[ewg] [PATCH v4] libibverbs: ibv_fork_init() and huge pages

[Alexander Schmidt]

On Mon, 19 Jul 2010 13:34:29 +0200
Alexander Schmidt <alexs at linux.vnet.ibm.com> wrote:

> Hi Roland,
> 
> this is the fourth version of the huge pages support, please see below for
> change log + description, we appreciate your comments.

Hi Roland,

did you have a chance to take a look at the code? We would like to get this
into upstream libibverbs, so any comments are appreciated.

Regards,
Alex

> 
> Problem description:
> 
> When fork support is enabled in libibverbs, madvise() is called for every
> memory page that is registered as a memory region. Memory ranges that
> are passed to madvise() must be page aligned and the size must be a
> multiple of the page size. libibverbs uses sysconf(_SC_PAGESIZE) to find
> out the system page size and rounds all ranges passed to reg_mr() according
> to this page size. When memory from libhugetlbfs is passed to reg_mr(), this
> does not work as the page size for this memory range might be different
> (e.g. 16Mb). So libibverbs would have to use the huge page size to
> calculate a page aligned range for madvise.
> 
> As huge pages are provided to the application "under the hood" when
> preloading libhugetlbfs, the application does not have any knowledge about
> when it registers a huge page or a usual page.
> 
> To work around this issue, detect the use of huge pages in libibverbs and
> align memory ranges passed to madvise according to the huge page size.
> 
> Changes since v1:
> 
> - detect use of huge pages at ibv_fork_init() time by walking through
>   /sys/kernel/mm/hugepages/
> - read huge page size from /proc/pid/smaps, which contains the page
>   size of the mapping (thereby enabling support for mutliple huge
>   page sizes)
> - code is independent of libhugetlbfs now, so huge pages can be provided
>   to the application by any library
> 
> Changes since v2:
> 
> - reading from /proc/ to determine the huge page size is now only done
>   when a call to madvise() using the system page size fails. So there
>   is no overhead introduced when registering non-huge-page memory.
> 
> Changes since v3:
> 
> - determining the page size of a given memory range by watching madvise()
>   fail has proven to be unreliable. So we introduce the RDMAV_HUGEPAGES_SAFE
>   environment variable to let the user decide if the page size should be
>   checked on every reg_mr() call or not. This requires the user to be aware
>   if huge pages are used by the running application or not.
> 
> I did not add an aditional API call to enable this, as applications can use
> setenv() + ibv_fork_init() to enable checking for huge pages in the code.
> 
> Signed-off-by: Alexander Schmidt <alexs at linux.vnet.ibm.com>
> ---
>  src/memory.c |   94 +++++++++++++++++++++++++++++++++++++++++++++++++++++++----
>  1 file changed, 88 insertions(+), 6 deletions(-)
> 
> --- libibverbs.git.orig/src/memory.c
> +++ libibverbs.git/src/memory.c
> @@ -40,6 +40,10 @@
>  #include <unistd.h>
>  #include <stdlib.h>
>  #include <stdint.h>
> +#include <stdio.h>
> +#include <string.h>
> +#include <dirent.h>
> +#include <limits.h>
> 
>  #include "ibverbs.h"
> 
> @@ -68,12 +72,71 @@ struct ibv_mem_node {
>  static struct ibv_mem_node *mm_root;
>  static pthread_mutex_t mm_mutex = PTHREAD_MUTEX_INITIALIZER;
>  static int page_size;
> +static int huge_page_enabled;
>  static int too_late;
> 
> +static unsigned long smaps_page_size(FILE *file)
> +{
> +	int n;
> +	unsigned long size = page_size;
> +	char buf[1024];
> +
> +	while (fgets(buf, sizeof(buf), file) != NULL) {
> +		if (!strstr(buf, "KernelPageSize:"))
> +			continue;
> +
> +		n = sscanf(buf, "%*s %lu", &size);
> +		if (n < 1)
> +			continue;
> +
> +		/* page size is printed in Kb */
> +		size = size * 1024;
> +
> +		break;
> +	}
> +
> +	return size;
> +}
> +
> +static unsigned long get_page_size(void *base)
> +{
> +	unsigned long ret = page_size;
> +	pid_t pid;
> +	FILE *file;
> +	char buf[1024];
> +
> +	pid = getpid();
> +	snprintf(buf, sizeof(buf), "/proc/%d/smaps", pid);
> +
> +	file = fopen(buf, "r");
> +	if (!file)
> +		goto out;
> +
> +	while (fgets(buf, sizeof(buf), file) != NULL) {
> +		int n;
> +		uintptr_t range_start, range_end;
> +
> +		n = sscanf(buf, "%lx-%lx", &range_start, &range_end);
> +
> +		if (n < 2)
> +			continue;
> +
> +		if ((uintptr_t) base >= range_start && (uintptr_t) base < range_end) {
> +			ret = smaps_page_size(file);
> +			break;
> +		}
> +	}
> +	fclose(file);
> +
> +out:
> +	return ret;
> +}
> +
>  int ibv_fork_init(void)
>  {
> -	void *tmp;
> +	void *tmp, *tmp_aligned;
>  	int ret;
> +	unsigned long size;
> 
>  	if (mm_root)
>  		return 0;
> @@ -88,8 +151,21 @@ int ibv_fork_init(void)
>  	if (posix_memalign(&tmp, page_size, page_size))
>  		return ENOMEM;
> 
> -	ret = madvise(tmp, page_size, MADV_DONTFORK) ||
> -	      madvise(tmp, page_size, MADV_DOFORK);
> +	if (getenv("RDMAV_HUGEPAGES_SAFE"))
> +		huge_page_enabled = 1;
> +	else
> +		huge_page_enabled = 0;
> +
> +	if (huge_page_enabled) {
> +		size = get_page_size(tmp);
> +		tmp_aligned = (void *)((uintptr_t)tmp & ~(size - 1));
> +	} else {
> +		size = page_size;
> +		tmp_aligned = tmp;
> +	}
> +
> +	ret = madvise(tmp_aligned, size, MADV_DONTFORK) ||
> +	      madvise(tmp_aligned, size, MADV_DOFORK);
> 
>  	free(tmp);
> 
> @@ -529,13 +605,19 @@ static int ibv_madvise_range(void *base,
>  	int inc;
>  	int rolling_back = 0;
>  	int ret = 0;
> + 	unsigned long range_page_size;
> 
>  	if (!size)
>  		return 0;
> 
> -	start = (uintptr_t) base & ~(page_size - 1);
> -	end   = ((uintptr_t) (base + size + page_size - 1) &
> -		 ~(page_size - 1)) - 1;
> + 	if (huge_page_enabled)
> + 		range_page_size = get_page_size(base);
> + 	else
> + 		range_page_size = page_size;
> +
> + 	start = (uintptr_t) base & ~(range_page_size - 1);
> + 	end   = ((uintptr_t) (base + size + range_page_size - 1) &
> + 		 ~(range_page_size - 1)) - 1;
> 
>  	pthread_mutex_lock(&mm_mutex);
>  again:
> --
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