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From:   Dan Williams <dan.j.williams@intel.com>
Date:   Wed, 12 Sep 2018 09:41:42 -0700
Message-ID: <CAPcyv4ggwdBdT0-znPxcGn=2G=VuFfJaQcZiBqb8VHV2oUQ5LQ@mail.gmail.com>
Subject: Re: [PATCH 2/2] ext4, dax: set ext4_dax_aops for dax files
To:     "Kani, Toshi" <toshi.kani@hpe.com>
Cc:     "jack@suse.cz" <jack@suse.cz>,
        "adilger.kernel@dilger.ca" <adilger.kernel@dilger.ca>,
        "linux-ext4@vger.kernel.org" <linux-ext4@vger.kernel.org>,
        "linux-nvdimm@lists.01.org" <linux-nvdimm@lists.01.org>,
        "linux-kernel@vger.kernel.org" <linux-kernel@vger.kernel.org>,
        "tytso@mit.edu" <tytso@mit.edu>
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On Wed, Sep 12, 2018 at 9:08 AM, Kani, Toshi <toshi.kani@hpe.com> wrote:
> 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@hpe.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@vger.kernel.org>
>> >
>> >
>> > > Signed-off-by: Toshi Kani <toshi.kani@hpe.com>
>> > > Cc: Jan Kara <jack@suse.cz>
>> > > Cc: Dan Williams <dan.j.williams@intel.com>
>> > > Cc: "Theodore Ts'o" <tytso@mit.edu>
>> > > Cc: Andreas Dilger <adilger.kernel@dilger.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.
>

Yes, Jan's proposal looks best to me.