mm/mglru: fix div-by-zero in vmpressure_calc_level()

Vulnerability Description

In the Linux kernel, the following vulnerability has been resolved: mm/mglru: fix div-by-zero in vmpressure_calc_level() evict_folios() uses a second pass to reclaim folios that have gone through page writeback and become clean before it finishes the first pass, since folio_rotate_reclaimable() cannot handle those folios due to the isolation. The second pass tries to avoid potential double counting by deducting scan_control->nr_scanned. However, this can result in underflow of nr_scanned, under a condition where shrink_folio_list() does not increment nr_scanned, i.e., when folio_trylock() fails. The underflow can cause the divisor, i.e., scale=scanned+reclaimed in vmpressure_calc_level(), to become zero, resulting in the following crash: [exception RIP: vmpressure_work_fn+101] process_one_work at ffffffffa3313f2b Since scan_control->nr_scanned has no established semantics, the potential double counting has minimal risks. Therefore, fix the problem by not deducting scan_control->nr_scanned in evict_folios().

References

• https://git.kernel.org/stable/c/8de7bf77f21068a5f602bb1e59adbc5ab533509d
• https://git.kernel.org/stable/c/d6510f234c7d117790397f9bb150816b0a954a04
• https://git.kernel.org/stable/c/a39e38be632f0e1c908d70d1c9cd071c03faf895
• https://git.kernel.org/stable/c/8b671fe1a879923ecfb72dda6caf01460dd885ef