Re: [PATCH v9 1/3] block: introduce bdev_flush_by_dev()

[Christoph Hellwig <hch@infradead.org>]

On Fri, Oct 02, 2026 at 01:10:42PM +0200, Jan Kara wrote:
> FWIW I share Julian's concern here. It would be fine to use AS_KERNEL_FILE
> for ext4 metadata but I think just unconditionally setting AS_KERNEL_FILE
> for bdev mappings will cause issues because some users may be using bdevs
> directly for their workloads and they could still expect proper memcg
> accounting to work in that case.

Agreed that it should not set unconditionally.

> We could set AS_KERNEL_FILE when opening bdev for a filesystem (and remove
> it when releasing bdev) but that would have to make sure there are no
> folios in the bdev mapping when changing the flag as otherwise the
> accounting would go wrong. Looks it might be doable but getting all the
> cornercases right will be hairy and overall not very appealing to me...

I'd rather not support special case writeback code just for this legacy
fs abuses bdev buffer cache case.  And we basically need to tear
down pagecache at unmount anyway, as i_blkbits can change, so while
we do need to be careful, I don't think it really is a major issue.

We might be able to restrict to setting it when
CONFIG_BLK_DEV_WRITE_MOUNTED is disabled to avoid the problem of non-fs
shared mmap writers, as anyone using a modern kernel and cgroups really
should have that disabled.