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erofs: cap LZMA stream pool size

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fs/erofs/decompressor_lzma.c sizes the module-global MicroLZMA stream
pool from num_possible_cpus() when the lzma_streams module parameter is
unset, then z_erofs_load_lzma_config() preallocates one image-supplied
dictionary per stream, accepting dictionaries up to 8 MiB.  On high-CPU
systems, a small EROFS image can pin hundreds of MiB of vmalloc-backed
decoder state until the erofs module is unloaded.

Impact: An EROFS image mounted by the system can pin up to 8 MiB of
vmalloc memory per LZMA stream, either as intended or unexpectedly.

Bound the default stream count by a new
CONFIG_EROFS_FS_ZIP_LZMA_DEFAULT_MAX_STREAMS option, default 16, so the
worst-case default preallocation is 128 MiB if the number of CPUs is no
less than 16 while preserving the existing per-image dictionary limit.
An explicit lzma_streams module parameter is still honoured as-is, so
administrators who deliberately size the pool are not affected.

Fixes: 622ceaddb7 ("erofs: lzma compression support")
Cc: stable@vger.kernel.org
Assisted-by: Claude:claude-opus-4-8
Signed-off-by: Michael Bommarito <michael.bommarito@gmail.com>
Reviewed-by: Gao Xiang <hsiangkao@linux.alibaba.com>
Signed-off-by: Gao Xiang <hsiangkao@linux.alibaba.com>
This commit is contained in:
Michael Bommarito
2026-07-14 07:47:29 -04:00
committed by Gao Xiang
parent 96b2dbbe58
commit c9b47e6b23
2 changed files with 16 additions and 1 deletions
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14
fs/erofs/Kconfig
View File

@@ -131,6 +131,20 @@ config EROFS_FS_ZIP_LZMA
Say N if you want to disable LZMA compression support.
config EROFS_FS_ZIP_LZMA_DEFAULT_MAX_STREAMS
int "EROFS LZMA default maximum decompression streams"
depends on EROFS_FS_ZIP_LZMA
range 1 NR_CPUS
default 16
help
By default EROFS allocates one LZMA decompression stream per CPU.
Each stream can hold a dictionary of up to 8 MiB taken from the
mounted image, so on systems with many CPUs this can reserve a lot
of memory. This caps the default; the lzma_streams module parameter
still overrides it.
If unsure, keep the default of 16.
config EROFS_FS_ZIP_DEFLATE
bool "EROFS DEFLATE compressed data support"
depends on EROFS_FS_ZIP

3
fs/erofs/decompressor_lzma.c
View File

@@ -51,7 +51,8 @@ static int __init z_erofs_lzma_init(void)
/* by default, use # of possible CPUs instead */
if (!z_erofs_lzma_nstrms)
z_erofs_lzma_nstrms = num_possible_cpus();
z_erofs_lzma_nstrms = min_t(unsigned int, num_possible_cpus(),
CONFIG_EROFS_FS_ZIP_LZMA_DEFAULT_MAX_STREAMS);
for (i = 0; i < z_erofs_lzma_nstrms; ++i) {
struct z_erofs_lzma *strm = kzalloc_obj(*strm);