CVE-2026-72132: NFS: 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

nfsfoliomarkunstable() and nfsfolioclearcommit() charge and uncharge NRWRITEBACK/WBWRITEBACK by folionrpages(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 pgtest 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.