Re: [PATCH v2] setup: dynamically detect default huge page size

Prateek <[email protected]> writes:

>     Replaces the hardcoded 2MB huge page size with dynamic detection by
>     parsing /proc/meminfo. This fixes no-mmap allocation failures on
>     architectures with different default huge page sizes (like ARM64
>     which often uses 512MB) or x86 systems configured for 1GB pages.
>
>     - Safely parses /proc/meminfo without allocating memory.
>     - Adds a __uring_memcmp shim for CONFIG_NOLIBC builds, allowing
>       setup.c to use standard memcmp for the Hugepagesize: match.
>     - Drops the MAP_HUGE_2MB mmap flag to allow the kernel to correctly
>       apply the system's default huge page size.
>     - Falls back safely to 2MB if /proc/meminfo is unreadable.

Reviewed-by: Gabriel Krisman Bertazi <[email protected]>

>
> Signed-off-by: Prateek <[email protected]>
> ---
> Changes in v2:
> - Initialized hps explicitly to 0.
> - Replaced the char-by-char Hugepagesize comparison with a new __uring_memcmp helper.
> - Removed the redundant ret variable and simplified the fallback assignment using a ternary operator.
>
>  src/lib.h    |  2 ++
>  src/nolibc.c | 19 +++++++++++++
>  src/setup.c  | 75 +++++++++++++++++++++++++++++++++++++++++-----------
>  3 files changed, 80 insertions(+), 16 deletions(-)
>
> diff --git a/src/lib.h b/src/lib.h
> index 4d32d3e1..463dd4b5 100644
> --- a/src/lib.h
> +++ b/src/lib.h
> @@ -41,10 +41,12 @@
>  void *__uring_memset(void *s, int c, size_t n);
>  void *__uring_malloc(size_t len);
>  void __uring_free(void *p);
> +int __uring_memcmp(const void *s1, const void *s2, size_t n);
>  
>  #define malloc(LEN)		__uring_malloc(LEN)
>  #define free(PTR)		__uring_free(PTR)
>  #define memset(PTR, C, LEN)	__uring_memset(PTR, C, LEN)
> +#define memcmp(S1, S2, LEN)	__uring_memcmp(S1, S2, LEN)
>  #endif
>  
>  #endif /* #ifndef LIBURING_LIB_H */
> diff --git a/src/nolibc.c b/src/nolibc.c
> index 88b1494a..14ede500 100644
> --- a/src/nolibc.c
> +++ b/src/nolibc.c
> @@ -25,6 +25,25 @@ void *__uring_memset(void *s, int c, size_t n)
>  	return s;
>  }
>  
> +int __uring_memcmp(const void *s1, const void *s2, size_t n)
> +{
> +	size_t i;
> +	const unsigned char *p1 = s1, *p2 = s2;
> +
> +	for (i = 0; i < n; i++) {
> +		if (p1[i] != p2[i])
> +			return p1[i] - p2[i];
> +
> +		/*
> +		 * An empty inline ASM to avoid auto-vectorization
> +		 * because it's too bloated for liburing.
> +		 */
> +		__asm__ volatile ("");
> +	}
> +
> +	return 0;
> +}
> +
>  struct uring_heap {
>  	size_t		len;
>  	char		user_p[] __attribute__((__aligned__));
> diff --git a/src/setup.c b/src/setup.c
> index ea6f11fd..88f86784 100644
> --- a/src/setup.c
> +++ b/src/setup.c
> @@ -220,15 +220,58 @@ __cold int io_uring_ring_dontfork(struct io_uring *ring)
>  	return 0;
>  }
>  
> -#ifndef MAP_HUGE_SHIFT
> -#define MAP_HUGE_SHIFT	26
> -#endif
> -#ifndef MAP_HUGE_2MB
> -#define MAP_HUGE_2MB	(21U << MAP_HUGE_SHIFT)
> -#endif
>  
> -/* FIXME */
> -static size_t huge_page_size = 2 * 1024 * 1024;
> +static size_t get_huge_page_size(void)
> +{
> +	static size_t hps = 0;
> +	char buf[4096];
> +	char *p, *end;
> +	unsigned long val = 0;
> +	ssize_t n;
> +	int fd;
> +
> +	if (hps)
> +		return hps;
> +
> +	fd = __sys_open("/proc/meminfo", O_RDONLY, 0);
> +	if (fd < 0)
> +		goto out;
> +
> +	n = __sys_read(fd, buf, sizeof(buf) - 1);
> +	__sys_close(fd);
> +	if (n <= 0)
> +		goto out;
> +	buf[n] = '\0';
> +
> +	/*
> +	 * Scan line-by-line for "Hugepagesize:".
> +	 */
> +	p = buf;
> +	end = buf + n;
> +	while (p < end) {
> +		/* Check if this line starts with "Hugepagesize:" (13 chars) */
> +		if (p + 13 <= end && !memcmp(p, "Hugepagesize:", 13)) {
> +			p += 13;
> +			while (p < end && (*p == ' ' || *p == '\t'))
> +				p++;
> +			val = 0;
> +			while (p < end && *p >= '0' && *p <= '9') {
> +				val = val * 10 + (*p - '0');
> +				p++;
> +			}
> +			break;
> +		}
> +		/* Advance to next line */
> +		while (p < end && *p != '\n')
> +			p++;
> +		if (p < end)
> +			p++;
> +	}
> +out:
> +	hps = val ? val * 1024 : 2 * 1024 * 1024;
> +	return hps;
> +}
> +
>  
>  #define KRING_SIZE	64
>  
> @@ -261,13 +304,13 @@ static int io_uring_alloc_huge(unsigned entries, struct io_uring_params *p,
>  	mem_used = (mem_used + page_size - 1) & ~(page_size - 1);
>  
>  	/*
> -	 * A maxed-out number of CQ entries with IORING_SETUP_CQE32 fills a 2MB
> -	 * huge page by itself, so the SQ entries won't fit in the same huge
> -	 * page. For SQEs, that shouldn't be possible given KERN_MAX_ENTRIES,
> +	 * A maxed-out number of CQ entries with IORING_SETUP_CQE32 can fill a
> +	 * single huge page by itself, so the SQ entries won't fit in the same
> +	 * huge page. For SQEs, that shouldn't be possible given KERN_MAX_ENTRIES,
>  	 * but check that too to future-proof (e.g. against different huge page
>  	 * sizes). Bail out early so we don't overrun.
>  	 */
> -	if (!buf && (sqes_mem > huge_page_size || ring_mem > huge_page_size))
> +	if (!buf && (sqes_mem > get_huge_page_size() || ring_mem > get_huge_page_size()))
>  		return -ENOMEM;
>  
>  	if (buf) {
> @@ -279,8 +322,8 @@ static int io_uring_alloc_huge(unsigned entries, struct io_uring_params *p,
>  		if (sqes_mem <= page_size)
>  			buf_size = page_size;
>  		else {
> -			buf_size = huge_page_size;
> -			map_hugetlb = MAP_HUGETLB | MAP_HUGE_2MB;
> +			buf_size = get_huge_page_size();
> +			map_hugetlb = MAP_HUGETLB;
>  		}
>  		sqes_size = buf_size;
>  		ptr = __sys_mmap(NULL, sqes_size, PROT_READ|PROT_WRITE,
> @@ -302,8 +345,8 @@ static int io_uring_alloc_huge(unsigned entries, struct io_uring_params *p,
>  		if (ring_mem <= page_size)
>  			buf_size = page_size;
>  		else {
> -			buf_size = huge_page_size;
> -			map_hugetlb = MAP_HUGETLB | MAP_HUGE_2MB;
> +			buf_size = get_huge_page_size();
> +			map_hugetlb = MAP_HUGETLB;
>  		}
>  		ptr = __sys_mmap(NULL, buf_size, PROT_READ|PROT_WRITE,
>  					MAP_SHARED|MAP_ANONYMOUS|map_hugetlb,
> -- 
> 2.43.0
>

-- 
Gabriel Krisman Bertazi