Re: [RFC PATCH v2 1/3] mm/zsmalloc: encode class index in obj value for lockless class lookup

[Barry Song]

On Wed, May 27, 2026 at 8:02 PM Wenchao Hao <haowenchao22@xxxxxxxxx> wrote:
>
> Encode the size_class index (class_idx) into the obj value so that
> zs_free() can determine the correct size_class without dereferencing
> the handle->obj->PFN->zpdesc->zspage->class chain under pool->lock.
> class_idx is invariant across page migration (only PFN is rewritten),
> so a lockless read of obj always yields a valid class_idx.
>
> The space below the PFN field in obj is over-provisioned on 64-bit
> systems, with more bits than obj_idx needs. Split that space into
> class_idx and obj_idx subfields:
>
>  |<-- _PFN_BITS -->|<-- ZS_OBJ_CLASS_BITS -->|<-- ZS_OBJ_IDX_BITS -->|
>  +-----------------+-------------------------+-----------------------+
>  |       PFN       |        class_idx        |        obj_idx        |
>  +-----------------+-------------------------+-----------------------+
> MSB                ^                                                LSB
>                    |
>                    +-- ZS_OBJ_PFN_SHIFT
>
> The macro layout changes as follows:
>
>     Before            After               Meaning
>     ----------------  ------------------  ----------------------------
>     OBJ_INDEX_BITS    ZS_OBJ_IDX_BITS     width of obj_idx subfield
>     OBJ_INDEX_MASK    ZS_OBJ_IDX_MASK     mask  of obj_idx subfield
>     (n/a)             ZS_OBJ_CLASS_BITS   width of class_idx subfield
>     (n/a)             ZS_OBJ_CLASS_MASK   mask  of class_idx subfield
>     (n/a)             ZS_OBJ_PFN_SHIFT    bit offset of PFN in obj
>
> On 32-bit systems there is no spare room for class_idx, so the
> encoding is disabled (ZS_OBJ_CLASS_BITS = 0) and the obj layout
> remains [PFN | obj_idx].
>
> Signed-off-by: Wenchao Hao <haowenchao@xxxxxxxxxx>
> ---

Much better than v1.

Please make sure that the From: field matches the Signed-off-by: line next time.

Please commit the patch on your machine using:

Wenchao Hao <haowenchao@xxxxxxxxxx>

as the author.
Then git send-email will automatically generate the correct From: line
in the email.