Medium [CVE-2026-72132] Charge unstable writes by request size, not folio size

In the Linux kernel, the following vulnerability has been resolved: NFS: Charge unstable writes by request size, not folio size nfs_folio_mark_unstable() and nfs_folio_clear_commit() charge and uncharge NR_WRITEBACK/WB_WRITEBACK by folio_nr_pages(folio) once per *request* added to or removed from a commit list.

This is correct only when a folio has a single associated request.

When pg_test splits a folio into N sub-folio requests (e.g. pNFS flexfiles striping with a stripe unit smaller than the folio size, or plain wsize-limited splitting), each of the N requests independently charges the whole folio's page count, inflating the accounting by a factor of N per folio.

With large folios and small stripe units this reaches multiple orders of magnitude: a 2 MiB folio split into 512 4 KiB requests can charge up to 512x its real size, pushing global dirty+writeback accounting past the system's dirty threshold and forcing every buffered writer on the host into the hard-throttle path, including unrelated in-kernel NFS server threads sharing the box.

Charge each request only for the pages it actually covers. A flaw was found in the Linux kernel's Network File System (NFS) component.

The `nfs_folio_mark_unstable()` and `nfs_folio_clear_commit()` functions incorrectly account for unstable writes, charging the full folio size for each sub-folio request.