linux-ext4 - Re: [PATCH 2/2] ext4, dax: set ext4_dax

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Date:   Wed, 12 Sep 2018 16:08:00 +0000
From:   "Kani, Toshi" <toshi.kani@....com>
To:     "dan.j.williams@...el.com" <dan.j.williams@...el.com>,
        "jack@...e.cz" <jack@...e.cz>
CC:     "adilger.kernel@...ger.ca" <adilger.kernel@...ger.ca>,
        "linux-ext4@...r.kernel.org" <linux-ext4@...r.kernel.org>,
        "linux-nvdimm@...ts.01.org" <linux-nvdimm@...ts.01.org>,
        "linux-kernel@...r.kernel.org" <linux-kernel@...r.kernel.org>,
        "tytso@....edu" <tytso@....edu>
Subject: Re: [PATCH 2/2] ext4, dax: set ext4_dax_aops for dax files

On Wed, 2018-09-12 at 11:31 +0200, Jan Kara wrote:
> On Tue 11-09-18 11:15:18, Dan Williams wrote:
> > On Tue, Sep 11, 2018 at 8:42 AM, Toshi Kani <toshi.kani@....com> wrote:
> > > Sync syscall to an existing DAX file needs to flush processor cache,
> > > but it does not currently.  This is because 'ext4_da_aops' is set to
> > > address_space_operations of existing DAX files, instead of 'ext4_dax_aops',
> > > since S_DAX flag is set after ext4_set_aops() in the open path.
> > > 
> > >   New file
> > >   --------
> > >   lookup_open
> > >     ext4_create
> > >       __ext4_new_inode
> > >         ext4_set_inode_flags   // Set S_DAX flag
> > >       ext4_set_aops            // Set aops to ext4_dax_aops
> > > 
> > >   Existing file
> > >   -------------
> > >   lookup_open
> > >     ext4_lookup
> > >       ext4_iget
> > >         ext4_set_aops          // Set aops to ext4_da_aops
> > >         ext4_set_inode_flags   // Set S_DAX flag
> > > 
> > > Change ext4_iget() to call ext4_set_inode_flags() before ext4_set_aops().
> > > 
> > > Fixes: 5f0663bb4a64f588f0a2dd6d1be68d40f9af0086
> > 
> > Same format nit:
> > 
> > Fixes: 5f0663bb4a64 ("ext4, dax: introduce ext4_dax_aops")
> > Cc: <stable@...r.kernel.org>
> > 
> > 
> > > Signed-off-by: Toshi Kani <toshi.kani@....com>
> > > Cc: Jan Kara <jack@...e.cz>
> > > Cc: Dan Williams <dan.j.williams@...el.com>
> > > Cc: "Theodore Ts'o" <tytso@....edu>
> > > Cc: Andreas Dilger <adilger.kernel@...ger.ca>
> > > ---
> > >  fs/ext4/inode.c |    3 ++-
> > >  1 file changed, 2 insertions(+), 1 deletion(-)
> > > 
> > > diff --git a/fs/ext4/inode.c b/fs/ext4/inode.c
> > > index 775cd9b4af55..93cbbb859c40 100644
> > > --- a/fs/ext4/inode.c
> > > +++ b/fs/ext4/inode.c
> > > @@ -4998,6 +4998,8 @@ struct inode *ext4_iget(struct super_block *sb, unsigned long ino)
> > >         if (ret)
> > >                 goto bad_inode;
> > > 
> > > +       ext4_set_inode_flags(inode);
> > > +
> > 
> > Hmm, does this have unintended behavior changes?
> > 
> > I notice that there are some checks for flags "IS_APPEND(inode) ||
> > IS_IMMUTABLE(inode)" *before* the call to ext4_set_inode_flags(). I
> > didn't look too much deeper at whether those checks are bogus, but it
> > would seem safer to do something like this for a lower risk fix.
> > 
> > Thoughts?
> 
> Well, safer but it would leave the landmine around for others to hit.
> Toshi, please move the ext4_set_inode_flags() call to be just after the
> assignment:
> 
> 	ei->i_flags = le32_to_cpu(raw_inode->i_flags);
> 
> in ext4_iget(). That way people won't introduce checks for i_flags that can
> never hit... And yes, it fixes also other bugs (mostly in sanity checks
> AFAICS) than the DAX issue.

Sure.  Assuming you think the implicit change Dan pointed out is not a
problem, yes, I will go with this cleaner approach.

Thanks!
-Toshi