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swapoff_one(): only check free pages count manually turning swap off

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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
This commit is contained in:
Konstantin Belousov 2021-11-27 01:22:27 +02:00
parent 8661e085fb
commit 0190c38b9d
1 changed files with 14 additions and 5 deletions
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19
sys/vm/swap_pager.c
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@ -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);
done:
sx_xunlock(&swdev_syscall_lock);
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);
else
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);