[PATCH] malloc: Optimize the madvise behaviour on the main heap

[Dev Jain]

On 14/11/25 9:57 pm, Adhemerval Zanella Netto wrote:
>
> On 09/11/25 04:29, Dev Jain wrote:
>> Linux handles virtual memory in Virtual Memory Areas (VMAs). The
>> madvise(MADV_HUGEPAGE) call works on a VMA granularity, which sets the
>> VM_HUGEPAGE flag on the VMA. Therefore, if we can guarantee that a VMA
>> has been marked with VM_HUGEPAGE already, then we do not need to call
>> madvise() on that VMA again.
>>
>> For mp_.thp_pagesize != 0, currently we align the new brk to the thp size.
>> This means that after the first extension, all such brk extensions are
>> guaranteed to produce an extension size >= thp size: madvise_thp() will
>> invoke the madvise() syscall only if size >= thp size, and the other
>> condition is related to the sysctl setting, wherein mp_.thp_mode will be
>> same throughout the lifetime of the process. Therefore, currently we invoke
>> the madvise() syscall on the heap on each extension, which is unnecessary.
>>
>> First, pass the total heap size, instead of the extension size, to
>> madvise_thp: Linux does not care about the size passed, in case the
>> madvise() syscall is invoked with MADV_HUGEPAGE flag, because the flag
>> will be set on the entire VMA, no matter for what portion of the VMA
>> the syscall is invoked.
>>
>> This enables us to do the following: if the old heap size >= thp size,
>> we can guarantee that madvise() was invoked on one of the previous
>> extensions of the heap. So, avoid making the syscall in this case.
>>
>> The tricky part is computing the size of the heap, i.e the current program
>> break minus the initial program break. In case the first ever attempt at
>> extending the break fails, mp_.sbrk_base will be set to an mmapped address.
>> Therefore, we need some other way of remembering the initial location
>> of the program break. We can reuse some code for this: MORECORE (0), when
>> invoked for the first time ever, will give us the initial program break.
>> ---
>> The patch applies on 259adb087dd9. Built on Aarch64, all malloc tests pass.
> I am not sure if the calculation of
>
>>   malloc/malloc.c | 23 +++++++++++++++++++++--
>>   1 file changed, 21 insertions(+), 2 deletions(-)
>>
>> diff --git a/malloc/malloc.c b/malloc/malloc.c
>> index 0b21bdf1bd..277fb9e9ec 100644
>> --- a/malloc/malloc.c
>> +++ b/malloc/malloc.c
>> @@ -1938,6 +1938,12 @@ struct malloc_par
>>     /* First address handed out by MORECORE/sbrk.  */
>>     char *sbrk_base;
>>   
>> +  /* The initial location of program break. This will most likely be equal
>> +     to sbrk_base; in case the first ever extension attempt of brk fails,
>> +     sbrk_base will point to an mmapped address (see sysmalloc_mmap_fallback),
>> +     in which case these two values will not be equal.  */
>> +  char *init_sbrk_base;
>> +
>>   #if USE_TCACHE
>>     /* Maximum number of small buckets to use.  */
>>     size_t tcache_small_bins;
>> @@ -2667,6 +2673,9 @@ sysmalloc (INTERNAL_SIZE_T nb, mstate av)
>>         if (__glibc_unlikely (mp_.thp_pagesize != 0))
>>   	{
>>   	  uintptr_t lastbrk = (uintptr_t) MORECORE (0);
>> +	  if (mp_.init_sbrk_base == NULL)
>> +	    mp_.init_sbrk_base = (char *) lastbrk;
>> +
>>   	  uintptr_t top = ALIGN_UP (lastbrk + size, mp_.thp_pagesize);
>>   	  size = top - lastbrk;
>>   	}
>> @@ -2682,8 +2691,18 @@ sysmalloc (INTERNAL_SIZE_T nb, mstate av)
>>         if ((ssize_t) size > 0)
>>           {
>>             brk = (char *) (MORECORE ((long) size));
>> -	  if (brk != (char *) (MORECORE_FAILURE))
>> -	    madvise_thp (brk, size);
>> +	  if (brk != (char *) (MORECORE_FAILURE)) {
>> +	    size_t old_size = (size_t) (brk - mp_.init_sbrk_base);
>> +
>> +	    /*
>> +	      If heap already marked with MADV_HUGEPAGE, skip madvise(). Note
>> +	      that, we don't need to check mp_.init_sbrk_base != NULL; if it
>> +	      is NULL, it implies that mp_.thp_pagesize == 0, in which case
>> +	      madvise_thp() will not invoke madvise().
>> +	     */
>> +	    if (old_size < mp_.thp_pagesize)
>> +	    madvise_thp (brk, old_size + size);
> I am not sure if this calculation is fully correct, with a simple testcase:
>
> $ cat t.c
> #include <stdlib.h>
> #include <pthread.h>
>
> static void *tf (void* arg)
> {
>    for (int i = 0; i < 1024; i++)
>      malloc (4096);
>    return NULL;
> }
>
> int main ()
> {
>    for (int i = 0; i < 1024; i++)
>      malloc (4096);
>
>    pthread_t t;
>    pthread_create (&t, NULL, tf, NULL);
>    pthread_join (t, NULL);
> }
> $ strace -e madvise -f -E GLIBC_TUNABLES=glibc.malloc.hugetlb=1 elf/ld.so --library-path . ./t
> madvise(0xaaaade400000, 3354624, MADV_HUGEPAGE) = -1 ENOMEM (Cannot allocate memory)
> madvise(0xf7fa28ed0000, 65536, 0x66 /* MADV_??? */) = 0
> strace: Process 313335 attached
> [pid 313335] madvise(0xf7fa24000000, 2097152, MADV_HUGEPAGE) = 0
> [pid 313335] madvise(0xf7fa28ed0000, 8314880, MADV_DONTNEED) = 0
> [pid 313335] +++ exited with 0 +++
> +++ exited with 0 +++
>
> Where without this patch:
>
> $ strace -e madvise -f -E GLIBC_TUNABLES=glibc.malloc.hugetlb=1 ./t
> madvise(0xc3da42fb9000, 2097152, MADV_HUGEPAGE) = 0
> madvise(0xc3da43200000, 2097152, MADV_HUGEPAGE) = 0
> strace: Process 313347 attached
> [pid 313347] madvise(0xfa19510a0000, 8314880, MADV_DONTNEED) = 0
> [pid 313347] +++ exited with 0 +++
> +++ exited with 0 +++

Thanks for testing! Although without the patch, I get this:

madvise(0xfffff8000000, 2097152, MADV_HUGEPAGE) = 0
madvise(0xfffff8000000, 4194304, MADV_HUGEPAGE) = 0
madvise(0xfffff8000000, 6291456, MADV_HUGEPAGE) = 0
madvise(0xfffff7400000, 65536, 0x66 /* MADV_??? */) = -1 EINVAL (Invalid argument)
strace: Process 3162240 attached
[pid 3162240] madvise(0xfffff0000000, 2097152, MADV_HUGEPAGE) = 0
[pid 3162240] madvise(0xfffff7400000, 8314880, MADV_DONTNEED) = 0
[pid 3162240] +++ exited with 0 +++
+++ exited with 0 +++

I don't know where the EINVAL is coming from, that shouldn't be happening, because we
have hardcoded MADV_HUGEPAGE everywhere. I played around and I think that when madvise() is not invoked
ever, then strace shows it returning EINVAL. I am not familiar with how strace actually
works and how to interpret the output.

But, there is an obvious error in this patch. I shouldn't be making the madvise() call
on brk pointer, but on init_sbrk_base. Calling it on brk will cross the heap VMA, and
the target range may include a hole. I checked the kernel source and it returns -ENOMEM
if it encounters a gap.

But I have a question. In madvise_thp, we align the pointer down if it is not aligned to
pagesize. If before that pointer, there is a gap, then madvise() will return -ENOMEM,
although, the madvise() operation will continue on the VMA which we have specified.
So, the operation will do what we want it to do, but return -ENOMEM. Do you think
this is something which needs to be fixed? The fix is simple: Align the pointer up
instead of down, and subtract from size instead of add. Then we can check again
whether size >= thp_pagesize, and continue with the syscall.

>
>
> The second madvise call 'madvise(0xf7fa28ed0000, 65536, 0x66 /* MADV_??? */)'
> is indeed even bogus.  With gdb the value passed on the syscall are indeed:
>
> (gdb) catch syscall madvise
> (gdb) r
> [...]
> (gdb) i r
> x0             0xfffffffffffffff4  -12
> x1             0x355000            3493888
> x2             0xe                 14
> [...]
>
>> +	  }
>>             LIBC_PROBE (memory_sbrk_more, 2, brk, size);
>>           }
>>