Re: [PATCH] block: Allow REQ_FUA|REQ_READ

[Niklas Cassel <cassel@kernel.org>]

Hello Kent,

On Tue, Mar 11, 2025 at 09:35:16AM -0400, Kent Overstreet wrote:
> REQ_FUA|REQ_READ means "do a read that bypasses the controller cache",
> the same as writes.
> 
> This is useful for when the filesystem gets a checksum error, it's
> possible that a bit was flipped in the controller cache, and when we
> retry we want to retry the entire IO, not just from cache.

Looking at ATA Command Set - 6 (ACS-6),
7.23 READ FPDMA QUEUED - 60h

"""
If the Forced Unit Access (FUA) bit is set to one, the device shall retrieve
the data from the non-volatile media regardless of whether the device holds
the requested information in the volatile cache.

If the device holds a modified copy of the requested data as a result of
having volatile cached writes, the modified data shall be written to the
non-volatile media before being retrieved from the non-volatile media as
part of this operation.
"""

So IIUC, at least for ATA, if something is corrupted in the volatile
write cache, setting the FUA bit will ensure that the corruption will
get propagated to the non-volatile media.


Kind regards,
Niklas