On 22.01.2026 14:05, Roger Pau Monné wrote:
> 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 <[email protected]>
>>>> ---
>>>> 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).
Hmm, fair point. Let's keep it separate then.
Jan
