On Fri, Mar 9, 2018 at 12:18 AM, Vivek Goyal <vgoyal@xxxxxxxxxx> wrote:
> d->last signifies that this is the last layer we are looking into and there
> is no more. And that means this allows for some optimzation opportunities
> during lookup. For example, in ovl_lookup_single() we don't have to check
> for opaque xattr of a directory is this is the last layer we are looking
> into (d->last = true).
>
> But knowing for sure whether we are looking into last layer can be very
> tricky. If redirects are not enabled, then we can look at poe->numlower
> and figure out if the lookup we are about to is last layer or not. But
> if redircts are enabled then it is possible poe->numlower suggests that
> we are looking in last layer, but there is an absolute redirect present
> in found element and that redirects us to a layer in root and that means
> lookup will continue in lower layers further.
>
> For example, consider following.
>
> /upperdir/pure (opaque=y)
> /upperdir/pure/foo (opaque=y,redirect=/bar)
> /lowerdir/bar
>
> In this case pure is "pure upper". When we look for "foo", that time
> poe->numlower=0. But that alone does not mean that we will not search
> for a merge candidate in /lowerdir. Absolute redirect changes that.
>
> IOW, d->last should not be set just based on poe->numlower if redirects
> are enabled. That can lead to setting d->last while it should not have
> and that means we will not check for opaque xattr while we should have.
>
> So do this.
>
> - If redirects are not enabled, then continue to rely on poe->numlower
> information to determine if it is last layer or not.
>
> - If redirects are enabled, then set d->last = true only if this is the
> last layer in root ovl_entry (roe).
>
> Suggested-by: Amir Goldstein <amir73il@xxxxxxxxx>
> Signed-off-by: Vivek Goyal <vgoyal@xxxxxxxxxx>
Reviewed-by: Amir Goldstein <amir73il@xxxxxxxxx>
Much better description than my RFC patch :-)
One minor error
> ---
> fs/overlayfs/namei.c | 8 ++++++--
> 1 file changed, 6 insertions(+), 2 deletions(-)
>
> diff --git a/fs/overlayfs/namei.c b/fs/overlayfs/namei.c
> index de3e6da1d5a5..2e173cfbda0e 100644
> --- a/fs/overlayfs/namei.c
> +++ b/fs/overlayfs/namei.c
> @@ -815,7 +815,7 @@ struct dentry *ovl_lookup(struct inode *dir, struct dentry *dentry,
> .is_dir = false,
> .opaque = false,
> .stop = false,
> - .last = !poe->numlower,
> + .last = ofs->config.redirect_follow ? false : !poe->numlower,
> .redirect = NULL,
> };
>
> @@ -873,7 +873,11 @@ struct dentry *ovl_lookup(struct inode *dir, struct dentry *dentry,
> for (i = 0; !d.stop && i < poe->numlower; i++) {
> struct ovl_path lower = poe->lowerstack[i];
>
> - d.last = i == poe->numlower - 1;
> + if (!ofs->config.redirect_follow)
> + d.last = i == poe->numlower - 1;
> + else
> + d.last = lower.layer->idx == roe->numlower - 1;
> +
Should be lower.layer->idx == roe->numlower. (idx 0 is upper)
But to be honest I did not verify that xattr checks are optimized away with
my RFC patch, just that the test case above behaves as expected.
Thanks,
Amir.
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