On 3/11/20 11:13 AM, Joonsoo Kim wrote: >> >> Few years ago Joonsoo wanted to fix these kinds of weird MIGRATE_CMA corner >> cases by using ZONE_MOVABLE instead [1]. Unfortunately it was reverted due to >> unresolved bugs. Perhaps the idea could be resurrected now? >> >> [1] >> https://lore.kernel.org/linux-mm/1512114786-5085-1-git-send-email-iamjoonsoo.kim@xxxxxxx/ > > Thanks for ccing, Vlastimil. No problem. Also, welcome back :) > Recently, I'm working for resurrecting this idea. > I will send the preparation patches in this or next week. Good to hear! > Unresolved bugs of my patchset comes from the fact that ZONE_MOVABLE > which is used for > serving CMA memory in my patchset could have both lowmem(direct mapped) and > highmem(no direct mapped) pages on CONFIG_HIGHMEM enabled system. > > For someone to use this memory, PageHighMem() should be called to > check if there is direct > mapping or not. Current PageHighMem() implementation is: > > #define PageHighMem(__p) is_highmem_idx(page_zonenum(__p)) > > Since ZONE_MOVABLE has both typed pages, ZONE_MOVABLE should be considered > as highmem zone. In this case, PageHighMem() always returns TRUE for > all pages on > ZONE_MOVABLE and lowmem pages on ZONE_MOVABLE could make some troubles. Doh, HighMem brings only troubles these days [1]. [1] https://lwn.net/Articles/813201/ > My plan to fix this problem is to change the PageHighMem() implementation. > > #define PageHighMem(__p) (page_to_pfn(__p) >= max_low_pfn) > > In fact, PageHighMem() is used to check whether direct mapping exists or not. > With this new implementation, regardless of the zone type of the page, we can > correctly check if the page is direct mapped or not. Changing the > name, PageHighMem(), > to !PageDirectMapped() is also planned but it will be done after > everything have settle down. > > Unfortunately, before changing the implementation, I should check the > all call-sites > of PageHighMem() since there is some callers who use PageHighMem() to check > the zone type. > > What my preparation patch will does is to correct this PageHighMem() usage. > After fixing it, I will try to merge the patchset [1]. > > Thanks. >
