class VmCompressedStorage

Attempts to store the data in |page| of size |len|. The data is assumed to start at offset 0 in

the page, and |len| cannot exceed kPageSize. The |page| becomes owned by this

VmCompressedStorage instance, although ownership may be returned via the return result.

The return value is an optional reference to compressed data, as well as an optional vm_page_t,

with a nullptr for the vm_page_t being used to indicate absence. If a vm_page_t is returned the

caller owns it, and is responsible for freeing it.

If the data is stored successfully then a valid CompressedRef will be returned and will be

retained until that reference is passed to |Free|. Otherwise a nullopt is returned.

Regardless of the success or failure of the storage, a vm_page_t might be returned, and if one

is returned it may or may not be the same as the |page| passed in. The returned vm_page_t has

undefined content and is now freely owned by the caller and can be used as the caller likes, or

returned to the pmm.

The expected usage of this is that a compressor puts data in a buffer page and passes it here

as |page|, then takes any returned vm_page_t as its next buffer page, or if none was returned

allocates a new one. This provides the storage implementation with the freedom to either copy

the data out of |page| and return it, or take the page to add to its storage instead of going

to the PMM (and then returning a nullptr).

Free the data referenced by a |CompressedRef| that was returned from |Store|. After calling

this the reference is no longer valid and must not be passed to |CompressedData|, and any

previous result of |CompressedData| must not be used.

Retrieve a reference to original data that was stored. The metadata and length of the data are

also returned, alleviating the need to retain them separately.

The return address remains valid as long as this specific reference is not freed, and otherwise

any other calls to |Store| or |Free| do not invalidate.

TODO(https://fxbug.dev/42138396): Restrict this if de-fragmentation of the compressed storage

becomes necessary.

No guarantee on alignment of the data is provided, and callers must tolerate arbitrary byte

alignment.

TODO(https://fxbug.dev/42138396): Consider providing an alignment guarantee if needed by

decompressors.

Retrieve the metadata for the original page referred to by ref.

Set the metadata for the original page referred to by ref.

Perform an information dump of the internal state to the debuglog.