Re: [PATCH v2] ntfs: fix $MFTMirr write offset when it spans multiple folios

[Namjae Jeon]

On Tue, Sep 8, 2026 at 5:41 AM 朱 天浩 <zhutianhao75@xxxxxxxxxxx> wrote:
>
> When commit 115380f9a2f9 ("ntfs: update mft operations") refactored the
> MFT code to use folios, it assumed $MFTMirr is allocated contiguously,
> which is indeed what mkfs arranges.  However, the folio rewrite kept a
> leftover from the old buffer-head/runlist based implementation:
> vol->cluster_size_mask is still applied to the mirror write offset.
> In fact it is no longer needed -- and in some configurations it is
> actively wrong.
>
> The bug triggers whenever the four mirror records do not fit in a
> single folio, i.e. when mft_record_size exceeds PAGE_SIZE / 4 (for
> example mft_record_size > 1KiB on 4KiB pages).
>
> For example, on the built-in 4Kn SSD (Apple SSD AP0256J) of a
> MacBookPro14,1 with 4096-byte MFT records, mirror record 3 is still
> written to the location of record 0.  $MFTMirr therefore gets out of
> sync with $MFT and the following warning is printed on remount:
>
>         ntfs: (device nvme0n1p3): check_mft_mirror(): $MFT and $MFTMirr
>               record 0 do not match.  Run chkdsk.
>
> Fix it by always adding the folio offset (folio->index << PAGE_SHIFT)
> to the base LCN address instead of applying the cluster_size_mask
> truncation.  This is the standard file-offset calculation and is
> correct for every combination of cluster size, page size and MFT
> record size, provided $MFTMirr data is contiguous from mftmirr_lcn
> (which mkfs always arranges).
>
> Tested on the volume above: before the change mirror records 1-3 are
> all written to record 0's sector; after the change the write-back
> probe reports:
>
>         ntfs: NTFSDBG pre mft_no=0x3 folio_idx=0x3 ofs=0x0 mftmirr_lcn=0x2540c5 clu_bits=12 rec_bits=12 PAGE_SHIFT=12 sect=0x12a0640
>         ntfs: NTFSDBG pre mft_no=0x0 folio_idx=0x0 ofs=0x0 mftmirr_lcn=0x2540c5 clu_bits=12 rec_bits=12 PAGE_SHIFT=12 sect=0x12a0628
>
> which matches the expected sectors computed as:
>
>         (NTFS_CLU_TO_B(mftmirr_lcn) + (folio->index << PAGE_SHIFT)) >> SECTOR_SHIFT
>
>         record 0: (0x2540c5 << 12) + 0x0000 = 0x2540c5000 >> 9 = 0x12a0628
>         record 3: (0x2540c5 << 12) + 0x3000 = 0x2540c8000 >> 9 = 0x12a0640
>
> A 13 MB file write succeeds on this volume and the file is byte-for-byte
> identical after a umount/mount cycle.
>
> Fixes: 115380f9a2f9 ("ntfs: update mft operations")
> Assisted-by: UOS-AI
> Assisted-by: CodeBuddy:Hy4 preview
> Signed-off-by: Zhu Tianhao <zhutianhao75@xxxxxxxxxxx>
Applied it to #ntfs-next.
Thanks!