[PATCH] mm: pass mm to grab_swap_token

Hugh Dickins
2009-06-21T13:02:57Z

If a kthread happens to use get_user_pages() on an mm (as KSM does),
there's a chance that it will end up trying to read in a swap page,
then oops in grab_swap_token() because the kthread has no mm: GUP
passes down the right mm, so grab_swap_token() ought to be using it.

We have not identified a stronger case than KSM's daemon (not yet
in mainline), but the issue must have come up before, since RHEL has
included a fix for this for years (though a different fix, they just
back out of grab_swap_token if current->mm is unset: which is what
we first proposed, but using the right mm here seems more correct).

Reported-by: Izik Eidus <ieidus@redhat.com>
Signed-off-by: Johannes Weiner <hannes@cmpxchg.org>
Signed-off-by: Hugh Dickins <hugh.dickins@tiscali.co.uk>
Acked-by: Rik van Riel <riel@redhat.com>
---
Includes a small swap.h tidyup inspired by checkpatch.pl.
Due to crossed mails and crossed wires, there's a competing
patch-pair from Hannes:

http://lkml.org/lkml/2009/6/16/623
http://lkml.org/lkml/2009/6/16/624

We're all agreed that either this or those should go into 2.6.31:
personally, I don't rate arg typechecking of null functions when
!CONFIG_SWAP very highly, and prefer the one-line macros of Rik's
original; but you may well disagree, and prefer to take Hannes's.

include/linux/swap.h | 12 ++++++------
mm/memory.c | 2 +-
mm/thrash.c | 32 +++++++++++++++-----------------
3 files changed, 22 insertions(+), 24 deletions(-)

--- 2.6.30-git17/include/linux/swap.h 2009-06-21 07:48:18.000000000 +0100
+++ linux/include/linux/swap.h 2009-06-21 09:35:51.000000000 +0100
@@ -298,8 +298,8 @@ extern int try_to_free_swap(struct page
struct backing_dev_info;

/* linux/mm/thrash.c */
-extern struct mm_struct * swap_token_mm;
-extern void grab_swap_token(void);
+extern struct mm_struct *swap_token_mm;
+extern void grab_swap_token(struct mm_struct *);
extern void __put_swap_token(struct mm_struct *);

static inline int has_swap_token(struct mm_struct *mm)
@@ -419,10 +419,10 @@ static inline swp_entry_t get_swap_page(
}

/* linux/mm/thrash.c */
-#define put_swap_token(x) do { } while(0)
-#define grab_swap_token() do { } while(0)
-#define has_swap_token(x) 0
-#define disable_swap_token() do { } while(0)
+#define put_swap_token(mm) do { } while (0)
+#define grab_swap_token(mm) do { } while (0)
+#define has_swap_token(mm) 0
+#define disable_swap_token() do { } while (0)

static inline void
mem_cgroup_uncharge_swapcache(struct page *page, swp_entry_t ent)
--- 2.6.30-git17/mm/memory.c 2009-06-21 07:48:19.000000000 +0100
+++ linux/mm/memory.c 2009-06-21 09:35:51.000000000 +0100
@@ -2516,7 +2516,7 @@ static int do_swap_page(struct mm_struct
delayacct_set_flag(DELAYACCT_PF_SWAPIN);
page = lookup_swap_cache(entry);
if (!page) {
- grab_swap_token(); /* Contend for token _before_ read-in */
+ grab_swap_token(mm); /* Contend for token _before_ read-in */
page = swapin_readahead(entry,
GFP_HIGHUSER_MOVABLE, vma, address);
if (!page) {
--- 2.6.30-git17/mm/thrash.c 2007-07-09 00:32:17.000000000 +0100
+++ linux/mm/thrash.c 2009-06-21 09:35:51.000000000 +0100
@@ -26,47 +26,45 @@ static DEFINE_SPINLOCK(swap_token_lock);
struct mm_struct *swap_token_mm;
static unsigned int global_faults;

-void grab_swap_token(void)
+void grab_swap_token(struct mm_struct *mm)
{
int current_interval;

global_faults++;

- current_interval = global_faults - current->mm->faultstamp;
+ current_interval = global_faults - mm->faultstamp;

if (!spin_trylock(&swap_token_lock))
return;

/* First come first served */
if (swap_token_mm == NULL) {
- current->mm->token_priority = current->mm->token_priority + 2;
- swap_token_mm = current->mm;
+ mm->token_priority = mm->token_priority + 2;
+ swap_token_mm = mm;
goto out;
}

- if (current->mm != swap_token_mm) {
- if (current_interval < current->mm->last_interval)
- current->mm->token_priority++;
+ if (mm != swap_token_mm) {
+ if (current_interval < mm->last_interval)
+ mm->token_priority++;
else {
- if (likely(current->mm->token_priority > 0))
- current->mm->token_priority--;
+ if (likely(mm->token_priority > 0))
+ mm->token_priority--;
}
/* Check if we deserve the token */
- if (current->mm->token_priority >
- swap_token_mm->token_priority) {
- current->mm->token_priority += 2;
- swap_token_mm = current->mm;
+ if (mm->token_priority > swap_token_mm->token_priority) {
+ mm->token_priority += 2;
+ swap_token_mm = mm;
}
} else {
/* Token holder came in again! */
- current->mm->token_priority += 2;
+ mm->token_priority += 2;
}

out:
- current->mm->faultstamp = global_faults;
- current->mm->last_interval = current_interval;
+ mm->faultstamp = global_faults;
+ mm->last_interval = current_interval;
spin_unlock(&swap_token_lock);
-return;
}

/* Called on process exit. */