[v2,1/3] sparc64: NG4 memset 32 bits overflow

Early in boot Linux patches memset and memcpy to branch to platform
optimized versions of these routines. The NG4 (Niagra 4) versions are
currently used on  all platforms starting from T4. Recently, there were M7
optimized routines added into UEK4 but not into mainline yet. So, even with
M7 optimized routines NG4 are still going to be used on T4, T5, M5, and M6
processors.

While investigating how to improve initialization time of dentry_hashtable
which is 8G long on M6 ldom with 7T of main memory, I noticed that memset()
does not reset all the memory in this array, after studying the code, I
realized that NG4memset() branches use %icc register instead of %xcc to
check compare, so if value of length is over 32-bit long, which is true for
8G array, these routines fail to work properly.

The fix is to replace all %icc with %xcc in these routines. (Alternative is
to use %ncc, but this is misleading, as the code already has sparcv9 only
instructions, and cannot be compiled on 32-bit).

This is important to fix this bug, because even older T4-4 can have 2T of
memory, and there are large memory proportional data structures in kernel
which can be larger than 4G in size. The failing of memset() is silent and
corruption is hard to detect.

Signed-off-by: Pavel Tatashin <pasha.tatashin@oracle.com>
Reviewed-by: Babu Moger <babu.moger@oracle.com>
---
 arch/sparc/lib/NG4memset.S |   26 +++++++++++++-------------
 1 files changed, 13 insertions(+), 13 deletions(-)

Pavel Tatashin <pasha.tatashin@oracle.com> writes:
>
> While investigating how to improve initialization time of dentry_hashtable
> which is 8G long on M6 ldom with 7T of main memory, I noticed that memset()

I don't think a 8G dentry (or other kernel) hash table makes much
sense. I would rather fix the hash table sizing algorithm to have some
reasonable upper limit than to optimize the zeroing.

I believe there are already boot options for it, but it would be better
if it worked out of the box.

-Andi
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On 2017-02-28 19:24, Andi Kleen wrote:
> Pavel Tatashin <pasha.tatashin@oracle.com> writes:
>>
>> While investigating how to improve initialization time of dentry_hashtable
>> which is 8G long on M6 ldom with 7T of main memory, I noticed that memset()
>
> I don't think a 8G dentry (or other kernel) hash table makes much
> sense. I would rather fix the hash table sizing algorithm to have some
> reasonable upper limit than to optimize the zeroing.
>
> I believe there are already boot options for it, but it would be better
> if it worked out of the box.
>
> -Andi


Hi Andi,

I agree that there should be some smarter cap for maximum hash table 
sizes, and as you said it is already possible to set the limits via 
parameters. I still think, however, this HASH_ZERO patch makes sense for 
the following reasons:

- Even if the default maximum size is reduced the size of these tables 
should still be tunable, as it really depends on the way machine is 
used, and in it is possible that for some use patterns large hash tables 
are necessary.

- Most of them are initialized before smp_init() call. The time from 
bootloader to smp_init() should be minimized as parallelization is not 
available yet. For example, LDOM domain on which I tested this patch 
with few more optimization takes 8.5 seconds to get from grub to 
smp_init() (760CPUs and 7T of memory), out of these 8.5 seconds 3.1s 
(vs. 11.8s before this patch) are spent initializing these hash tables. 
So, even 3.1s is still significant, and should be improved further by 
changing the default maximums, but that should be a different patch.

Thank you,
Pasha


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> - Even if the default maximum size is reduced the size of these
> tables should still be tunable, as it really depends on the way
> machine is used, and in it is possible that for some use patterns
> large hash tables are necessary.

I consider it very unlikely that a 8G dentry hash table ever makes
sense. I cannot even imagine a workload where you would have that
many active files. It's just a bad configuration that should be avoided.

And when the tables are small enough you don't need these hacks.

-Andi
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Hi Andi,

Thank you for your comment, I am thinking to limit the default maximum 
hash tables sizes to 512M.

If it is bigger than 512M, we would still need my patch to improve the 
performance. This is because it would mean that initialization of hash 
tables would still take over 1s out of 6s in bootload to smp_init() 
interval on larger machines.

I am not sure HASH_ZERO is a hack because if you look at the way 
pv_lock_hash is allocated, it assumes that the memory is already zeroed 
since it provides HASH_EARLY flag. It quietly assumes that the memblock 
boot allocator zeroes the memory for us. On the other hand, in other 
places where HASH_EARLY is specified we still explicitly zero the 
hashes. At least with HASH_ZERO flag this becomes a defined interface, 
and in the future if memblock allocator is changed to zero memory only 
on demand (as it really should), the HASH_ZERO flag can be passed there 
the same way it is passed to vmalloc() in my patch.

Does something like this look OK to you? If yes, I will send out a new 
patch.


  index 1b0f7a4..5ddf741 100644
  --- a/mm/page_alloc.c
  +++ b/mm/page_alloc.c
  @@ -79,6 +79,12 @@
   EXPORT_PER_CPU_SYMBOL(numa_node);
   #endif

  +/*
  + * This is the default maximum number of entries system hashes can 
have, the
  + * value can be overwritten by setting hash table sizes via kernel 
parameters.
  + */
  +#define SYSTEM_HASH_MAX_ENTRIES                (1 << 26)
  +
   #ifdef CONFIG_HAVE_MEMORYLESS_NODES
   /*
    * N.B., Do NOT reference the '_numa_mem_' per cpu variable directly.
  @@ -7154,6 +7160,11 @@ static unsigned long __init 
arch_reserved_kernel_pages(void)
                  if (PAGE_SHIFT < 20)
                          numentries = round_up(numentries, 
(1<<20)/PAGE_SIZE);

  +               /* Limit default maximum number of entries */
  +               if (numentries > SYSTEM_HASH_MAX_ENTRIES) {
  +                       numentries = SYSTEM_HASH_MAX_ENTRIES;
  +               }
  +
                  /* limit to 1 bucket per 2^scale bytes of low memory */
                  if (scale > PAGE_SHIFT)
                          numentries >>= (scale - PAGE_SHIFT);

Thank you
Pasha

On 2017-03-01 10:19, Andi Kleen wrote:
>> - Even if the default maximum size is reduced the size of these
>> tables should still be tunable, as it really depends on the way
>> machine is used, and in it is possible that for some use patterns
>> large hash tables are necessary.
>
> I consider it very unlikely that a 8G dentry hash table ever makes
> sense. I cannot even imagine a workload where you would have that
> many active files. It's just a bad configuration that should be avoided.
>
> And when the tables are small enough you don't need these hacks.
>
> -Andi
>
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On Wed, Mar 01, 2017 at 11:34:10AM -0500, Pasha Tatashin wrote:
> Hi Andi,
> 
> Thank you for your comment, I am thinking to limit the default
> maximum hash tables sizes to 512M.
> 
> If it is bigger than 512M, we would still need my patch to improve

Even 512MB seems too large. I wouldn't go larger than a few tens
of MB, maybe 32MB.

Also you would need to cover all the big hashes.

The most critical ones are likely the network hash tables, these
maybe be a bit larger (but certainly also not 0.5TB) 

-Andi
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Hi Andi,

After thinking some more about this issue, I figured that I would not 
want to set default maximums.

Currently, the defaults are scaled with system memory size, which seems 
like the right thing to do to me. They are set to size hash tables one 
entry per page and, if a scale argument is provided, scale them down to 
1/2, 1/4, 1/8 entry per page etc.

So, in some cases the scale argument may be wrong, and dentry, inode, or 
some other client of alloc_large_system_hash() should be adjusted.

For example, I am pretty sure that scale value in most places should be 
changed from literal value (inode scale = 14, dentry scale = 13, etc to: 
(PAGE_SHIFT + value): inode scale would become (PAGE_SHIFT + 2), dentry 
scale would become (PAGE_SHIFT + 1), etc. This is because we want 1/4 
inodes and 1/2 dentries per every page in the system.
In alloc_large_system_hash() we have basically this:
nentries = nr_kernel_pages >> (scale - PAGE_SHIFT);

This is basically a bug, and would not change the theory, but I am sure 
that changing scales without at least some theoretical backup is not a 
good idea and would most likely lead to regressions, especially on some 
smaller configurations.

Therefore, in my opinion having one fast way to zero hash tables, as 
this patch tries to do, is a good thing. In the next patch revision I 
can go ahead and change scales to be (PAGE_SHIFT + val) from current 
literals.

Thank you,
Pasha

On 2017-03-01 12:31, Andi Kleen wrote:
> On Wed, Mar 01, 2017 at 11:34:10AM -0500, Pasha Tatashin wrote:
>> Hi Andi,
>>
>> Thank you for your comment, I am thinking to limit the default
>> maximum hash tables sizes to 512M.
>>
>> If it is bigger than 512M, we would still need my patch to improve
>
> Even 512MB seems too large. I wouldn't go larger than a few tens
> of MB, maybe 32MB.
>
> Also you would need to cover all the big hashes.
>
> The most critical ones are likely the network hash tables, these
> maybe be a bit larger (but certainly also not 0.5TB)
>
> -Andi
>

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> For example, I am pretty sure that scale value in most places should
> be changed from literal value (inode scale = 14, dentry scale = 13,
> etc to: (PAGE_SHIFT + value): inode scale would become (PAGE_SHIFT +
> 2), dentry scale would become (PAGE_SHIFT + 1), etc. This is because
> we want 1/4 inodes and 1/2 dentries per every page in the system.

This is still far too much for a large system. The algorithm
simply was not designed for TB systems.

It's unlikely to have nowhere near that many small files active, as it's 
better to use the memory for something that is actually useful.

Also even a few hops in the open hash table are normally not a problems
dentry/inode; it is not that file lookups are that critical.

For networking the picture may be different, but I suspect GBs worth of
hash tables are still overkill there (Dave et.al. may have stronger opinions on this) 

I think a upper size (with user override which already exists) is fine,
but if you really don't want to do it then scale the factor down 
very aggressively for larger sizes, so that we don't end up with more
than a few tens of MB.

> This is basically a bug, and would not change the theory, but I am
> sure that changing scales without at least some theoretical backup

One dentry per page would only make sense if the files are zero sized.
If the file even has one byte then it already needs more than 1 page just to
cache the contents (even ignoring inodes and other caches)

With larger files that need multiple pages it makes even less sense.

So clearly one dentry per page theory is nonsense if the files are actually
used.

There is the "make find / + stat fast" case (where only the entries 
and inodes are cached). But even there it is unlikely that the TB system
has a much larger file system with more files than the 100GB system, so
I once a reasonable plateau is reached I don't see why you would want 
to exceed that.

Also the reason to make hash tables big is to minimize collisions,
but we have fairly good hash functions and a few hops worse case 
are likely not a problem for an already expensive file access
or open.

BTW the other option would be to switch all the large system hashes to a
rhashtable and do the resizing only when it is actually needed. 
But that would be more work than just adding a reasonable upper limit.

-Andi
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On Wed, Mar 01, 2017 at 04:20:28PM -0500, Pasha Tatashin wrote:
> Hi Andi,
> 
> After thinking some more about this issue, I figured that I would not want
> to set default maximums.
> 
> Currently, the defaults are scaled with system memory size, which seems like
> the right thing to do to me. They are set to size hash tables one entry per
> page and, if a scale argument is provided, scale them down to 1/2, 1/4, 1/8
> entry per page etc.

I disagree that it's the right thing to do.  You want your dentry cache
to scale with the number of dentries in use.  Scaling with memory size
is a reasonable approximation for smaller memory sizes, but allocating
8GB of *hash table entries* for dentries is plainly ridiculous, no matter
how much memory you have.  You won't have half a billion dentries active
in most uses of such a large machine.
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Hi Andi,

>
> I think a upper size (with user override which already exists) is fine,
> but if you really don't want to do it then scale the factor down
> very aggressively for larger sizes, so that we don't end up with more
> than a few tens of MB.
>

I have scaled it, I do not think setting a default upper limit is a 
future proof strategy.

Thank you,
Pasha
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