On Tue, 2024-01-23 at 11:53 +0200, Amir Goldstein wrote:
> On Tue, Jan 23, 2024 at 11:05 AM Alexander Larsson <alexl@xxxxxxxxxx>
> wrote:
> >
> > On Mon, 2024-01-22 at 21:51 +0200, Amir Goldstein wrote:
> > > An opaque directory cannot have xwhiteouts, so instead of marking
> > > an
> > > xwhiteouts directory with a new xattr, overload overlay.opaque
> > > xattr
> > > for marking both opaque dir ('y') and xwhiteouts dir ('x').
> > >
> > > This is more efficient as the overlay.opaque xattr is checked
> > > during
> > > lookup of directory anyway.
> > >
> > > This also prevents unnecessary checking the xattr when reading a
> > > directory without xwhiteouts, i.e. most of the time.
> > >
> > > Note that the xwhiteouts marker is not checked on the upper layer
> > > and
> > > on the last layer in lowerstack, where xwhiteouts are not
> > > expected.
> > >
> > > Fixes: bc8df7a3dc03 ("ovl: Add an alternative type of whiteout")
> > > Cc: <stable@xxxxxxxxxxxxxxx> # v6.7
> > > Signed-off-by: Amir Goldstein <amir73il@xxxxxxxxx>
> > > ---
> > >
> > > Miklos,
> > >
> > > This v4 is a combination of your v2 and my v3 patches to optimize
> > > xwhiteouts readdir for the intersection of a dentry with
> > > xwhiteouts
> > > on any layer and a layer with any xwhiteouts on any directory.
> > >
> > > Alex,
> > >
> > > Please re-review/test.
> >
> > Looks good to me. The only thing I worry about is the atomicity of
> > ovl_layer_set_xwhiteouts(). Doesn't that need a barrier or
> > something?
> >
>
> I think it does not because:
>
> ovl_layer_set_xwhiteouts() is called before adding the overlay
> dir
> dentry to dcache, while readdir of that same directory happens
> after the overlay dir dentry is in dcache, so if some cpu
> observes
> ovl_dentry_is_xwhiteouts(), it will also observe layer-
> >has_xwhiteouts
> for the layers where xwhiteouts marker was found in that merge
> dir.
I see what you mean. A layer getting has_xwhiteouts updated
concurrently via a different dentry may be missed, but that doesn't
matter for the readdir result, because we're guaranteed to see it if it
actually affected the readdir dentry and that is the only thing that
matters for the results.
--
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Alexander Larsson Red Hat,
Inc
alexl@xxxxxxxxxx alexander.larsson@xxxxxxxxx
He's an impetuous vegetarian stage actor from the Mississippi delta.
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