Miklos Szeredi posted a request for comments titled "fuse writable mmap design". He explained, "writable shared memory mappings for fuse are something I've been trying to implement forever. Now hopefully I've got it all worked out, it survives indefinitely with bash-shared-mapping and fsx-linux. And I'd like to solicit comments about the approach." He went on to describe the patch:

"fuse_writepage() allocates a new temporary page with GFP_NOFS|__GFP_HIGHMEM. It copies the contents of the original page, and queues a WRITE request to the userspace filesystem using this temp page. From the VM's point of view, the writeback is finished instantly: the page is removed from the radix trees, and the PageDirty and PageWriteback flags are cleared. The per-bdi writeback count is not decremented until the writeback truly completes. [...] On dirtying the page, fuse waits for a previous write to finish before proceeding. This makes sure, there can only be one temporary page used at a time for one cached page."