RE: FreeBSD 14-0 file swapping broken.

Artem Hevorhian <artemhevorhian_at_gmail.com> wrote on
Date: Sun, 09 Jun 2024 15:30:21 UTC :

> I would like to report that, likely, in FreeBSD version 14.0-stable, file
> swapping is broken. To confirm, here is what I tried to do and what I
> achieved. In order to reproduce the problem, please follow the following
> steps.
> 
> I was following this tutorial
> https://www.cyberciti.biz/faq/create-a-freebsd-swap-file/
> 
> I created a large swap file (8192 MiB) and saved it to /root/swap.8G.bin.
> 
> After that, I ran
> 
> sudo chmod 0600 /root/swap.8G.bin
> 
> After that, I updated fstab by adding the following line to the end.
> 
> md42 none swap sw,file=/root/swap.8G.bin 0 0
> 
> On running
> 
> sudo swapon -aq
> 
> I got the swap file working initially, and I saw it after running swapinfo.
> But on reboot, it disappeared.

Going a different direction than how to enable using of swap files is the
following. It is not FreeBSD version specific for any supported version
(RELEASE or STABLE) or for main [future: 15.*] and has a long history going
back into now long unsupported versions.

QUOTE ( of https://bugs.freebsd.org/bugzilla/show_bug.cgi?id=206048#c7 )
On 2017-Feb-13, at 7:20 PM, Konstantin Belousov <kostikbel at gmail.com> wrote
on the freebsd-arm list:

. . .

swapfile write requires the write request to come through the filesystem
write path, which might require the filesystem to allocate more memory
and read some data. E.g. it is known that any ZFS write request
allocates memory, and that write request on large UFS file might require
allocating and reading an indirect block buffer to find the block number
of the written block, if the indirect block was not yet read.

As result, swapfile swapping is more prone to the trivial and unavoidable
deadlocks where the pagedaemon thread, which produces free memory, needs
more free memory to make a progress. Swap write on the raw partition over
simple partitioning scheme directly over HBA are usually safe, while e.g.
zfs over geli over umass is the worst construction.
END QUOTE


Summary consequence: I recommend only using swap partitions, not swap
                      files.


Yes, I have suffered deadlocks from attempted swap file use, with just
UFS over umass (USB SSD) being what held the the swap file in question.

===
Mark Millard
marklmi at yahoo.com