Messages in this thread | | | Subject | Re: [PATCH v2 2/4] mm/migration: remove unneeded lock page and PageMovable check | From | Miaohe Lin <> | Date | Mon, 9 May 2022 16:51:41 +0800 |
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On 2022/4/29 18:07, David Hildenbrand wrote: > On 25.04.22 15:27, Miaohe Lin wrote: >> When non-lru movable page was freed from under us, __ClearPageMovable must >> have been done. Even if it's not done, ClearPageIsolated here won't hurt >> as page will be freed anyway. So we can thus remove unneeded lock page and >> PageMovable check here. >> >> Signed-off-by: Miaohe Lin <linmiaohe@huawei.com> >> Reviewed-by: Christoph Hellwig <hch@lst.de> >> --- >> mm/migrate.c | 8 ++------ >> 1 file changed, 2 insertions(+), 6 deletions(-) >> >> diff --git a/mm/migrate.c b/mm/migrate.c >> index b779646665fe..0fc4651b3e39 100644 >> --- a/mm/migrate.c >> +++ b/mm/migrate.c >> @@ -1093,12 +1093,8 @@ static int unmap_and_move(new_page_t get_new_page, >> /* page was freed from under us. So we are done. */ >> ClearPageActive(page); >> ClearPageUnevictable(page); >> - if (unlikely(__PageMovable(page))) { >> - lock_page(page); >> - if (!PageMovable(page)) >> - ClearPageIsolated(page); >> - unlock_page(page); >> - } >> + if (unlikely(__PageMovable(page))) >> + ClearPageIsolated(page); >> goto out; >> } > > Hm, that code+change raises a couple of questions. > > We're doing here the same as in putback_movable_pages(). So I guess the > difference here is that the caller did release the reference while the > page was isolated, while we don't assume the same in > putback_movable_pages().
Agree.
> > > Shouldn't whoever owned the page have cleared that? IOW, is it even > valid that we see a movable or isolated page here (WARN/BUG?)? > > At least for balloon compaction, I remember that __PageMovable() is > properly cleared before freeing it via balloon_page_delete().
z3fold, zsmalloc will do __ClearPageMovable when the page is going to be released. So I think we shouldn't see a movable page here:
void __ClearPageMovable(struct page *page) { VM_BUG_ON_PAGE(!PageMovable(page), page); /* * Clear registered address_space val with keeping PAGE_MAPPING_MOVABLE * flag so that VM can catch up released page by driver after isolation. * With it, VM migration doesn't try to put it back. */ page->mapping = (void *)((unsigned long)page->mapping & PAGE_MAPPING_MOVABLE); }
But it seems there is no guarantee for PageIsolated flag. Or am I miss something?
> > > Also, I am not sure how reliable that page count check is here: if we'd > have another speculative reference to the page, we might see > "page_count(page) > 1" and not take that path, although the previous > owner released the last reference.
IIUC, there should not be such speculative reference. The driver should have taken care of it.
Thanks!
> >
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