fuse: add writeback documentation

Fuse supports the following I/O modes:

- direct-io

- cached

  + write-through

  + writeback-cache

The direct-io mode can be selected with the FOPEN_DIRECT_IO flag in the

FUSE_OPEN reply.

In direct-io mode the page cache is completely bypassed for reads and writes.

No read-ahead takes place. Shared mmap is disabled.

In cached mode reads may be satisfied from the page cache, and data may be

read-ahead by the kernel to fill the cache.  The cache is always kept consistent

after any writes to the file.  All mmap modes are supported.

The cached mode has two sub modes controlling how writes are handled.  The

write-through mode is the default and is supported on all kernels.  The

writeback-cache mode may be selected by the FUSE_WRITEBACK_CACHE flag in the

FUSE_INIT reply.

In write-through mode each write is immediately sent to userspace as one or more

WRITE requests, as well as updating any cached pages (and caching previously

uncached, but fully written pages).  No READ requests are ever sent for writes,

so when an uncached page is partially written, the page is discarded.

In writeback-cache mode (enabled by the FUSE_WRITEBACK_CACHE flag) writes go to

the cache only, which means that the write(2) syscall can often complete very

fast.  Dirty pages are written back implicitly (background writeback or page

reclaim on memory pressure) or explicitly (invoked by close(2), fsync(2) and

when the last ref to the file is being released on munmap(2)).  This mode

assumes that all changes to the filesystem go through the FUSE kernel module

(size and atime/ctime/mtime attributes are kept up-to-date by the kernel), so

it's generally not suitable for network filesystems.  If a partial page is

written, then the page needs to be first read from userspace.  This means, that

even for files opened for O_WRONLY it is possible that READ requests will be

generated by the kernel.