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Date:   Thu, 27 Aug 2020 22:39:00 +0100
From:   Al Viro <viro@zeniv.linux.org.uk>
To:     Christoph Hellwig <hch@infradead.org>
Cc:     Jan Kara <jack@suse.cz>, linux-fsdevel@vger.kernel.org,
        yebin <yebin10@huawei.com>, linux-block@vger.kernel.org,
        Jens Axboe <axboe@kernel.dk>
Subject: Re: [PATCH RFC 2/2] block: Do not discard buffers under a mounted
 filesystem
Message-ID: <20200827213900.GG1236603@ZenIV.linux.org.uk>
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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?