Compressed files & the page cache

[Barry Song]

On Wed, Jul 16, 2025 at 7:32 AM Gao Xiang <hsiangkao at linux.alibaba.com> wrote:
[...]
>
> I don't see this will work for EROFS because EROFS always supports
> variable uncompressed extent lengths and that will break typical
> EROFS use cases and on-disk formats.
>
> Other thing is that large order folios (physical consecutive) will
> caused "increase the latency on UX task with filemap_fault()"
> because of high-order direct reclaims, see:
> https://android-review.googlesource.com/c/kernel/common/+/3692333
> so EROFS will not set min-order and always support order-0 folios.

Regarding Hailong's Android hook, it's essentially a complaint about
the GFP mask used to allocate large folios for files. I'm wondering
why the page cache hasn't adopted the same approach that's used for
anon large folios:

    gfp = vma_thp_gfp_mask(vma);

Another concern might be that the allocation order is too large,
which could lead to memory fragmentation and waste. Ideally, we'd
have "small" large folios—say, with order <= 4—to strike a better
balance.

>
> I think EROFS will not use this new approach, vmap() interface is
> always the case for us.
>
> Thanks,
> Gao Xiang
>
> >
>

Thanks
Barry