diff options
authorKonstantin Belousov <kib@FreeBSD.org>2021-11-26 23:22:27 +0000
committerKonstantin Belousov <kib@FreeBSD.org>2021-11-29 16:38:02 +0000
commit0190c38b9dfaa16de1bc61e829b9a1221fed7896 (patch)
parent8661e085fb953855dbc7059f21a64a05ae61b22c (diff)
swapoff_one(): only check free pages count manually turning swap off
When swap is turned off due to system shutdown or reboot, ignore the check. Problem is that the check is not accurate by any means, free page count can legitimately be low while system still able to page in everything from the swap. Then, we turn swap off if swapping on real file or some non-standard geom provider, and typically panic when system appears to actually need to unavailable page. For syscall, it is better to be safe than sorry. Reported and tested by: peterj Reviewed by: markj Sponsored by: The FreeBSD Foundation MFC after: 1 week Differential revision: https://reviews.freebsd.org/D33147
1 files changed, 14 insertions, 5 deletions
diff --git a/sys/vm/swap_pager.c b/sys/vm/swap_pager.c
index 4cfdb3fd2cc8..f313d8b2d014 100644
--- a/sys/vm/swap_pager.c
+++ b/sys/vm/swap_pager.c
@@ -469,7 +469,8 @@ static bool swp_pager_swblk_empty(struct swblk *sb, int start, int limit);
static void swp_pager_free_empty_swblk(vm_object_t, struct swblk *sb);
static int swapongeom(struct vnode *);
static int swaponvp(struct thread *, struct vnode *, u_long);
-static int swapoff_one(struct swdevt *sp, struct ucred *cred);
+static int swapoff_one(struct swdevt *sp, struct ucred *cred,
+ bool ignore_check);
* Swap bitmap functions
@@ -2523,14 +2524,14 @@ sys_swapoff(struct thread *td, struct swapoff_args *uap)
error = EINVAL;
goto done;
- error = swapoff_one(sp, td->td_ucred);
+ error = swapoff_one(sp, td->td_ucred, false);
return (error);
static int
-swapoff_one(struct swdevt *sp, struct ucred *cred)
+swapoff_one(struct swdevt *sp, struct ucred *cred, bool ignore_check)
u_long nblks;
#ifdef MAC
@@ -2552,8 +2553,16 @@ swapoff_one(struct swdevt *sp, struct ucred *cred)
* available virtual memory in the system will fit the amount
* of data we will have to page back in, plus an epsilon so
* the system doesn't become critically low on swap space.
+ * The vm_free_count() part does not account e.g. for clean
+ * pages that can be immediately reclaimed without paging, so
+ * this is a very rough estimation.
+ *
+ * On the other hand, not turning swap off on swapoff_all()
+ * means that we can lose swap data when filesystems go away,
+ * which is arguably worse.
- if (vm_free_count() + swap_pager_avail < nblks + nswap_lowat)
+ if (!ignore_check &&
+ vm_free_count() + swap_pager_avail < nblks + nswap_lowat)
return (ENOMEM);
@@ -2603,7 +2612,7 @@ swapoff_all(void)
devname = devtoname(sp->sw_vp->v_rdev);
devname = "[file]";
- error = swapoff_one(sp, thread0.td_ucred);
+ error = swapoff_one(sp, thread0.td_ucred, true);
if (error != 0) {
printf("Cannot remove swap device %s (error=%d), "
"skipping.\n", devname, error);