AZL-88704

In the Linux kernel, the following vulnerability has been resolved:

ceph: fix num_ops off-by-one when crypto allocation fails

movedirtyfolioinpagearray() may fail if the file is encrypted, the dirty folio is not the first in the batch, and it fails to allocate a bounce buffer to hold the ciphertext. When that happens, cephprocessfoliobatch() simply redirties the folio and flushes the current batch -- it can retry that folio in a future batch.

However, if this failed folio is not contiguous with the last folio that did make it into the batch, then cephprocessfoliobatch() has already incremented ceph_wbc->num_ops; because it doesn't follow through and add the discontiguous folio to the array, cephsubmit_write() -- which expects that ceph_wbc->num_ops accurately reflects the number of contiguous ranges (and therefore the required number of "write extent" ops) in the writeback -- will panic the kernel:

BUG_ON(ceph_wbc->op_idx + 1 != req->r_num_ops);

This issue can be reproduced on affected kernels by writing to fscrypt-enabled CephFS file(s) with a 4KiB-written/4KiB-skipped/repeat pattern (total filesize should not matter) and gradually increasing the system's memory pressure until a bounce buffer allocation fails.

Fix this crash by decrementing ceph_wbc->num_ops back to the correct value when movedirtyfolioinpage_array() fails, but the folio already started counting a new (i.e. still-empty) extent.

The defect corrected by this patch has existed since 2022 (see first Fixes:), but another bug blocked multi-folio encrypted writeback until recently (see second Fixes:). The second commit made it into 6.18.16, 6.19.6, and 7.0-rc1, unmasking the panic in those versions. This patch therefore fixes a regression (panic) introduced by cac190c7674f.