swap_pager: indefinite wait buffer

[tech-lists]

Hello,

On Thu, Feb 27, 2020 at 12:29:39PM +0100, Luciano Mannucci wrote:
>
>Hello everybody!
>
>Since my last upgrade (to 11.3-RELASE-p6) I've noticed some messages
>in the daily security report of one of my PPC 64 virtual servers, that
>runs under KVM/Qemu, which says:
>
>Feb 26 20:17:53 Saguaro kernel: swap_pager: indefinite wait buffer: bufobj: 0, blkno: 1698063, size: 20480

I also see this - 12.1-stable r358260 and the load it happens with is
poudriere, but it'll only happen when building the largest packages. I have to
re-run the poudriere run. It'll eventually complete.

>and swapinfo says:
>
>Device          1K-blocks     Used    Avail Capacity
>/dev/md99         2097152   420564  1676588    20%
>/dev/md98         2097152   419352  1677800    20%
>Total             4194304   839916  3354388    20%

I also use a swapfile, but as well as that I have a 4GB swap partition. I've
had the swapfile up to 32GB and it still happens. If I turn off the swapfile,
the swap partition will fill up leading to out-of-swapspace errors. The
indefinite-wait-buffer messages only happen with a swapfile.

>Is there something I should check?
>Any kernel variables tunable via sysctl?

I haven't found a fix for this yet. I was thinking of reinstalling the system
but this time making a 32GB partition for swap (and dispensing with the
swapfile). I would have hoped that a swapfile could be used just like a swap
partition but alas it seems not. There are vm_swap tunables in sysctl but I
have no idea what they do and, as far as I can tell, appear not to be
documented.
-- 
J.
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