Practical Limit on EROFS lcluster size

[Gao Xiang]

On Mon, Dec 20, 2021 at 10:53:10PM +0800, Gao Xiang wrote:
> Hi Kelvin,
> 
> On Mon, Dec 20, 2021 at 08:45:42AM -0500, Kelvin Zhang wrote:
> > Hi Gao,
> >     I was playing with large pcluster sizes recently, I noticed a
> > quirk about EROFS. In summary, logical cluster size has a practical
> > limit of 8MB. Here's why:
> > 
> >    Looking at https://git.kernel.org/pub/scm/linux/kernel/git/xiang/erofs-utils.git/tree/lib/compress.c?h=experimental&id=564adb0a852b38a1790db20516862fc31bca314d#n92
> > , line 92, we see the following code:
> > 
> > if (d0 == 1 && erofs_sb_has_big_pcluster()) {
> >         type = Z_EROFS_VLE_CLUSTER_TYPE_NONHEAD;
> >         di.di_u.delta[0] = cpu_to_le16(ctx->compressedblks |
> >                 Z_EROFS_VLE_DI_D0_CBLKCNT); // This line
> >         di.di_u.delta[1] = cpu_to_le16(d1);
> > } else if (d0) {
> >         type = Z_EROFS_VLE_CLUSTER_TYPE_NONHEAD;
> > 
> >         di.di_u.delta[0] = cpu_to_le16(d0);  // and this line
> >         di.di_u.delta[1] = cpu_to_le16(d1);
> > }
> > 
> > When a compressed index has type NOHEAD, delta[0] stores d0(distance
> > to head block). But The 11th bit of d0 is also used as a flag bit to
> > indicate that d0 stores the pcluster size. This means d0 cannot exceed
> > Z_EROFS_VLE_DI_D0_CBLKCNT(2048), or else the parser will incorrectly
> > interpret d0 as pcluster size, rather than distance to head block.
> >     Is this an intentional design choice? It's not necessarily bad,
> > but it's something I think is worth documenting in code.
> 
> Thanks for this great insight! Actually on-disk EROFS format doesn't
> have such limitation by design, since if it looks back to the delta0
> lcluster and it's still a NONHEAD lcluster, it will look back with
> new delta0 again until finding the final HEAD lcluster.
> 
> But I'm not sure if mkfs code can handle > 8MiB lcluster properly yet,
> without modification since lcluster size is strictly limited with
> https://git.kernel.org/pub/scm/linux/kernel/git/xiang/erofs-utils.git/tree/include/erofs/compress.h#n14
> EROFS_CONFIG_COMPR_MAX_SZ * 2
> 
> Yeah, I have to admit the current document might not be so detailed,
> partially due to my somewhat bad English written speed, and limited
> time...

By the way, I'd like to correct some concepts here (sorry I didn't use
them correctly in my previous email).

A lcluster includes a HEAD or NONHEAD lcluster, currently only 4KiB
is supported.
A pcluster contains arbitrary compressed blocks, which can be
decompressed independently.
A compressed extent matches a pcluster, and several lclusters (maybe
partially).

So strictly speaking is "practical Limit on EROFS compressed extent
size"...

> 
> Thanks,
> Gao Xiang
> 
> > 
> > 
> > -- 
> > Sincerely,
> > 
> > Kelvin Zhang