Re: [PATCH] erofs: cap Zstandard stream pool size

[Gao Xiang <xiang@kernel.org>]

Hi Xusheng,

On Tue, Jul 28, 2026 at 10:18:31AM +0800, Zhan Xusheng wrote:
> From: Zhan Xusheng <zhanxusheng@xiaomi.com>
> 
> fs/erofs/decompressor_zstd.c sizes the module-global Zstandard stream
> pool from num_possible_cpus() when the zstd_streams module parameter is
> unset, and z_erofs_load_zstd_config() then preallocates one workspace per
> stream, grown to the largest dictionary of any mounted image (up to
> Z_EROFS_ZSTD_MAX_DICT_SIZE, i.e. Z_EROFS_PCLUSTER_MAX_SIZE).  On high-CPU
> systems this can pin a large amount of vmalloc-backed decoder state until
> the erofs module is unloaded, mirroring the LZMA case fixed in commit
> c9b47e6b2311 ("erofs: cap LZMA stream pool size").
> 
> Bound the default stream count by a new
> CONFIG_EROFS_FS_ZIP_ZSTD_DEFAULT_MAX_STREAMS option, default 16, while
> preserving the per-image workspace sizing.  An explicit zstd_streams
> module parameter is still honoured as-is.
> 
> Fixes: 7c35de4df105 ("erofs: Zstandard compression support")
> Signed-off-by: Zhan Xusheng <zhanxusheng@xiaomi.com>

Thanks for the patch.

Unlike LZMA, each Zstandard only takes 1MiB at most (although erofs-utils
only generates 4MiB LZ77 dictionary at most for LZMA, but on-disk format
allows 8MiB so just in case.)

So I think for Zstandard, 1MiB each stream should fulfill to most
platforms (and for embedded systems for example, the vendors should
control the dictionary size when generating the image; but for servers,
I think 1MiB at most for each server CPU is OK).

Unless there is a particular need, I don't expect every decompressor need
a strict customized Kconfig for this (of course, DEFLATE and LZ4 takes
32k/64k so it doesn't matter.)

Thanks,
Gao Xiang