Re: [PATCH v7] crypto: scompress: remove memcpy if sg_nents is 1 and pages are lowmem

[Herbert Xu]

On Sat, Mar 02, 2024 at 08:27:45AM +1300, Barry Song wrote:
> From: Barry Song <v-songbaohua@xxxxxxxx>
> 
> while sg_nents is 1, which is always true for the current kernel
> as the only user - zswap is this case, we might have a chance to
> remove memcpy, thus improve the performance.
> Though sg_nents is 1, its buffer might cross two pages. If those
> pages are highmem, we have no cheap way to map them to contiguous
> virtual address because kmap doesn't support more than one page
> (kmap single higmem page could be still expensive for tlb) and
> vmap is expensive.
> So we also test and enure page is not highmem in order to safely
> use page_to_virt before removing the memcpy. The good news is
> that in the most majority of cases, we are lowmem, and we are
> always lowmem in those modern and popular hardware.
> 
> Cc: Johannes Weiner <hannes@xxxxxxxxxxx>
> Cc: Nhat Pham <nphamcs@xxxxxxxxx>
> Cc: Yosry Ahmed <yosryahmed@xxxxxxxxxx>
> Signed-off-by: Barry Song <v-songbaohua@xxxxxxxx>
> Tested-by: Chengming Zhou <zhouchengming@xxxxxxxxxxxxx>
> ---
>  -v7:
>  * fix the problem pointed out by Herbert - flush all pages if dst
>    is longer than one page.
> 
>  crypto/scompress.c | 36 +++++++++++++++++++++++++++++-------
>  1 file changed, 29 insertions(+), 7 deletions(-)

Patch applied.  Thanks.
-- 
Email: Herbert Xu <herbert@xxxxxxxxxxxxxxxxxxx>
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