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Date: Sun, 31 Jan 2010 15:57:19 +0000 (GMT)
From: Hugh Dickins <hugh.dickins@...cali.co.uk>
To: Jarkko Lavinen <jarkko.lavinen@...ia.com>
cc: Andrew Morton <akpm@...l.org>, Nitin Gupta <ngupta@...are.org>,
Jakub Jelinek <jakub@...hat.com>, Karel Zak <kzak@...hat.com>,
linux-mm@...ck.org, linux-kernel@...r.kernel.org
Subject: Re: [PATCH] mm: Fix nr_good_pages calculation
On Wed, 20 Jan 2010, Jarkko Lavinen wrote:
> Swapon wastes one page of swap space to no effect.
>
> Mkswap stores the value 'pages - 1' into last_page field, where pages is the
> partition size in pages. When nr_good_pages is calculated, last_page + 1
> should be used for the number of all the pages header included.
>
> Signed-off-by: Jarkko Lavinen <jarkko.lavinen@...ia.com>
> ---
> mm/swapfile.c | 2 +-
> 1 files changed, 1 insertions(+), 1 deletions(-)
>
> diff --git a/mm/swapfile.c b/mm/swapfile.c
> index 6c0585b..50d90ca 100644
> --- a/mm/swapfile.c
> +++ b/mm/swapfile.c
> @@ -1961,7 +1961,7 @@ SYSCALL_DEFINE2(swapon, const char __user *, specialfile, int, swap_flags)
> if (error)
> goto bad_swap;
>
> - nr_good_pages = swap_header->info.last_page -
> + nr_good_pages = swap_header->info.last_page + 1 -
> swap_header->info.nr_badpages -
> 1 /* header page */;
>
> --
Sorry to be so slow to respond: I needed to do a little research, and
find time to cool myself down near absolute zero to consider offs-by-one.
You're right that there's an off-by-one there: Nitin and I also noticed
it last year; but I'm afraid I have to Nack your patch, just as I did
Nitin's similar patch last August.
Fixing up nr_good_pages like that makes the numbers displayed look right;
but you're then inconsistent with p->max, counting a page which can never
get to be used.
Here's what I believe is the right patch: is this something you and Nitin
can give your Acks to, or do you see further issues with it?
Thanks,
Hugh
[PATCH] mm: fix swapon size off-by-one
There's an off-by-one disagreement between mkswap and swapon about the
meaning of swap_header last_page: mkswap (in all versions I've looked
at: util-linux-ng and BusyBox and old util-linux; probably as far back
as 1999) consistently means the offset (in page units) of the last page
of the swap area, whereas kernel sys_swapon (as far back as 2.2 and 2.3)
strangely takes it to mean the size (in page units) of the swap area.
This disagreement is the safe way round; but it's worrying people,
and loses us one page of swap.
The fix is not just to add one to nr_good_pages: we need to get maxpages
(the size of the swap_map array) right before that; and though that is an
unsigned long, be careful not to overflow the unsigned int p->max which
later holds it (probably why header uses __u32 last_page instead of size).
Why did we subtract one from the maximum swp_offset to calculate maxpages?
Though it was probably me who made that change in 2.4.10, I don't get it:
and now we should be adding one (without risk of overflow in this case).
Fix the handling of swap_header badpages: it could have overrun the
swap_map when very large swap area used on a more limited architecture.
Remove pre-initializations of swap_header, nr_good_pages and maxpages:
those date from when sys_swapon was supporting other versions of header.
Reported-by: Nitin Gupta <ngupta@...are.org>
Reported-by: Jarkko Lavinen <jarkko.lavinen@...ia.com>
Signed-off-by: Hugh Dickins <hugh.dickins@...cali.co.uk>
---
mm/swapfile.c | 31 ++++++++++++++++++-------------
1 file changed, 18 insertions(+), 13 deletions(-)
--- 2.6.33-rc6/mm/swapfile.c 2009-12-18 11:42:55.000000000 +0000
+++ linux/mm/swapfile.c 2010-01-31 13:15:58.000000000 +0000
@@ -1759,11 +1759,11 @@ SYSCALL_DEFINE2(swapon, const char __use
unsigned int type;
int i, prev;
int error;
- union swap_header *swap_header = NULL;
- unsigned int nr_good_pages = 0;
+ union swap_header *swap_header;
+ unsigned int nr_good_pages;
int nr_extents = 0;
sector_t span;
- unsigned long maxpages = 1;
+ unsigned long maxpages;
unsigned long swapfilepages;
unsigned char *swap_map = NULL;
struct page *page = NULL;
@@ -1922,9 +1922,13 @@ SYSCALL_DEFINE2(swapon, const char __use
* swap pte.
*/
maxpages = swp_offset(pte_to_swp_entry(
- swp_entry_to_pte(swp_entry(0, ~0UL)))) - 1;
- if (maxpages > swap_header->info.last_page)
- maxpages = swap_header->info.last_page;
+ swp_entry_to_pte(swp_entry(0, ~0UL)))) + 1;
+ if (maxpages > swap_header->info.last_page) {
+ maxpages = swap_header->info.last_page + 1;
+ /* p->max is an unsigned int: don't overflow it */
+ if ((unsigned int)maxpages == 0)
+ maxpages = UINT_MAX;
+ }
p->highest_bit = maxpages - 1;
error = -EINVAL;
@@ -1948,23 +1952,24 @@ SYSCALL_DEFINE2(swapon, const char __use
}
memset(swap_map, 0, maxpages);
+ nr_good_pages = maxpages - 1; /* omit header page */
+
for (i = 0; i < swap_header->info.nr_badpages; i++) {
- int page_nr = swap_header->info.badpages[i];
- if (page_nr <= 0 || page_nr >= swap_header->info.last_page) {
+ unsigned int page_nr = swap_header->info.badpages[i];
+ if (page_nr == 0 || page_nr > swap_header->info.last_page) {
error = -EINVAL;
goto bad_swap;
}
- swap_map[page_nr] = SWAP_MAP_BAD;
+ if (page_nr < maxpages) {
+ swap_map[page_nr] = SWAP_MAP_BAD;
+ nr_good_pages--;
+ }
}
error = swap_cgroup_swapon(type, maxpages);
if (error)
goto bad_swap;
- nr_good_pages = swap_header->info.last_page -
- swap_header->info.nr_badpages -
- 1 /* header page */;
-
if (nr_good_pages) {
swap_map[0] = SWAP_MAP_BAD;
p->max = maxpages;
--
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