Re: [PATCH RFC 2/2] block: Do not discard buffers under a mounted filesystem

[Dave Chinner <david@fromorbit.com>]

On Thu, Aug 27, 2020 at 10:39:00PM +0100, Al Viro wrote:
> On Thu, Aug 27, 2020 at 08:16:03AM +0100, Christoph Hellwig wrote:
> > On Tue, Aug 25, 2020 at 04:50:56PM +0200, Jan Kara wrote:
> > > Do you mean that address_space filesystem uses to access its metadata on
> > > /dev/sda will be different from the address_space you will see when reading
> > > say /dev/sda?  Thus these will be completely separate (and incoherent)
> > > caches?
> > 
> > Yes.
> > 
> > > Although this would be simple it will break userspace I'm afraid.
> > > There are situations where tools read e.g. superblock of a mounted
> > > filesystem from the block device and rely on the data to be reasonably
> > > recent. Even worse e.g. tune2fs or e2fsck can *modify* superblock of a
> > > mounted filesystem through the block device (e.g. to set 'fsck after X
> > > mounts' fields and similar).
> > 
> > We've not had any problems when XFS stopped using the block device
> > address space 9.5 years ago.
> 
> How much writes from fsck use does xfs see, again?

All of them, because xfs_repair uses direct IO and caches what it
needs in userspace.

Cheers,

Dave.
-- 
Dave Chinner
david@fromorbit.com