From 196e18db462e56415965762a6bbd7217aeea792b Mon Sep 17 00:00:00 2001
From: Pantelis Antoniou <pantelis.antoniou@konsulko.com>
Date: Fri, 22 Sep 2023 17:58:08 +0300
Subject: [PATCH] linux file backing store probing

Signed-off-by: Pantelis Antoniou <pantelis.antoniou@konsulko.com>
---
 src/blake3/blake3_host_state.c | 122 +++++++++++++++++++++++++++++++--
 1 file changed, 118 insertions(+), 4 deletions(-)

diff --git a/src/blake3/blake3_host_state.c b/src/blake3/blake3_host_state.c
index 4f83666e..cf16166f 100644
--- a/src/blake3/blake3_host_state.c
+++ b/src/blake3/blake3_host_state.c
@@ -15,6 +15,7 @@
 #include <sys/types.h>
 #include <sys/stat.h>
 #include <sys/mman.h>
+#include <sys/sysmacros.h>
 
 #include "blake3.h"
 #include "blake3_impl.h"
@@ -222,6 +223,109 @@ void blake3_hasher_destroy(blake3_hasher *self)
 	free(self);
 }
 
+#if defined(__linux__)
+
+static int linux_block_dev_is_rotational(dev_t dev)
+{
+	char *path = NULL;
+	int rc, fd;
+	uint8_t c[2];
+	int rotational;
+
+	/* by default it's the filesize */
+	rotational = -1;	/* not found */
+
+	/* read the rotational attribute of the root */
+	rc = asprintf(&path, "/sys/dev/block/%u:%u/queue/rotational", major(dev), 0);
+	if (rc != -1) {
+		fd = open(path, O_RDONLY);
+		if (fd != -1) {
+			if (read(fd, c, 2) == 2)
+				rotational = c[0] == '1' && c[1] == '\n';
+			close(fd);
+		}
+		free(path);
+	}
+
+	return rotational;
+}
+
+static ssize_t linux_file_cached_size(int fd, void *mem, size_t filesize)
+{
+	long pagesize = sysconf(_SC_PAGESIZE);
+	size_t vec_size, resident_size, i;
+	unsigned char *vec;
+	int rc;
+
+	(void)rc;
+	vec_size = (filesize + pagesize - 1) / pagesize;
+	vec = malloc(vec_size + 1);
+	if (!vec)
+		return -1;
+
+	rc = mincore(mem, filesize, vec);
+	assert(!rc);
+
+	/* this could be parallelized */
+	resident_size = 0;
+	for (i = 0; i < vec_size; i++) {
+		if (vec[i] & 1)
+			resident_size += pagesize;
+	}
+
+	free(vec);
+
+	if (resident_size > filesize)
+		resident_size = filesize;
+
+	return (ssize_t)resident_size;
+}
+
+#define BLAKE3_MMAP_MIN_CHUNKSIZE	(1U << 20)	// minimum chunksize is 1MB
+
+static size_t blake3_mmap_file_chunksize(int fd, dev_t dev, void *mem, size_t filesize)
+{
+	size_t chunksize;
+	ssize_t cached_size;
+	int rotational;
+
+	/* if the file is small enough don't bother with the rest */
+	if (filesize <= BLAKE3_MMAP_MIN_CHUNKSIZE)
+		return BLAKE3_MMAP_MIN_CHUNKSIZE;
+
+	/* by default it's the filesize */
+	chunksize = filesize;
+
+	/* starting, check if the rotational attribute exists in this dev */
+	rotational = linux_block_dev_is_rotational(dev);
+	if (rotational == -1) {
+		/* attribute not found? check the non-partition */
+		rotational = linux_block_dev_is_rotational(makedev(major(dev), 0));
+	}
+
+	if (rotational == 1) {
+		/* OK, it's rotational, but is it in cache?
+		 * to avoid checking for the cached status of the whole file
+		 * we just probe the MIN_CHUNKSIZE
+		 * We will thrash in the case where the file is only cached
+		 * for the first few bytes, but this is generally unusual.
+		 */
+		cached_size = linux_file_cached_size(fd, mem, BLAKE3_MMAP_MIN_CHUNKSIZE);
+
+		if (cached_size <= 0)
+			chunksize = BLAKE3_MMAP_MIN_CHUNKSIZE;
+	}
+
+	return chunksize;
+}
+#else
+static size_t blake3_mmap_file_chunksize(int fd, dev_t dev, void *mem, size_t filesize)
+{
+	/* for all others, just use mmap at max */
+	return filesize;
+}
+#endif
+
 /* 256K threshold for using alloca */
 #define BLAKE3_ALLOCA_BUFFER_SIZE	(256U << 10)
 
@@ -230,11 +334,11 @@ int blake3_hash_file(blake3_hasher *hasher, const char *filename,
 {
 	blake3_host_state *hs;
 	FILE *fp = NULL;
-	void *mem = NULL;
-	void *buf = NULL;
+	void *mem = NULL, *buf = NULL, *p;
 	int fd = -1, ret = -1;
-	size_t rdn, filesize, bufsz = 0;
+	size_t rdn, filesize, bufsz = 0, max_chunk, left, chunk;
 	struct stat sb;
+	dev_t dev = 0;
 	int rc;
 
 	if (!hasher || !filename || !output)
@@ -278,6 +382,7 @@ int blake3_hash_file(blake3_hasher *hasher, const char *filename,
 			if (mem != MAP_FAILED) {
 				close(fd);
 				fd = -1;
+				dev = sb.st_dev;
 			} else
 				mem = NULL;
 		}
@@ -296,7 +401,16 @@ int blake3_hash_file(blake3_hasher *hasher, const char *filename,
 
 	/* mmap case, very simple */
 	if (mem) {
-		blake3_hasher_update(hasher, mem, filesize);
+		max_chunk = blake3_mmap_file_chunksize(fd, dev, mem, filesize);
+
+		p = mem;
+		left = filesize;
+		while (left > 0) {
+			chunk = left > max_chunk ? max_chunk : left;
+			blake3_hasher_update(hasher, p, chunk);
+			p += chunk;
+			left -= chunk;
+		}
 	} else {
 		/* slow path using file reads */