linux-kernel - Re: [PATCHv7 5/7] kho: don't unpreserve memory during abort

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Message-ID: <CA+CK2bAvKrfuOXTa-RWtcuSR8rkPMhurwCn41NcUm44_vT63rA@mail.gmail.com>
Date: Fri, 24 Oct 2025 09:28:33 -0400
From: Pasha Tatashin <pasha.tatashin@...een.com>
To: Mike Rapoport <rppt@...nel.org>
Cc: Pratyush Yadav <pratyush@...nel.org>, akpm@...ux-foundation.org, brauner@...nel.org, 
	corbet@....net, graf@...zon.com, jgg@...pe.ca, linux-kernel@...r.kernel.org, 
	linux-kselftest@...r.kernel.org, linux-mm@...ck.org, masahiroy@...nel.org, 
	ojeda@...nel.org, rdunlap@...radead.org, tj@...nel.org
Subject: Re: [PATCHv7 5/7] kho: don't unpreserve memory during abort

On Thu, Oct 23, 2025 at 3:21 AM Mike Rapoport <rppt@...nel.org> wrote:
>
> On Wed, Oct 22, 2025 at 01:15:30PM +0200, Pratyush Yadav wrote:
> > On Tue, Oct 21 2025, Pasha Tatashin wrote:
> >
> > > KHO allows clients to preserve memory regions at any point before the
> > > KHO state is finalized. The finalization process itself involves KHO
> > > performing its own actions, such as serializing the overall
> > > preserved memory map.
> > >
> > > If this finalization process is aborted, the current implementation
> > > destroys KHO's internal memory tracking structures
> > > (`kho_out.ser.track.orders`). This behavior effectively unpreserves
> > > all memory from KHO's perspective, regardless of whether those
> > > preservations were made by clients before the finalization attempt
> > > or by KHO itself during finalization.
> > >
> > > This premature unpreservation is incorrect. An abort of the
> > > finalization process should only undo actions taken by KHO as part of
> > > that specific finalization attempt. Individual memory regions
> > > preserved by clients prior to finalization should remain preserved,
> > > as their lifecycle is managed by the clients themselves. These
> > > clients might still need to call kho_unpreserve_folio() or
> > > kho_unpreserve_phys() based on their own logic, even after a KHO
> > > finalization attempt is aborted.
> >
> > I think you also need to update test_kho and reserve_mem to do this
> > since right now they assume all memory gets unpreserved on failure.
>
> I agree.

Hm, this makes no sense to me. So, KHO tried to finalize (i.e.,
convert xarray to sparse bitmap) and failed (e.g. due to OOM) or
aborted, so we aborted the finalization. But the preserved memory
stays preserved, and if user/caller retries finalization and it
succeeds, the preserved memory will still be passed to the next
kernel. Why would reserve_mem and test_kho depend on whether KHO
finalization succeeded or was canceled? It is possible that user
cancel only to add something else to preservation.

Pasha