linux-ext4 - Re: [PATCH 1/2] dax: Pass detailed error code from dax_iomap

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Date:   Tue, 2 Jan 2018 19:54:40 +0100
From:   Jan Kara <jack@...e.cz>
To:     Dan Williams <dan.j.williams@...el.com>
Cc:     Jan Kara <jack@...e.cz>, linux-ext4 <linux-ext4@...r.kernel.org>,
        Ted Tso <tytso@....edu>,
        Ross Zwisler <ross.zwisler@...ux.intel.com>,
        linux-fsdevel <linux-fsdevel@...r.kernel.org>,
        linux-nvdimm@...ts.01.org
Subject: Re: [PATCH 1/2] dax: Pass detailed error code from dax_iomap_fault()

On Thu 21-12-17 09:12:52, Dan Williams wrote:
> On Thu, Dec 21, 2017 at 8:30 AM, Jan Kara <jack@...e.cz> wrote:
> > Ext4 needs to pass through error from its iomap handler to the page
> > fault handler so that it can properly detect ENOSPC and force
> > transaction commit and retry the fault (and block allocation). Add
> > argument to dax_iomap_fault() for passing such error.
> >
> > Signed-off-by: Jan Kara <jack@...e.cz>
> > ---
> >  fs/dax.c            | 9 ++++++---
> >  fs/ext2/file.c      | 2 +-
> >  fs/ext4/file.c      | 2 +-
> >  fs/xfs/xfs_file.c   | 2 +-
> >  include/linux/dax.h | 2 +-
> >  5 files changed, 10 insertions(+), 7 deletions(-)
> >
> > diff --git a/fs/dax.c b/fs/dax.c
> > index 95981591977a..f3afa1d6156c 100644
> > --- a/fs/dax.c
> > +++ b/fs/dax.c
> > @@ -1096,7 +1096,7 @@ static bool dax_fault_is_synchronous(unsigned long flags,
> >  }
> >
> >  static int dax_iomap_pte_fault(struct vm_fault *vmf, pfn_t *pfnp,
> > -                              const struct iomap_ops *ops)
> > +                              int *iomap_errp, const struct iomap_ops *ops)
> >  {
> >         struct vm_area_struct *vma = vmf->vma;
> >         struct address_space *mapping = vma->vm_file->f_mapping;
> > @@ -1149,6 +1149,8 @@ static int dax_iomap_pte_fault(struct vm_fault *vmf, pfn_t *pfnp,
> >          * that we never have to deal with more than a single extent here.
> >          */
> >         error = ops->iomap_begin(inode, pos, PAGE_SIZE, flags, &iomap);
> > +       if (iomap_errp)
> > +               *iomap_errp = error;
> 
> Since we already have 'struct iomap' tracking the state of the iomap
> should we track the error status there as well?  I.e. move the on
> stack allocation of struct iomap to the per-fs dax fault handlers.

I don't think that's really adequate. Firstly because at least part of
struct iomap needs to be initialized inside dax_iomap_fault() and secondly
because by the time dax_iomap_fault() returns, mapping information in struct
iomap may be invalid (as we unlocked radix tree entry). So I think it is
really better to pass back only the error code and keep struct iomap
internal to dax_iomap_fault().

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