* Impose a maximum memfd size limit. In theory, this can be
2GB (wl_shm_create_pool() is the limiting factor - its size argument
is an int32_t). For now, use 256MB.
This is mainly to reduce the amount of virtual address space used by
the compositor, which keeps at least one mmapping (of the entire
memfd) around. One mmapping *per terminal window* that is.
Given that we have 128TB with 48-bit virtual addresses, we could
probably bump this to 2GB without any issues. However, 256MB should
be enough.
TODO: check how much we typically move the offset when scrolling in
a fullscreen window on a 4K monitor. 256MB may turn out to be too
small.
On 32-bit shm_scroll() is completely disabled. There simply isn't
enough address space.
* Wrapping is done by moving the offset to "the other end" of the
memfd, and copying the buffer contents to the new, wrapped offset.
The "normal" scrolling code then does the actual scrolling. This
means we'll re-instantiate all objects twice when wrapping.
This function "scrolls" the buffer by the specified number of (pixel)
rows.
The idea is move the image offset by re-sizing the underlying memfd
object. I.e. to scroll forward, increase the size of the memfd file,
and move the pixman image offset forward (and the Wayland SHM buffer
as well).
Only increasing the file size would, obviously, cause the memfd file
to grow indefinitely. To deal with this, we "punch" a whole from the
beginning of the file to the new offset. This frees the associated
memory.
Thus, while we have a memfd file whose size is (as seen by
e.g. fstat()) is ever growing, the actual file size is always the
original buffer size.
Some notes:
* FALLOC_FL_PUNCH_HOLE can be quite slow when the number of used pages
to drop is large.
* all normal fallocate() usages have been replaced with ftruncate(),
as this is *much* faster. fallocate() guarantees subsequent writes
wont fail. I.e. it actually reserves (disk) space. While it doesn't
allocate on-disk blocks for on-disk files, it *does* zero-initialize
the in-memory blocks. And this is slow. ftruncate() doesn't do this.
TODO: implement reverse scrolling (i.e. a negative row count).
