Re: [PATCH v3 10/11] zram: use rw_semaphore for decompression streams

[Bo Zhang <zhangbo0325@gmail.com>]

On Thu, 15 Oct 2026 04:46:27 +0800 Barry Song <baohua@kernel.org> wrote:
> is it due to the case that sg_is_last() is false, local_copy is needed?

Yes, exactly. Our tree isn't on the sg read model yet, so I detect the
single-page case via zs_obj_get_pages() (page1 == page2) rather than
sg_is_last(), but the condition is the same: the residual exclusive
reads are the cross-page objects that need the ->local_copy bounce.

> Also, with either deflate or zstd, create_dctx() will
> always be present. So, what algorithm are we actually testing?
> What happens if we switch to zstd?

We're testing lzo-rle, which is stateless (no create_dctx), so the
~77% shared figure is specific to it. You're right that with zstd or
deflate every read takes create_dctx != NULL and would fall onto the
exclusive path, so the shared benefit wouldn't apply there - as Sergey
noted, zstd mutates the decompression ctx and its dctx workspace is too
large to allocate per-request. So the shared-read win is really a
stateless-backend property; I should have called that out.

I'll also apply Sergey's vm_map_ram() patch (with the memalloc_noio
fixup) and re-run on-device. Note this only helps the cross-page part
of our exclusive reads on a stateless backend like lzo-rle; it won't
change zstd/deflate, which stay exclusive regardless because of
create_dctx. Will report the numbers separately.

Thanks,
Bo