Messages in this thread | | | Date | Wed, 13 Jul 2022 15:05:52 +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 07:47:58AM -0400, Phil Auld wrote: > 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.
That is why we had to change this to a binary file. Think about every-other CPU being there, that's a huge list. This already was broken on some systems which is why it had to be changed (i.e. we didn't change it for no reason at all.)
> > > 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.
Sure, pick a max size please, that's fine with me.
greg k-h
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