Re: [PATCH 4/9] mm/z3fold: throw warning on failure of trylock_page in z3fold_alloc

[Miaohe Lin <linmiaohe@xxxxxxxxxx>]

On 2022/5/19 15:10, Vitaly Wool wrote:
> On Fri, Apr 29, 2022 at 8:40 AM Miaohe Lin <linmiaohe@xxxxxxxxxx> wrote:
>>
>> If trylock_page fails, the page won't be non-lru movable page. When this
>> page is freed via free_z3fold_page, it will trigger bug on PageMovable
>> check in __ClearPageMovable. Throw warning on failure of trylock_page to
>> guard against such rare case just as what zsmalloc does.
> 
> I don't see how this is better than what we currently have. We can
> check if a page is movable before calling __ClearPageMovable instead.

Currently the z3fold page (unless headless page) is assumed to be non-lru movable.
We will always do __ClearPageMovable in free_z3fold_page which will trigger BUG_ON
!PageMovable now. And do you mean we should do something like below ?

diff --git a/mm/z3fold.c b/mm/z3fold.c
index f41f8b0d9e9a..a244bb5dcb34 100644
--- a/mm/z3fold.c
+++ b/mm/z3fold.c
@@ -417,9 +417,10 @@ static struct z3fold_header *init_z3fold_page(struct page *page, bool headless,
 /* Resets the struct page fields and frees the page */
 static void free_z3fold_page(struct page *page, bool headless)
 {
-       if (!headless) {
+       if (likely(__PageMovable(page))) {
                lock_page(page);
-               __ClearPageMovable(page);
+               if (PageMovable(page))
+                       __ClearPageMovable(page);
                unlock_page(page);
        }
        __free_page(page);

Thanks!

> 
> ~Vitaly
> 
>> Signed-off-by: Miaohe Lin <linmiaohe@xxxxxxxxxx>
>> ---
>>  mm/z3fold.c | 7 +++----
>>  1 file changed, 3 insertions(+), 4 deletions(-)
>>
>> diff --git a/mm/z3fold.c b/mm/z3fold.c
>> index 4e6814c5694f..b3b4e65c107f 100644
>> --- a/mm/z3fold.c
>> +++ b/mm/z3fold.c
>> @@ -1122,10 +1122,9 @@ static int z3fold_alloc(struct z3fold_pool *pool, size_t size, gfp_t gfp,
>>                 __SetPageMovable(page, pool->inode->i_mapping);
>>                 unlock_page(page);
>>         } else {
>> -               if (trylock_page(page)) {
>> -                       __SetPageMovable(page, pool->inode->i_mapping);
>> -                       unlock_page(page);
>> -               }
>> +               WARN_ON(!trylock_page(page));
>> +               __SetPageMovable(page, pool->inode->i_mapping);
>> +               unlock_page(page);
>>         }
>>         z3fold_page_lock(zhdr);
>>
>> --
>> 2.23.0
>>
> .
>