fmap: reduce amount of mmap() calls in fmap_aging.

Scanning /dev/shm on an old kernel is serialized due to the additional mmap
contention that fmap_aging generates.
With this change it regains some parallelism (still not optimal), and it also
helps on newer kernels (less mmap calls is always good).
This patch makes just one mmap call for an entire range of pages,
if we try to unmap adjacent pages.

libclamav/fmap.c

@@ -189,13 +189,33 @@ static void fmap_aging(fmap_t *m) {
 	    }
 	}
 	if(avail) { /* at least one page is paged and not locked */
+	    char *lastpage = NULL;
+	    char *firstpage = NULL;
 	    for(i=0; i<avail; i++) {
 		char *pptr = (char *)m + freeme[i] * m->pgsz + m->hdrsz;
 		/* we mark the page as seen */
 		fmap_bitmap[freeme[i]] = FM_MASK_SEEN;
 		/* and we mmap the page over so the kernel knows there's nothing good in there */
+		/* reduce number of mmap calls: if pages are adjacent only do 1 mmap call */
+		if (lastpage && pptr == lastpage) {
+			lastpage = pptr + m->pgsz;
+			continue;
+		}
+		if (!lastpage) {
+			firstpage = pptr;
+			lastpage = pptr + m->pgsz;
+			continue;
+		}
+		fmap_lock;
+		if(mmap(firstpage, lastpage - firstpage, PROT_READ | PROT_WRITE, MAP_FIXED|MAP_PRIVATE|ANONYMOUS_MAP, -1, 0) == MAP_FAILED)
+		    cli_dbgmsg("fmap_aging: kernel hates you\n");
+		fmap_unlock;
+		firstpage = pptr;
+		lastpage = pptr + m->pgsz;
+	    }
+	    if (lastpage) {
 		fmap_lock;
-		if(mmap(pptr, m->pgsz, PROT_READ | PROT_WRITE, MAP_FIXED|MAP_PRIVATE|ANONYMOUS_MAP, -1, 0) == MAP_FAILED)
+		if(mmap(firstpage, lastpage - firstpage, PROT_READ | PROT_WRITE, MAP_FIXED|MAP_PRIVATE|ANONYMOUS_MAP, -1, 0) == MAP_FAILED)
 		    cli_dbgmsg("fmap_aging: kernel hates you\n");
 		fmap_unlock;
 	    }