Re: [RFC PATCH v2 0/4] mm/swap: reserve swap areas for deliberate offload

[Matthias Goergens <matthias.goergens@gmail.com>]

Hi Christoph,

> Either way block devices must be safe to be called from paging paths,
> not just for swap but also for file system based paging.  So this is
> a bug in the implementation, and not something worked around in the
> swap code.

Agreed that a backend used for paging has to make progress under
pressure, and the zvol example was a poor lead.  The series isn't
meant to excuse a backend that can deadlock; that would still be a bug
to fix in the backend.

What I'm after is different: backends that are correct but need
memory to accept a write, where the better policy is to use them for
cold-page offload when memory isn't tight and keep pressure reclaim on
areas that don't need to allocate.  Two in-tree examples: zram
allocates on writes and fails with -ENOMEM, with no fallback, while its
logical size still shows free slots; and filesystem swapfiles must
give up copy-on-write, checksums and compression today, because swap
writes bypass the filesystem.  I'll make that the case for v3 instead.

Thanks,
Matthias