Re: [RFC PATCH 1/2] mm: swap: add SWAP_FLAG_SKIP_ZERO_CHECK to skip zero-filled page check

[YoungJun Park <youngjun.park@lge.com>]

On Tue, May 19, 2026 at 02:18:33AM -0700, Christoph Hellwig wrote:
> On Tue, May 19, 2026 at 04:08:35PM +0900, YoungJun Park wrote:
> > On Mon, May 18, 2026 at 11:30:52PM -0700, Christoph Hellwig wrote:
> > 
> > Hi Christoph! 
> > 
> > > Nothing in this patch ever sets the new flag.  And I'd rather not
> > 
> > I defined a new flag that can be set at swapon timing. Is your concern
> > that there is no mention of the user-space flag setting part?
> 
> Heh.  My main concert was that I assumed it's a kernel-only flag as
> SWAP_FLAG_* aren't defined in a uapi header.  But now that I know
> that they are user passed flag, my concern is that this is not something
> that should be exposed to the user because it is a depply internal
> implementation detail.
> 
> > 
> > > add new weird bdev hooks for zram when we really need to consolidate
> > > the compressed swap logic in the core swap code.
> >  
> > Are you mentioning we consolidate the same data pattern logic (including
> > compressed swap) into the core swap code?
> 
> That is the plan based on my understanding of the LSF/MM session.
> 
> > I am curious about what direction you have in mind. I can think of three
> > possible approaches.
> > 
> > - Handle it in individual devices
> > 
> > Can be optimized for each device. Currently, the swap layer
> > handles zero pages but not same data patterns, causing a mismatch. My
> > patch aims to resolve this. If we prefer this direction, we could even
> > remove the zero page check from the swap layer and let individual swap
> > devices handle it.
> 
> I don't think devices should care about this at all.  compression and
> dedup is core functionality that does not depend on the underlying
> device.
> 
> > I wonder which direction you are considering.
> 
> Please look at all the ongoing discussions on the linux-mm list.

I submitted this patch from the perspective of optimizing the current state,
but I see that it might not align with the future direction.

Thank you for the reply and clarification.

Best regards,
Youngjun Park