If you create a shared mapping of a sparse file, dirty it
and then run msync, all the file's blocks are laid out
backwards:
mnm:/mnt/sda6> 0 bmap foo
0-0: 530-530 (1)
1-1: 529-529 (1)
2-2: 528-528 (1)
3-3: 527-527 (1)
4-4: 526-526 (1)
5-5: 525-525 (1)
6-6: 524-524 (1)
7-7: 523-523 (1)
8-8: 522-522 (1)
9-9: 521-521 (1)
10-10: 520-520 (1)
11-11: 519-519 (1)
12-12: 518-518 (1)
13-13: 517-517 (1)
14-14: 516-516 (1)
15-15: 515-515 (1)
This is because filemap_sync puts the lowest-index page at
mapping->dirty_pages.prev and the highest at mapping->dirty_pages.next.
I think that by walking the dirty pages list in ascending file
offset order as we instantiate their disk mappings we will generally
get better layout.
mnm:/mnt/sda6> 0 bmap foo2
0-11: 531-542 (12)
12-15: 544-547 (4)
--- linux-2.5.6/mm/filemap.c Sat Mar 9 00:18:43 2002
+++ 25/mm/filemap.c Sat Mar 9 17:41:29 2002
@@ -550,7 +550,7 @@ int filemap_fdatasync(struct address_spa
spin_lock(&pagecache_lock);
while (!list_empty(&mapping->dirty_pages)) {
- struct page *page = list_entry(mapping->dirty_pages.next, struct page, list);
+ struct page *page = list_entry(mapping->dirty_pages.prev, struct page, list);
list_del(&page->list);
list_add(&page->list, &mapping->locked_pages);
#include <stdio.h>
#include <unistd.h>
#include <fcntl.h>
#include <stdlib.h>
#include <string.h>
#include <errno.h>
#include <sys/mman.h>
int main(int argc, char *argv[])
{
int fd;
loff_t size = 4096*16;
char *filename;
void *mapped_mem;
if (argc != 2) {
fprintf(stderr, "Usage; %s filename\n", argv[0]);
exit(1);
}
filename = argv[1];
fd = open(filename, O_RDWR|O_TRUNC|O_CREAT, 0666);
if (fd < 0) {
fprintf(stderr, "%s: Cannot open `%s': %s\n",
argv[0], filename, strerror(errno));
exit(1);
}
printf("%s: expanding `%s' to size %Ld\n", argv[0], filename, size);
if (ftruncate(fd, size) < 0) {
perror("ftruncate");
exit(1);
}
mapped_mem = mmap(0, size, PROT_READ|PROT_WRITE, MAP_SHARED, fd, 0);
if (mapped_mem == MAP_FAILED) {
perror("mmap");
exit(1);
}
printf("dirtying %Ld bytes of memory\n", size);
memset(mapped_mem, 0, size);
msync(mapped_mem, size, MS_SYNC);
exit(0);
}
-
-
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This archive was generated by hypermail 2b29 : Fri Mar 15 2002 - 22:00:13 EST