Re: [PATCH v4 6/9] f2fs: don't allow DIO reads but not DIO writes

[Jaegeuk Kim]

On 07/25, Eric Biggers wrote:
> On Sat, Jul 23, 2022 at 07:01:59PM -0700, Jaegeuk Kim wrote:
> > On 07/22, Eric Biggers wrote:
> > > From: Eric Biggers <ebiggers@xxxxxxxxxx>
> > > 
> > > Currently, if an f2fs filesystem is mounted with the mode=lfs and
> > > io_bits mount options, DIO reads are allowed but DIO writes are not.
> > > Allowing DIO reads but not DIO writes is an unusual restriction, which
> > > is likely to be surprising to applications, namely any application that
> > > both reads and writes from a file (using O_DIRECT).  This behavior is
> > > also incompatible with the proposed STATX_DIOALIGN extension to statx.
> > > Given this, let's drop the support for DIO reads in this configuration.
> > 
> > IIRC, we allowed DIO reads since applications complained a lower performance.
> > So, I'm afraid this change will make another confusion to users. Could
> > you please apply the new bahavior only for STATX_DIOALIGN?
> > 
> 
> Well, the issue is that the proposed STATX_DIOALIGN fields cannot represent this
> weird case where DIO reads are allowed but not DIO writes.  So the question is
> whether this case actually matters, in which case we should make STATX_DIOALIGN
> distinguish between DIO reads and DIO writes, or whether it's some odd edge case
> that doesn't really matter, in which case we could just fix it or make
> STATX_DIOALIGN report that DIO is unsupported.  I was hoping that you had some
> insight here.  What sort of applications want DIO reads but not DIO writes?
> Is this common at all?

I think there's no specific application to use the LFS mode at this
moment, but I'd like to allow DIO read for zoned device which will be
used for Android devices.

> 
> - Eric