On Mon, Aug 25, 2014 at 10:00:32AM +0200, Marek Szyprowski wrote: > Hello, > > On 2014-08-25 03:26, Minchan Kim wrote: > >Hello, > > > >On Thu, Aug 21, 2014 at 10:45:12AM +0200, Marek Szyprowski wrote: > >>Hello, > >> > >>Russell King recently noticed that limiting default CMA region only to > >>low memory on ARM architecture causes serious memory management issues > >>with machines having a lot of memory (which is mainly available as high > >>memory). More information can be found the following thread: > >>http://thread.gmane.org/gmane.linux.ports.arm.kernel/348441/ > >> > >>Those two patches removes this limit letting kernel to put default CMA > >>region into high memory when this is possible (there is enough high > >>memory available and architecture specific DMA limit fits). > >Agreed. It should be from the beginning because CMA page is effectly > >pinned if it is anonymous page and system has no swap. > > Nope. Even without swap, anonymous page can be correctly migrated to other > location. Migration code doesn't depend on presence of swap. I could be possible only if the zone has freeable page(ie, free pages + shrinkable page like page cache). IOW, if the zone is full with anon pages, it's efffectively pinned. > > >>This should solve strange OOM issues on systems with lots of RAM > >>(i.e. >1GiB) and large (>256M) CMA area. > >I totally agree with the patchset although I didn't review code > >at all. > > > >Another topic: > >It means it should be a problem still if system has CMA in lowmem > >by some reason(ex, hardware limit or other purpose of CMA > >rather than DMA subsystem)? > > > >In that case, an idea that just popped in my head is to migrate > >pages from cma area to highest zone because they are all > >userspace pages which should be in there but not sure it's worth > >to implement at this point because how many such cripple platform > >are. > > > >Just for the recording. > > Moving pages between low and high zone is not that easy. If I remember > correctly you cannot migrate a page from low memory to high zone in > generic case, although it should be possible to add exception for > anonymous pages. This will definitely improve poor low memory > handling in low zone when CMA is enabled. Yeb, it's possible for anonymous pages but I just wonder it's worth to add more complexitiy to mm and and you are answering it's worth. Okay. May I understand your positive feedback means such platform( ie, DMA works with only lowmem) are still common? Thanks. > > Best regards > -- > Marek Szyprowski, PhD > Samsung R&D Institute Poland > > -- > 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> -- Kind regards, Minchan Kim -- 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>