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charset="UTF-8" Content-Transfer-Encoding: quoted-printable On Mon, Oct 5, 2026 at 9:21=E2=80=AFPM Sergey Senozhatsky wrote: > > When a dictionary is configured, zstd allocates all of its memory (C/D > dictionaries, compression and decompression contexts and compression > workspaces) via zstd_custom_alloc(). zstd_custom_alloc() uses > kvzalloc(), which falls back to vmalloc() only when kmalloc() fails, > and kmalloc() rounds such sizes up to a power-of-two number of pages. > > For example, on x86_64 with a 110K dictionary, switching from > kvzalloc() to __vmalloc() reduces allocations from: > - CDict (level 3): 512 KB -> 400 KB > - CDict (levels 12+): 2048 KB -> 1552 KB > - DCtx (per-CPU): 128 KB -> 96 KB > - CCtx workspace (level 3): 128 KB -> 84 KB (per-CPU) > - CCtx workspace (level 14+): 1024 KB -> 764 KB (per-CPU) > > Use __vmalloc() for allocations larger than PAGE_SIZE (and kzalloc() > for smaller ones), which is also how zram allocates zstd workspaces > when no dictionary is configured. zstd_custom_free() already uses > kvfree(), which handles both. > > Signed-off-by: Sergey Senozhatsky Acked-by: Brian Geffon > --- > drivers/block/zram/backend_zstd.c | 9 ++++++++- > 1 file changed, 8 insertions(+), 1 deletion(-) > > diff --git a/drivers/block/zram/backend_zstd.c b/drivers/block/zram/backe= nd_zstd.c > index b367049f131d..7a8b5ff186f2 100644 > --- a/drivers/block/zram/backend_zstd.c > +++ b/drivers/block/zram/backend_zstd.c > @@ -26,10 +26,17 @@ struct zstd_params { > /* > * For C/D dictionaries we need to provide zstd with zstd_custom_mem, > * which zstd uses internally to allocate/free memory when needed. > + * > + * zstd allocations (C/D dictionaries and contexts) are large and > + * long-lived. Don't use kvzalloc() for them: when enough physically > + * contiguous memory is available it serves them from kmalloc(), which > + * rounds them up to a power-of-two number of pages. > */ > static void *zstd_custom_alloc(void *opaque, size_t size) > { > - return kvzalloc(size, GFP_NOIO | __GFP_NOWARN); > + if (size <=3D PAGE_SIZE) > + return kzalloc(size, GFP_NOIO | __GFP_NOWARN); > + return __vmalloc(size, GFP_NOIO | __GFP_NOWARN | __GFP_ZERO); > } > > static void zstd_custom_free(void *opaque, void *address) > -- > 2.56.0.rc1.315.gc6ed9934b7-goog >