Re: [PATCH v15 5/9] erofs: introduce the page cache share feature

[Christoph Hellwig <hch@lst.de>]

On Tue, Jan 20, 2026 at 03:19:21PM +0800, Gao Xiang wrote:
>> It will be very hard to change unless we move to physical indexing of
>> the page cache, which has all kinds of downside.s
>
> I'm not sure if it's really needed: I think the final
> folio adaption plan is that folio can be dynamic
> allocated? then why not keep multiple folios for a
> physical memory, since folios are not order-0 anymore.

Having multiple folios for the same piece of memory can't work,
at we'd have unsynchronized state.

> Using physical indexing sounds really inflexible on my
> side, and it can be even regarded as a regression for me.

I'm absolutely not arguing for that..

>>> So that let's face the reality: this feature introduces
>>> on-disk xattrs called "fingerprints." --- Since they're
>>> just xattrs, the EROFS on-disk format remains unchanged.
>>
>> I think the concept of using a backing file of some sort for the shared
>> pagecache (which I have no problem with at all), vs the imprecise
>
> In that way (actually Jingbo worked that approach in 2023),
> we have to keep the shared data physically contiguous and
> even uncompressed, which cannot work for most cases.

Why does that matter?

> On the other side, I do think `fingerprint` from design
> is much like persistent NFS file handles in some aspect
> (but I don't want to equal to that concept, but very
> similar) for a single trusted domain, we should have to
> deal with multiple filesystem sources and mark in a
> unique way in a domain.

I don't really thing they are similar in any way.