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Date:   Thu, 6 May 2021 08:18:36 +0100
From:   Christoph Hellwig <hch@infradead.org>
To:     "Darrick J. Wong" <djwong@kernel.org>
Cc:     Leah Rumancik <leah.rumancik@gmail.com>,
        linux-ext4@vger.kernel.org, tytso@mit.edu
Subject: Re: [PATCH v3 2/3] ext4: add ioctl EXT4_IOC_CHECKPOINT
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 <20210505212711.GA8532@magnolia>
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On Wed, May 05, 2021 at 02:27:11PM -0700, Darrick J. Wong wrote:
> Er... what specifically does "data" mean?  File data, or just the dirent
> blocks?
> 
> I think this is only true if discard_zeroes_data == 1, right?  The last
> I looked, ext4 was calling REQ_OP_DISCARD, not REQ_OP_WRITE_ZEROES.
> 
> Also, there are some SSDs that "implement" discard as nop, which means
> that the old contents can still be read by re-reading the LBAs.  What
> about those?

Not just some, but most at least for corner cases.  ATA TRIM, SCSI UNMAP
and NVMe Deallocate all explicitly allow for keeping some of the old
data, and devices make use of that when the discard requests does not
map to their internal granularities.

> (Also wondering if this is where FS_SECRM_FL files should get their
> freed file blocks erased with REQ_OP_SECURE_ERASE...)

Only implemented for mmc..