Re: [PATCH] fuse: fix fuseblk i_blkbits for iomap partial writes

[Joanne Koong <joannelkoong@gmail.com>]

On Mon, Aug 11, 2025 at 6:49 PM Joanne Koong <joannelkoong@gmail.com> wrote:
>
> On regular fuse filesystems, i_blkbits is set to PAGE_SHIFT which means
> any iomap partial writes will mark the entire folio as uptodate. However
> fuseblk filesystems work differently and allow the blocksize to be less
> than the page size. As such, this may lead to data corruption if fuseblk
> sets its blocksize to less than the page size, uses the writeback cache,
> and does a partial write, then a read and the read happens before the
> write has undergone writeback, since the folio will not be marked
> uptodate from the partial write so the read will read in the entire
> folio from disk, which will overwrite the partial write.
>
> The long-term solution for this, which will also be needed for fuse to
> enable large folios with the writeback cache on, is to have fuse also
> use iomap for folio reads, but until that is done, the cleanest
> workaround is to use the page size for fuseblk's internal kernel
> blksize/blkbits values while maintaining current behavior for stat().
>
> This was verified using ntfs-3g:
> $ sudo mkfs.ntfs -f -c 512 /dev/vdd1
> $ sudo ntfs-3g /dev/vdd1 ~/fuseblk
> $ stat ~/fuseblk/hi.txt
> IO Block: 512
>
> Signed-off-by: Joanne Koong <joannelkoong@gmail.com>
> Fixes: a4c9ab1d4975 ("fuse: use iomap for buffered writes")

Please ignore this version of the patch. This is superseded by the
newer version here:
https://lore.kernel.org/linux-fsdevel/20250812214614.2674485-1-joannelkoong@gmail.com/T/#t

Thanks,
Joanne