linux-ext4 - Re: [PATCH v3 2/3] ext4: add ioctl EXT4_IOC

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Message-ID: <20210506150816.GE8532@magnolia>
Date:   Thu, 6 May 2021 08:08:16 -0700
From:   "Darrick J. Wong" <djwong@...nel.org>
To:     Christoph Hellwig <hch@...radead.org>
Cc:     Leah Rumancik <leah.rumancik@...il.com>,
        linux-ext4@...r.kernel.org, tytso@....edu
Subject: Re: [PATCH v3 2/3] ext4: add ioctl EXT4_IOC_CHECKPOINT

On Thu, May 06, 2021 at 08:18:36AM +0100, Christoph Hellwig wrote:
> 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.

Heh, so that's a "stable" behavior. :)

> > (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..

<shrug> If the wording got tweaked to "...not readable via LBA interface
after delete" then you could also REQ_OP_WRITE_ZEROES, which would work
on a broader range of hardware.

--D