Re: [PATCH v2 3/3] xen/mm: limit non-scrubbed allocations to a specific order

[Roger Pau Monné <roger.pau@xxxxxxxxxx>]

On Tue, Jan 20, 2026 at 08:25:49AM +0100, Jan Beulich wrote:
> On 19.01.2026 17:13, Jan Beulich wrote:
> > On 15.01.2026 12:18, Roger Pau Monne wrote:
> >> The current logic allows for up to 1G pages to be scrubbed in place, which
> >> can cause the watchdog to trigger in practice.  Reduce the limit for
> >> in-place scrubbed allocations to a newly introduced define:
> >> CONFIG_DIRTY_MAX_ORDER.  This currently defaults to CONFIG_DOMU_MAX_ORDER
> >> on all architectures.  Also introduce a command line option to set the
> >> value.
> >>
> >> Fixes: 74d2e11ccfd2 ("mm: Scrub pages in alloc_heap_pages() if needed")
> >> Signed-off-by: Roger Pau Monné <roger.pau@xxxxxxxxxx>
> >> ---
> >> Changes since v1:
> >>  - Split from previous patch.
> >>  - Introduce a command line option to set the limit.
> >> ---
> >>  docs/misc/xen-command-line.pandoc |  9 +++++++++
> >>  xen/common/page_alloc.c           | 23 ++++++++++++++++++++++-
> >>  2 files changed, 31 insertions(+), 1 deletion(-)
> > 
> > If you confine the change to page_alloc.c, won't this mean that patch 2's
> > passing of MEMF_no_scrub will then also be bounded (in which case the need
> > for patch 2 would largely disappear)?
> 
> This was rubbish, sorry. Besides my being thick-headed I can only attribute
> this to the double negation in !(memflags & MEMF_no_scrub).
> 
> I have another concern, though: You effectively undermine ptdom_max_order,
> which is even more of a problem as that would also affect Dom0's ability to
> obtain larger contiguous I/O buffers. Perhaps DIRTY_MAX_ORDER ought to
> default to PTDOM_MAX_ORDER (if HAS_PASSTHROUGH)?

OK, yes, I can default to PTDOM_MAX_ORDER instead of DOMU_MAX_ORDER.

> Yet then command line
> options may also need tying together, such that people using
> "memop-max-order=" to alter (increase) ptdom_max_order won't need to
> additionally use "max-order-dirty="? At which point maybe the new option
> shouldn't be a standalone one, but be added to "memop-max-order=" (despite
> it being effected in alloc_heap_pages())?

I had concerns about adding it to "memop-max-order=" because it's effect
is not limited to "issued by the various kinds of domain", this is an
option that affects all allocations.  I could try expanding the option
description to reflect that, but I wasn't sure whether it would lead
to confusion (as all options there are per-domain currently).

Also if added to "memop-max-order=" the parsing function needs to be
adjust a bit to consume an extra parameter in the !HAS_PASSTHROUGH
case (which is not much of an issue).

Thanks, Roger.