linux-ext4 - Re: [PATCH 11/13] dax, iomap: Add support for synchronous faults

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Message-ID: <20170822094655.GE4909@quack2.suse.cz>
Date:   Tue, 22 Aug 2017 11:46:55 +0200
From:   Jan Kara <jack@...e.cz>
To:     Ross Zwisler <ross.zwisler@...ux.intel.com>
Cc:     Jan Kara <jack@...e.cz>, linux-fsdevel@...r.kernel.org,
        linux-nvdimm@...ts.01.org, Andy Lutomirski <luto@...nel.org>,
        linux-ext4@...r.kernel.org, linux-xfs@...r.kernel.org,
        Christoph Hellwig <hch@...radead.org>,
        Dan Williams <dan.j.williams@...el.com>,
        Boaz Harrosh <boazh@...app.com>
Subject: Re: [PATCH 11/13] dax, iomap: Add support for synchronous faults

On Mon 21-08-17 12:58:30, Ross Zwisler wrote:
> On Thu, Aug 17, 2017 at 06:08:13PM +0200, Jan Kara wrote:
> > Add a flag to iomap interface informing the caller that inode needs
> > fdstasync(2) for returned extent to become persistent and use it in DAX
> > fault code so that we map such extents only read only. We propagate the
> > information that the page table entry has been inserted write-protected
> > from dax_iomap_fault() with a new VM_FAULT_RO flag. Filesystem fault
> > handler is then responsible for calling fdatasync(2) and updating page
> > tables to map pfns read-write. dax_iomap_fault() also takes care of
> > updating vmf->orig_pte to match the PTE that was inserted so that we can
> > safely recheck that PTE did not change while write-enabling it.
> 
> This changelog needs a little love.  s/VM_FAULT_RO/VM_FAULT_NEEDDSYNC/, the
> new path doesn't do the RO mapping, but instead just does the entire RW
> mapping after the fdatasync is complete, the vmf->orig_pte manipulation went
> away, etc.

Yeah, fixed. Thanks for noticing.

> > Signed-off-by: Jan Kara <jack@...e.cz>
> > ---
> >  fs/dax.c              | 31 +++++++++++++++++++++++++++++++
> >  include/linux/iomap.h |  2 ++
> >  include/linux/mm.h    |  6 +++++-
> >  3 files changed, 38 insertions(+), 1 deletion(-)
> > 
> > diff --git a/fs/dax.c b/fs/dax.c
> > index bc040e654cc9..ca88fc356786 100644
> > --- a/fs/dax.c
> > +++ b/fs/dax.c
> > @@ -1177,6 +1177,22 @@ static int dax_iomap_pte_fault(struct vm_fault *vmf,
> >  			goto error_finish_iomap;
> >  		}
> >  
> > +		/*
> > +		 * If we are doing synchronous page fault and inode needs fsync,
> > +		 * we can insert PTE into page tables only after that happens.
> > +		 * Skip insertion for now and return the pfn so that caller can
> > +		 * insert it after fsync is done.
> > +		 */
> > +		if (write && (vma->vm_flags & VM_SYNC) &&
> > +		    (iomap.flags & IOMAP_F_NEEDDSYNC)) {
> 
> Just a small nit, but I don't think we really need to check for 'write' here.
> The fact that IOMAP_F_NEEDDSYNC is set tells us that we are doing a write.
> 
> 	if ((flags & IOMAP_WRITE) &&
> 	    !jbd2_transaction_committed(EXT4_SB(inode->i_sb)->s_journal,
> 					EXT4_I(inode)->i_datasync_tid))
> 		iomap->flags |= IOMAP_F_NEEDDSYNC;
> 
> Ditto for the PMD case.

OK, done.

> With that one simplification and a cleaned up changelog, you can add:
> 
> Reviewed-by: Ross Zwisler <ross.zwisler@...ux.intel.com>

Thanks.

								Honza
-- 
Jan Kara <jack@...e.com>
SUSE Labs, CR