On 8/18/20 8:18 PM, Andrew Morton wrote: > On Fri, 14 Aug 2020 09:49:26 -0700 Doug Berger <opendmb@xxxxxxxxx> wrote: > >> The lowmem_reserve arrays provide a means of applying pressure >> against allocations from lower zones that were targeted at >> higher zones. Its values are a function of the number of pages >> managed by higher zones and are assigned by a call to the >> setup_per_zone_lowmem_reserve() function. >> >> The function is initially called at boot time by the function >> init_per_zone_wmark_min() and may be called later by accesses >> of the /proc/sys/vm/lowmem_reserve_ratio sysctl file. >> >> The function init_per_zone_wmark_min() was moved up from a >> module_init to a core_initcall to resolve a sequencing issue >> with khugepaged. Unfortunately this created a sequencing issue >> with CMA page accounting. >> >> The CMA pages are added to the managed page count of a zone >> when cma_init_reserved_areas() is called at boot also as a >> core_initcall. This makes it uncertain whether the CMA pages >> will be added to the managed page counts of their zones before >> or after the call to init_per_zone_wmark_min() as it becomes >> dependent on link order. With the current link order the pages >> are added to the managed count after the lowmem_reserve arrays >> are initialized at boot. >> >> This means the lowmem_reserve values at boot may be lower than >> the values used later if /proc/sys/vm/lowmem_reserve_ratio is >> accessed even if the ratio values are unchanged. >> >> In many cases the difference is not significant, but for example >> an ARM platform with 1GB of memory and the following memory layout >> [ 0.000000] cma: Reserved 256 MiB at 0x0000000030000000 >> [ 0.000000] Zone ranges: >> [ 0.000000] DMA [mem 0x0000000000000000-0x000000002fffffff] >> [ 0.000000] Normal empty >> [ 0.000000] HighMem [mem 0x0000000030000000-0x000000003fffffff] >> >> would result in 0 lowmem_reserve for the DMA zone. This would allow >> userspace to deplete the DMA zone easily. > > Sounds fairly serious for thos machines. Was a cc:stable considered? Since there is a Fixes: tag, it may have been assumed that the patch would be picked up and as soon as it reaches Linus' tree it would be picked up by the stable selection. -- Florian
