Messages in this thread | | | Date | Wed, 13 Jul 2022 07:37:27 -0400 | From | Phil Auld <> | Subject | Re: [PATCH] drivers/base/node.c: fix userspace break from using bin_attributes for cpumap and cpulist |
| |
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.
> > > > after: > > > > -r--r--r--. 1 root root 40960 Jul 12 16:48 /sys/devices/system/node/node0/cpulist > > -r--r--r--. 1 root root 4096 Jul 12 15:50 /sys/devices/system/node/node0/cpumap > > if we finally set a size which might be improper, it seems we defeat the > purpose we start to move to bin_attr? > > > > > Fixes: 75bd50fa841 ("drivers/base/node.c: use bin_attribute to break the size limitation of cpumap ABI") > > Cc: Greg Kroah-Hartman <gregkh@linuxfoundation.org> > > Cc: "Rafael J. Wysocki" <rafael@kernel.org> > > Signed-off-by: Phil Auld <pauld@redhat.com> > > --- > > drivers/base/node.c | 4 ++-- > > 1 file changed, 2 insertions(+), 2 deletions(-) > > > > diff --git a/drivers/base/node.c b/drivers/base/node.c > > index 0ac6376ef7a1..291c69671f23 100644 > > --- a/drivers/base/node.c > > +++ b/drivers/base/node.c > > @@ -45,7 +45,7 @@ static inline ssize_t cpumap_read(struct file *file, struct kobject *kobj, > > return n; > > } > > > > -static BIN_ATTR_RO(cpumap, 0); > > +static BIN_ATTR_RO(cpumap, PAGE_SIZE); > > PAGE_SIZE is probably big enough, will we still calculate to get it rather than > hard coding?
This one is actually wrong. I did not realize how big a NR_CPUS people were actually using. It should be something like (NR_CPUS/4 + NR_CPUS/32).
> > > > > static inline ssize_t cpulist_read(struct file *file, struct kobject *kobj, > > struct bin_attribute *attr, char *buf, > > @@ -66,7 +66,7 @@ static inline ssize_t cpulist_read(struct file *file, struct kobject *kobj, > > return n; > > } > > > > -static BIN_ATTR_RO(cpulist, 0); > > +static BIN_ATTR_RO(cpulist, (((NR_CPUS * 5) > PAGE_SIZE) ? NR_CPUS *5 : PAGE_SIZE)); > > I am still not sure why it is NR_CPUS * 5. Is 5 bytes big enough to > describe the number > of cpu id? technically it seems not, for example, for cpuid=100000, > we need at least 6 > bytes.
Sure. As I said in the comment I wanted to do NR_CPUS * (ceil(log10(NR_CPUS)) + 1) but doing that math in the kernel was messy. So I used 5. Even that is probably way bigger than needed. Are there really 100000 cpus on one node with discontiguous cpuids? "0-99999" is only, what, 9 characters?
We can put whatever number you want that is >= the size the read will return.
Thanks, Phil
> > BTW, my silly question is that what if we set the size to MAXIMUM int? > Will it fix > the userspace fsstat? > > > > > /** > > * struct node_access_nodes - Access class device to hold user visible > > -- > > 2.31.1 > > > > Thanks > Barry >
--
| |