[PATCH] mm/z3fold.c: Lock z3fold page before __SetPageMovable()

[PATCH] mm/z3fold.c: Lock z3fold page before __SetPageMovable()

[Henry Burns]

__SetPageMovable() expects it's page to be locked, but z3fold.c doesn't
lock the page. Following zsmalloc.c's example we call trylock_page() and
unlock_page(). Also makes z3fold_page_migrate() assert that newpage is
passed in locked, as documentation.

Signed-off-by: Henry Burns <henryburns@google.com>
---
 mm/z3fold.c | 3 +++
 1 file changed, 3 insertions(+)

diff --git a/mm/z3fold.c b/mm/z3fold.c
index e174d1549734..5bc404dbbb4a 100644
--- a/mm/z3fold.c
+++ b/mm/z3fold.c
@@ -918,7 +918,9 @@ static int z3fold_alloc(struct z3fold_pool *pool, size_t size, gfp_t gfp,
 		set_bit(PAGE_HEADLESS, &page->private);
 		goto headless;
 	}
+	WARN_ON(!trylock_page(page));
 	__SetPageMovable(page, pool->inode->i_mapping);
+	unlock_page(page);
 	z3fold_page_lock(zhdr);
 
 found:
@@ -1325,6 +1327,7 @@ static int z3fold_page_migrate(struct address_space *mapping, struct page *newpa
 
 	VM_BUG_ON_PAGE(!PageMovable(page), page);
 	VM_BUG_ON_PAGE(!PageIsolated(page), page);
+	VM_BUG_ON_PAGE(!PageLocked(newpage), newpage);
 
 	zhdr = page_address(page);
 	pool = zhdr_to_pool(zhdr);
-- 
2.22.0.410.gd8fdbe21b5-goog

Re: [PATCH] mm/z3fold.c: Lock z3fold page before __SetPageMovable()

[Andrew Morton]

On Mon,  1 Jul 2019 14:23:03 -0700 Henry Burns <henryburns@google.com> wrote:

> __SetPageMovable() expects it's page to be locked, but z3fold.c doesn't
> lock the page.

So this triggers the VM_BUG_ON_PAGE(!PageLocked(page), page) in
__SetPageMovable(), yes?

> Following zsmalloc.c's example we call trylock_page() and
> unlock_page(). Also makes z3fold_page_migrate() assert that newpage is
> passed in locked, as documentation.
> 
> ...
>
> --- a/mm/z3fold.c
> +++ b/mm/z3fold.c
> @@ -918,7 +918,9 @@ static int z3fold_alloc(struct z3fold_pool *pool, size_t size, gfp_t gfp,
>  		set_bit(PAGE_HEADLESS, &page->private);
>  		goto headless;
>  	}
> +	WARN_ON(!trylock_page(page));

If this warn triggers then someone else has locked the page.

>	__SetPageMovable(page, pool->inode->i_mapping);
> + 	unlock_page(page);

and we proceed to undo their lock.  So that other code path will then
perform an unlock of an unlocked page.  Etcetera.

It would be much much better to do a plain old lock_page() here.  If
that results in a deadlock then let's find out why and fix it without
trylock hacks.