Re: [RFC PATCH 0/2] mm: swap: allow per-device skipping of zero-filled page check

[Usama Arif <usama.arif@linux.dev>]

On Mon, 18 May 2026 16:34:53 +0900 Youngjun Park <youngjun.park@lge.com> wrote:

> Currently, the swap layer checks whether a page is entirely zero-filled
> before writing it out to the swap device. However, some swap backends,
> such as zram and our custom swap device, already perform their own
> same-filled page checking internally. This results in redundant CPU operations 
> checking same page pattern.

Hello!

So IMO, we should go in the other direction and remove same-filled page
support from zram. When we added zero-filled support to swap, we removed
it from zswap, because its not worth the extra overhead of maintaining
this in zswap.

I did some analysis at that time, and I remember more than 90% of the
same-filled pages on server workloads were zero-filled. I think someone
else did analysis on android (maybe Barry?) and it was about 85%.

IMO we should just remove same-filled page support from zram, and keep
this centralized in swap, so that everyone benefits.


> 
> This patchset introduces a new swapon flag, SWAP_FLAG_SKIP_ZERO_CHECK,
> to eliminate this redundancy. I introduce this as a per-device flag
> rather than a global setting because traditional swap devices still
> benefit from the swap layer's zero page check to avoid unnecessary I/O.
> By using this flag, userspace can selectively disable the zero check
> only for specific backends.
> 
> Furthermore, on certain architectures where the zero map is managed via
> a separate bitmap, skipping this check allows bypassing
> the bitmap allocation entirely (saving memory).
> 
> This modification is based on the previous discussion with Nhat Pham [1].
> Additionally, this patchset is built on top of Kairui Song's recent
> patchset regarding swap table and zeromap modifications [2].
> 
> Tested simply with zram on QEMU to verify zero-filled page handling.
> 
> References:
> [1] https://lore.kernel.org/linux-mm/acQvNRLpHwnHt7i+@yjaykim-PowerEdge-T330/
> [2] https://lore.kernel.org/linux-mm/20260517-swap-table-p4-v5-0-88ae43e064c7@tencent.com/T/#t
> 
> Youngjun Park (2):
>   mm: swap: add SWAP_FLAG_SKIP_ZERO_CHECK to skip zero-filled page check
>   mm: swap: do not allocate zero_bitmap if zero check is skipped
> 
>  include/linux/swap.h |  4 +++-
>  mm/page_io.c         |  7 ++++++-
>  mm/swap.h            | 12 ++++++++++++
>  mm/swapfile.c        | 14 ++++++++++----
>  4 files changed, 31 insertions(+), 6 deletions(-)
> 
> -- 
> 2.34.1
> 
>