On Tue, Dec 21, 2021 at 10:40 AM Matthew Wilcox [off-list ref] wrote:

On Tue, Dec 21, 2021 at 10:24:27AM -0800, Yang Shi wrote:

quoted

It seems the THP is split during smaps walk. The reproducer does call
MADV_FREE on partial THP which may split the huge page.

The below fix (untested) should be able to fix it.

Did you read the rest of the thread on this?  If the page is being
migrated, we should still account it ... also, you've changed the

Yes, the being migrated pages may be skipped. We should be able to add
a new flag to smaps_account() to indicate this is a migration entry
then don't elevate the page count.

refcount, so this:

        if (page_count(page) == 1) {
                smaps_page_accumulate(mss, page, size, size << PSS_SHIFT, dirty,
                        locked, true);
                return;
        }

will never trigger.

The get_page_unless_zero() is called after this block.