When a pages file is provided to `runsc restore`, reads from that file are
asynchronous (via statefile.AsyncReader) in order to maximize throughput.
However, all such reads must complete before Kernel.LoadFrom() returns, so
applications cannot execute before MemoryFile loading is complete. The main
objective of this CL is to allow reads to continue after Kernel.LoadFrom()
returns, allowing applications to execute while MemoryFile loading is still in
progress. This behavior is user-visible: it affects whether deleting the pages
file frees disk space immediately on POSIX filesystems, may affect whether
deletion is possible on non-POSIX filesystems, and prevents unmounting
regardless. Thus it is flag-guarded as `runsc restore --background`.
MemoryFile ranges that have yet to be loaded, but that are being waited-for by
applications, should be prioritized over ranges for which no application is
waiting. This requires that application requests for data (calls to
MemoryFile.(memmap.File).DataFD/MapInternal()) are able to determine which
ranges have not yet been loaded, request reads for such ranges with elevated
priority, and wait for only those reads to be completed; none of these are
supported by the existing statefile.AsyncReader.
Thus:
- Add //pkg/sentry/pgalloc/aio, which provides an async I/O API that is
designed to be easily implementable using a goroutine pool, Linux native AIO,
or io_uring, though only includes a goroutine pool implementation. (io_uring
is widely disabled due to security vulnerabilities. In my testing, Linux
native AIO is slower than the goroutine pool, but this may change with lower
GOMAXPROCS which needs further testing.)
- Move I/O scheduling into pgalloc: introduce an async page loader goroutine
that is started by MemoryFile.LoadFrom() when async page loading is requested
(implicitly, via the existence of a pages file), which is responsible for
driving submission of read requests and handling their completions.
PiperOrigin-RevId: 679321884
