[DISCUSSION] Preventing ENOSPC/EDQUOT writeback errors on network filesystems

[Piyush Sachdeva <s.piyush1024@gmail.com>]

Hi,
There have been plenty of discussions on how to handle writeback errors for
network filesystems, but most have focused on error reporting after the fact.
I'd like to start a discussion around preventing writeback errors specifically
ENOSPC and EDQUOT, before they cause silent data loss.

The problem:
With buffered writes on network filesystems (cifs, nfs, etc.), the write()
syscall copies data into the page cache and returns success immediately. The
actual upload to the server happens later during writeback. If the server is
out of space at that point, the write fails with ENOSPC. The netfs/writeback
layer records this error via mapping_set_error(), but critically the folio's
writeback flag is cleared and the page is now clean. Under memory pressure, the
VM can reclaim these clean pages, permanently losing data that the application
believes was successfully written. Meanwhile, i_size has already been updated
to reflect the new file size. So stat() shows a file size inclusive of the data
that was never persisted. Another inconsistency here is that total free space
hasn't been modified for the file system on the server, leading to incorrect
values in statfs() output from the client's pov (assuming statfs() calls go
to the server).
To illustrate with real-world scenarios:

- A user or application can keep issuing writes to an fd well beyond the
  available space, since buffered writes return success as soon as data is
  copied to the page cache. A significant amount of data, exceeding the
  available quota can accumulate before fsync() is called, at which point
  critical data loss is nearly certain.

- A malicious user can exploit this to keep resources pinned and memory
  oversubscribed, impacting other applications.

The error is technically observable: fsync() will return it, and close()
surfaces it through the flush callback. But in practice, many applications
check neither, and the POSIX "just call fsync()" answer isn't satisfying for
users who lose data silently.

Local filesystems largely avoid this because they can check available space
synchronously in write_begin() and fail the write() syscall directly. Network
filesystems can't do this cheaply — a round-trip per write to check server
space would negate the benefits of buffered I/O.

Through recent development, netfs is becoming a central layer for network
filesystem I/O. It already has retry logic for transient failures (EAGAIN,
ECONNABORTED), but ENOSPC/EDQUOT remain hard failures. This affects every
network filesystem using buffered writes.

I am curious to know if NFS has a solution to this and what the approach is
towards this specific problem by NFS community?

This problem is worth solving for all network filesystems. I have a few
thoughts on approaches, combining cached statfs() output with
fallocate()-style pre-allocation on the write path:

1. Pre-allocate space on the server before writing to the page cache,
   analogous to fallocate() on the write path. This guarantees server-side
   space for page cache data.

2. Since per-write fallocate() calls require a server round-trip, effectively
   negating the benefit of buffered I/O. Use cached statfs() output to gate
   when pre-allocation is triggered. For example, once free space drops below
   20% of total space, enable fallocate() on the write path. Otherwise, let
   writes proceed as normal.

3. Handle refresh and synchronization of the cached statfs() data separately
   to avoid staleness.

I'd appreciate feedback from the community on viable approaches.

--
Regards,
Piyush