linux-ext4 - Re: [PATCH 4/7] dax: Make dax_insert_mapping() return VM_FAULT

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Message-ID: <20170728094314.GD29433@quack2.suse.cz>
Date:   Fri, 28 Jul 2017 11:43:14 +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-ext4@...r.kernel.org,
        Dan Williams <dan.j.williams@...el.com>,
        Andy Lutomirski <luto@...nel.org>, linux-nvdimm@...ts.01.org,
        linux-xfs@...r.kernel.org, Christoph Hellwig <hch@...radead.org>,
        Dave Chinner <david@...morbit.com>
Subject: Re: [PATCH 4/7] dax: Make dax_insert_mapping() return VM_FAULT_ state

On Thu 27-07-17 16:22:30, Ross Zwisler wrote:
> On Thu, Jul 27, 2017 at 03:12:42PM +0200, Jan Kara wrote:
> > Currently dax_insert_mapping() returns normal error code which is later
> > converted to VM_FAULT_ state. Since we will need to do more state
> > modifications specific to dax_insert_mapping() it does not make sense to
> > push them up to the caller of dax_insert_mapping(). Instead make
> > dax_insert_mapping() return VM_FAULT_ state the same way as
> > dax_pmd_insert_mapping() does that.
> > 
> > Signed-off-by: Jan Kara <jack@...e.cz>
> > ---
> >  fs/dax.c | 45 +++++++++++++++++++++++++--------------------
> >  1 file changed, 25 insertions(+), 20 deletions(-)
> > 
> > diff --git a/fs/dax.c b/fs/dax.c
> > index 0673efd72f53..9658975b926a 100644
> > --- a/fs/dax.c
> > +++ b/fs/dax.c
> > @@ -814,6 +814,15 @@ int dax_writeback_mapping_range(struct address_space *mapping,
> >  }
> >  EXPORT_SYMBOL_GPL(dax_writeback_mapping_range);
> >  
> > +static int dax_fault_return(int error)
> > +{
> > +	if (error == 0)
> > +		return VM_FAULT_NOPAGE;
> > +	if (error == -ENOMEM)
> > +		return VM_FAULT_OOM;
> > +	return VM_FAULT_SIGBUS;
> > +}
> > +
> >  static sector_t dax_iomap_sector(struct iomap *iomap, loff_t pos)
> >  {
> >  	return iomap->blkno + (((pos & PAGE_MASK) - iomap->offset) >> 9);
> > @@ -828,7 +837,7 @@ static int dax_insert_mapping(struct vm_fault *vmf, struct iomap *iomap,
> >  	unsigned long vaddr = vmf->address;
> >  	void *ret, *kaddr;
> >  	pgoff_t pgoff;
> > -	int id, rc;
> > +	int id, rc, vmf_ret;
> >  	pfn_t pfn;
> >  
> >  	rc = bdev_dax_pgoff(iomap->bdev, sector, PAGE_SIZE, &pgoff);
> > @@ -850,9 +859,18 @@ static int dax_insert_mapping(struct vm_fault *vmf, struct iomap *iomap,
> >  
> >  	trace_dax_insert_mapping(mapping->host, vmf, ret);
> >  	if (vmf->flags & FAULT_FLAG_WRITE)
> > -		return vm_insert_mixed_mkwrite(vma, vaddr, pfn);
> > +		rc = vm_insert_mixed_mkwrite(vma, vaddr, pfn);
> >  	else
> > -		return vm_insert_mixed(vma, vaddr, pfn);
> > +		rc = vm_insert_mixed(vma, vaddr, pfn);
> > +
> > +	/* -EBUSY is fine, somebody else faulted on the same PTE */
> > +	if (rc == -EBUSY)
> > +		rc = 0;
> > +
> > +	vmf_ret = dax_fault_return(rc);
> 
> Prior to this point where we convert our 'rc' (which is a normal error code
> like 0, -ENOMEM, etc) to a 'vmf_ret' (which is a VM return code like
> VM_FAULT_NOPAGE, VM_FAULT_SIGBUS, etc), there are several paths in
> dax_insert_mapping() where we could still return a normal error code.  I think
> this will confuse the fault handler because this code will get returned all
> the way up to do_fault() which expects to get a VM return code.
> 
> So, I think we either need to:
> 
> a) Make sure all return paths through dax_insert_mapping() end up going
> through dax_fault_return(), as we do in the PMD case, or

Yeah, this was the intention (as my eventually I'd like to make
dax_insert_mapping() and dax_pmd_insert_mapping() one function - they are
already currently very similar). I'll fix the missed cases.

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