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SubjectAdding compression before/above swapcache
I'd like some feedback on how possible/useful, or not, it might be to
add compression into the page handling code before pages are added to
the swapcache. My thought is that adding a compressed cache at that
point may have (at least) two advantages over the existing page
compression, zswap and zram, which are both in the swap path.

1) Both zswap and zram are limited in the amount of memory that they
can compress/store:
-zswap is limited both in the amount of pre-compressed pages, by the
total amount of swap configured in the system, and post-compressed
pages, by its max_pool_percentage parameter. These limitations aren't
necessarily a bad thing, just requirements for the user (or distro
setup tool, etc) to correctly configure them. And for optimal
operation, they need to coordinate; for example, with the default
post-compressed 20% of memory zswap's configured to use, the amount of
swap in the system must be at least 40% of system memory (if/when
zswap is changed to use zsmalloc that number would need to increase).
The point being, there is a clear possibility of misconfiguration, or
even a simple lack of enough disk space for actual swap, that could
artificially reduce the amount of total memory zswap is able to
compress. Additionally, most of that real disk swap is wasted space -
all the pages stored compressed in zswap aren't actually written on
the disk.
-zram is limited only by its pre-compressed size, and of course the
amount of actual system memory it can use for compressed storage. If
using without dm-cache, this could allow essentially unlimited
compression until no more compressed pages can be stored; however that
requires the zram device to be configured as larger than the actual
system memory. If using with dm-cache, it may not be obvious what the
optimal zram size is.

Pre-swapcache compression would theoretically require no user
configuration, and the amount of compressed pages would be unlimited
(until there is no more room to store compressed pages).

2) Both zswap and zram (with dm-cache) write uncompressed pages to disk:
-zswap rejects any pages being sent to swap that don't compress well
enough, and they're passed on to the swap disk in uncompressed form.
Also, once zswap is full it starts uncompressing its old compressed
pages and writing them back to the swap disk.
-zram, with dm-cache, can pass pages on to the swap disk, but IIUC
those pages must be uncompressed first, and then written in compressed
form on disk. (Please correct me here if that is wrong).

A compressed cache that comes before the swap cache would be able to
push pages from its compressed storage to the swap disk, that contain
multiple compressed pages (and/or parts of compressed pages, if
overlapping page boundries). I think that would be able to,
theoretically at least, improve overall read/write times from a
pre-compressed perspective, simply because less actual data would be
transferred. Also, less actual swap disk space would be
used/required, which on systems with a very large amount of system
memory may be beneficial.


Additionally, a couple other random possible benefits:
-like zswap but unlike zram, a pre-swapcache compressed cache would be
able to select which pages to store compressed, either based on poor
compression results or some other criteria - possibly userspace could
madvise that certain pages were or weren't likely compressible.
-while zram and zswap are only able to compress and store pages that
are passed to them by zswapd or direct reclaim, a pre-swap compressed
cache wouldn't necessarily have to wait until the low watermark is
reached.

Any feedback would be greatly appreciated!


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