Re: [PATCH v3] ovl: mark xwhiteouts directory with overlay.opaque='x'

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On Sun, 21 Jan 2024 at 16:05, Amir Goldstein <amir73il@xxxxxxxxx> 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,
> Alex has reported a problem with your suggested approach of requiring
> xwhiteouts xattr on layers root dir [1].
> Following counter proposal, amortizes the cost of checking opaque xattr
> on directories during lookup to also check for xwhiteouts.

Concept looks good overall.

overlayfs.rst needs updating with the new format.   BTW the nesting
format should also be documented, but that's a separate patch.

> @@ -292,7 +292,11 @@ static int ovl_lookup_single(struct dentry *base, struct ovl_lookup_data *d,
>                 if (d->last)
>                         goto out;
> -               if (ovl_is_opaquedir(OVL_FS(d->sb), &path)) {
> +               /* overlay.opaque=x means xwhiteouts directory */
> +               val = ovl_get_opaquedir_val(ofs, &path);
> +               if (last_element && !is_upper && val == 'x') {
> +                       d->xwhiteouts = true;

Maybe I'm missing something, but can't we set the flag on the layer?


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