Re: [RFC PATCH 00/11] fs-verity support for XFS

[Eric Biggers <ebiggers@kernel.org>]

On Fri, Dec 16, 2022 at 07:57:37AM +1100, Dave Chinner wrote:
> > 
> > Well, that's why my patch uses kvmalloc() to allocate the bitmap.
> > 
> > I did originally think it was going to have to be a sparse bitmap that ties into
> > the shrinker so that pages of it can be evicted.  But if you do the math, the
> > required bitmap size is only 1 / 2^22 the size of the file, assuming the Merkle
> > tree uses SHA-256 and 4K blocks.  So a 100MB file only needs a 24-byte bitmap,
> > and the bitmap for any file under 17GB fits in a 4K page.
> > 
> > My patch puts an arbitrary limit at a 1 MiB bitmap, which would be a 4.4TB file.
> > 
> > It's not ideal to say "4 TB Ought To Be Enough For Anybody".  But it does feel
> > that it's not currently worth the extra complexity and runtime overhead of
> > implementing a full-blown sparse bitmap with cache eviction support, when no one
> > currently has a use case for fsverity on files anywhere near that large.
> 
> I think we can live with that for the moment, but I suspect that 4TB
> filesize limit will become an issue sooner rather than later. What
> will happen if someone tries to measure a file larger than this
> limit? What's the failure mode?
> 

FS_IOC_ENABLE_VERITY will fail with EFBIG.

- Eric