goof to assume that ZONE_DMA was less prevalent than it turns out to be. >> We want a single >> kernel image to be able to support legacy devices as well as next >> generation persistent memory platforms. > > yup. > >> Towards this end, alias ZONE_DMA and ZONE_DEVICE to work around needing >> to maintain a unique zone number for ZONE_DEVICE. Record the geometry >> of ZONE_DMA at init (->init_spanned_pages) and use that information in >> is_zone_device_page() to differentiate pages allocated via >> devm_memremap_pages() vs true ZONE_DMA pages. Otherwise, use the >> simpler definition of is_zone_device_page() when ZONE_DMA is turned off. >> >> Note that this also teaches the memory hot remove path that the zone may >> not have sections for all pfn spans (->zone_dyn_start_pfn). >> >> A user visible implication of this change is potentially an unexpectedly >> high "spanned" value in /proc/zoneinfo for the DMA zone. > > Well, all these icky tricks are to avoid increasing ZONES_SHIFT, yes? > Is it possible to just use ZONES_SHIFT=3? Last I tried I hit this warning in mm/memory.c #warning Unfortunate NUMA and NUMA Balancing config, growing page-frame for last_cpupid. > Also, this "dynamically added pfn of the zone" thing is a new concept > and I think it should be more completely documented somewhere in the > code. Ok, I'll take a look. -- 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>