Messages in this thread | | | Date | Wed, 13 Jul 2022 07:47:58 -0400 | From | Phil Auld <> | Subject | Re: [PATCH] drivers/base/node.c: fix userspace break from using bin_attributes for cpumap and cpulist |
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Hi Greg,
On Wed, Jul 13, 2022 at 08:06:02AM +0200 Greg Kroah-Hartman wrote: > On Tue, Jul 12, 2022 at 05:43:01PM -0400, Phil Auld 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. > > Does userspace care about that size, or can we just put any value in > there and it will be ok? How about just returning to the original > PAGE_SIZE value to keep things looking identical, will userspace not > read more than that size from the file then? >
I'll go look. But I think the point of pre-reading the size with fstat is to allocate a buffer to read into. So that may be a problem.
That said, I believe in this case it's the cpulist file which given the use of ranges is very unlikely to actually get that big.
> > On an 80 cpu 4-node sytem (NR_CPUS == 8192) > > We have systems running Linux with many more cpus than that, and your > company knows this :)
The 80 cpus here don't matter and we only build with NR_CPUS = 8192 :)
But yes, I realize now that the cpumap part I posted is broken for larger NR_CPUS. I originally had it as NR_CPUS, but as I said in my reply to Barry, it wants to be ~= NR_CPUS/4 + NR_CPUS/32. I'll change that.
I think we should decide on a max for each and use that.
Cheers, Phil
> > thanks, > > greg k-h >
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