Date: Fri, 12 Nov 2021 14:50:00 -0500 From: Chris Ross <cross+freebsd@distal.com> To: Warner Losh <imp@bsdimp.com> Cc: Ronald Klop <ronald-lists@klop.ws>, freebsd-fs <freebsd-fs@freebsd.org> Subject: Re: swap_pager: cannot allocate bio Message-ID: <4008C512-31F1-4BE3-B674-A270CF674757@distal.com> In-Reply-To: <CANCZdfpW3YJ7c_EO82BYwLCFhDXdCp2W_fxmxAXzYvr7HNmnZQ@mail.gmail.com> References: <9FE99EEF-37C5-43D1-AC9D-17F3EDA19606@distal.com> <09989390-FED9-45A6-A866-4605D3766DFE@distal.com> <op.1cpimpsmkndu52@joepie> <4E5511DF-B163-4928-9CC3-22755683999E@distal.com> <42006135.15.1636709757975@mailrelay> <7B41B7D7-0C74-4F87-A49C-A666DB970CC3@distal.com> <CANCZdfpW3YJ7c_EO82BYwLCFhDXdCp2W_fxmxAXzYvr7HNmnZQ@mail.gmail.com>
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> On Nov 12, 2021, at 11:15, Warner Losh <imp@bsdimp.com> wrote:
> So the root cause of this problem is well known. You have a memory =
shortage, so you want to page out dirty pages to reclaim memory.
> However, there's not enough memory to allocate the structures you need =
to do I/O and so the swapout I/O fails half way down
> the stack not being able to allocate a bio. Some paths through the =
swapper cope with this well, other parts that execute less
> often cope less well.
>=20
> There's some hacks in the tree today to help with the GELI case: we =
prioritize swapping I/O. But there's no g_alloc_bio_swapping() interface
> for swapping I/O to get priority on allocating a bio to start with. =
Places that use g_clone_bio() could have the clone's copy allocated
> from a special swap pool, but that starts to get messy and isn't done =
today. And the upper layers like geom_cfs and ZFS are
> inconsistent in allocations, so there's work needed to make it robust =
in ZFS, but I have only a vague notion of what's needed. At the very
> least, the swapping I/O that comes into the top of ZFS won't have =
swapping I/O marked coming out the bottom because the
> BIO_SWAP flag is quite new.
>=20
> So until then, swapping on zvols is fraught with deadlocks like this =
and in the past there's been a strong admonishment
> against it.
Apologies, Warner, but I=E2=80=99m not sure I=E2=80=99m understanding =
this last statement. If you mean swapping _onto_ zvols, I=E2=80=99m not =
doing that. If you mean swapping in any way, while having zvols, then =
yes I am doing that. =20
My swap is on a partition on the non-ZFS disk. A physical disk as far =
as the kernel knows, hardware RAID1.
# pstat -s
Device 1K-blocks Used Avail Capacity
/dev/da0p3 445682648 1018524 444664124 0%
Let me know if what you=E2=80=99re saying above is true to my case, and =
any advice as to how I can avoid it. I had a =E2=80=9Cnot enough swap =
space=E2=80=9D a while back, and accordingly increased the size of my =
swap partition. I have 128GB of memory, though between the ARC and the =
big process I was running, that fills it easily.
- Chris
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