Messages in this thread | | | Date | Wed, 13 Jul 2022 15:06:25 +0200 | From | Greg Kroah-Hartman <> | Subject | Re: [PATCH] drivers/base/node.c: fix userspace break from using bin_attributes for cpumap and cpulist |
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On Wed, Jul 13, 2022 at 07:37:27AM -0400, Phil Auld wrote: > On Wed, Jul 13, 2022 at 11:18:59AM +1200 Barry Song wrote: > > On Wed, Jul 13, 2022 at 9:58 AM Phil Auld <pauld@redhat.com> wrote: > > > > > > Using bin_attributes with a 0 size causes fstat and friends to return that 0 size. > > > This breaks userspace code that retrieves the size before reading the file. Rather > > > than reverting 75bd50fa841 ("drivers/base/node.c: use bin_attribute to break the size > > > limitation of cpumap ABI") let's put in a size value at compile time. Use direct > > > comparison and a worst-case maximum to ensure compile time constants. For cpulist the > > > max is on the order of NR_CPUS * (ceil(log10(NR_CPUS)) + 1) which for 8192 is 40960. > > > In order to get near that you'd need a system with every other CPU on one node or > > > something similar. e.g. (0,2,4,... 1024,1026...). We set it to a min of PAGE_SIZE > > > to retain the older behavior. For cpumap, PAGE_SIZE is plenty big. > > > > > > On an 80 cpu 4-node system (NR_CPUS == 8192) > > > > > > before: > > > > > > -r--r--r--. 1 root root 0 Jul 12 14:08 /sys/devices/system/node/node0/cpulist > > > -r--r--r--. 1 root root 0 Jul 11 17:25 /sys/devices/system/node/node0/cpumap > > > > it is a fundamental problem of bin_attr, isn't it? when we don't know the > > exact size of an attribute, and this size might become more than one > > PAGE_SIZE, we use bin_attr to break the limitation. but the fact is that > > we really don't know or it is really hard to know the actual size of the > > attribute. > > > > But it broke userspace applications. I figured rather than revert it maybe > we can find a max size to put in there and make it continue to work.
Yes, we need to do this, we can't break userspace.
thanks,
greg k-h
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