On Mon, Jan 22, 2024 at 3:35 PM Miklos Szeredi <miklos@xxxxxxxxxx> wrote: > > On Mon, 22 Jan 2024 at 14:18, Amir Goldstein <amir73il@xxxxxxxxx> wrote: > > > > Something like this looks ok? > > > > --- a/Documentation/filesystems/overlayfs.rst > > +++ b/Documentation/filesystems/overlayfs.rst > > @@ -145,7 +145,9 @@ filesystem, an overlay filesystem needs to record > > in the upper filesystem > > that files have been removed. This is done using whiteouts and opaque > > directories (non-directories are always opaque). > > > > -A whiteout is created as a character device with 0/0 device number. > > +A whiteout is created as a character device with 0/0 device number or > > +as a regular file with the xattr "trusted.overlay.whiteout". > > It should also refer to the "whiteouts and opaque directories" section. ok. > > > + > > When a whiteout is found in the upper level of a merged directory, any > > matching name in the lower level is ignored, and the whiteout itself > > is also hidden. > > @@ -154,6 +156,11 @@ A directory is made opaque by setting the xattr > > "trusted.overlay.opaque" > > to "y". Where the upper filesystem contains an opaque directory, any > > directory in the lower filesystem with the same name is ignored. > > > > +An opaque directory should not conntain any whiteouts, because they do not > > +serve any purpose. A merge directory containing regular files with the xattr > > +"trusted.overlay.whiteout", should be additionally marked by setting the xattr > > +"trusted.overlay.opaque" to "x" on the merge directory itself. > > I think it's worth noting that this can have a performance impact on > readdir, so people don't think xwhiteouts are a drop in replacement. > Ok, I will write something for v4. > > Alex already did that: > > > > https://docs.kernel.org/filesystems/overlayfs.html#nesting-overlayfs-mounts > > Indeed, thanks. > > > We do not currently have per-directory-per-layer flags in ovl_lowerstack(). > > > > Your patch was optimizing only per-layer check_xwhiteout. > > My patch is optimizing only per-directory check_xwhiteout. > > > > The important thing is that for the common case (no xwhiteouts) > > xwhiteout will never be checked. > > > > Are you concerned about optimizing check_xwhiteout in a multi lower > > overlayfs nested over a composefs overlay mount for the case that > > one of the merge dirs in the stack have xwhiteouts and the other do not?? > > > > I guess we can use a combination of your patch (v2) and my patch (v3) > > and do something like this: > > > > if (last_element && !is_upper && val == 'x') { > > d->xwhiteouts = d->layer->xwhiteouts = true; > > > > ... > > > > to mark the dentry as OVL_E_XWHITEOUTS > > AND mark the layer as having xwhiteouts > > and then in readdir we check that > > BOTH dentry has xwhiteouts (in some layer) > > AND the layer has xwhiteouts (in some directory). > > > > Is that what you meant? > > I didn't think it through, but yeah, this sounds good. > > This way we can also remove the checks against layer not being upper > and lowest, right? I think we can. Let me try this out. Thanks, Amir.
