在 2024/7/4 4:08, Andrew Morton 写道:
On Wed, 3 Jul 2024 20:02:33 +0800 yangge1116@xxxxxxx wrote:
From: yangge <yangge1116@xxxxxxx>
If a large number of CMA memory are configured in system (for example, the
CMA memory accounts for 50% of the system memory), starting a virtual
virtual machine with device passthrough, it will
call pin_user_pages_remote(..., FOLL_LONGTERM, ...) to pin memory.
Normally if a page is present and in CMA area, pin_user_pages_remote()
will migrate the page from CMA area to non-CMA area because of
FOLL_LONGTERM flag. But the current code will cause the migration failure
due to unexpected page refcounts, and eventually cause the virtual machine
fail to start.
If a page is added in LRU batch, its refcount increases one, remove the
page from LRU batch decreases one. Page migration requires the page is not
referenced by others except page mapping. Before migrating a page, we
should try to drain the page from LRU batch in case the page is in it,
however, folio_test_lru() is not sufficient to tell whether the page is
in LRU batch or not, if the page is in LRU batch, the migration will fail.
To solve the problem above, we modify the logic of adding to LRU batch.
Before adding a page to LRU batch, we clear the LRU flag of the page so
that we can check whether the page is in LRU batch by folio_test_lru(page).
Seems making the LRU flag of the page invisible a long time is no problem,
because a new page is allocated from buddy and added to the lru batch,
its LRU flag is also not visible for a long time.
Thanks.
I'll add this to the mm-hotfixes branch for additional testing. Please
continue to work with David on the changelog enhancements.
In mm-hotfixes I'd expect to send it to Linus next week. I could move
it into mm-unstable (then mm-stable) for merging into 6.11-rc1. This
is for additional testing time - it will still be backported into
earlier kernels. We can do this with any patch.
Ok, thanks.
