On 2025/10/30 05:14, Nico Pache wrote:
On Wed, Oct 29, 2025 at 12:56 PM Lorenzo Stoakes
<[email protected]> wrote:
On Wed, Oct 29, 2025 at 10:09:43AM +0800, Baolin Wang wrote:
I finally finished reading through the discussions across multiple
threads:), and it looks like we've reached a preliminary consensus (make
0/511 work). Great and thanks!
Yes we're getting there :) it's a sincere effort to try to find a way to move
forwards.
IIUC, the strategy is, configuring it to 511 means always enabling mTHP
collapse, configuring it to 0 means collapsing mTHP only if all PTEs are
non-none/zero, and for other values, we issue a warning and prohibit mTHP
collapse (avoid Lorenzo's concern about silently changing max_ptes_none).
Then the implementation for collapse_max_ptes_none() should be as follows:
static int collapse_max_ptes_none(unsigned int order, bool full_scan)
{
/* ignore max_ptes_none limits */
if (full_scan)
return HPAGE_PMD_NR - 1;
if (order == HPAGE_PMD_ORDER)
return khugepaged_max_ptes_none;
/*
* To prevent creeping towards larger order collapses for mTHP
collapse,
* we restrict khugepaged_max_ptes_none to only 511 or 0,
simplifying the
* logic. This means:
* max_ptes_none == 511 -> collapse mTHP always
* max_ptes_none == 0 -> collapse mTHP only if we all PTEs are
non-none/zero
*/
if (!khugepaged_max_ptes_none || khugepaged_max_ptes_none ==
HPAGE_PMD_NR - 1)
return khugepaged_max_ptes_none >> (HPAGE_PMD_ORDER -
order);
pr_warn_once("mTHP collapse only supports khugepaged_max_ptes_none
configured as 0 or %d\n", HPAGE_PMD_NR - 1);
return -EINVAL;
}
So what do you think?
Yeah I think something like this.
Though I'd implement it more explicitly like:
/* Zero/non-present collapse disabled. */
if (!khugepaged_max_ptes_none)
return 0;
/* Collapse the maximum number of zero/non-present PTEs. */
if (khugepaged_max_ptes_none == HPAGE_PMD_NR - 1)
return (1 << order) - 1;
Then we can do away with this confusing (HPAGE_PMD_ORDER - order) stuff.
This looks cleaner/more explicit given the limits we are enforcing!
I'll go for something like that.
A quick check in google sheets suggests my maths is ok here but do correct me if
I'm wrong :)
LGTM!
LGTM. Thanks.
