linux-kernel - Re: [PATCH 5/9] revert "mm/z3fold.c: allow __GFP_HIGHMEM in z3fold

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Date:   Thu, 19 May 2022 19:34:01 +0800
From:   Miaohe Lin <linmiaohe@...wei.com>
To:     Vitaly Wool <vitaly.wool@...sulko.com>
CC:     Andrew Morton <akpm@...ux-foundation.org>,
        Linux-MM <linux-mm@...ck.org>,
        LKML <linux-kernel@...r.kernel.org>
Subject: Re: [PATCH 5/9] revert "mm/z3fold.c: allow __GFP_HIGHMEM in
 z3fold_alloc"

On 2022/5/19 15:12, Vitaly Wool wrote:
> On Fri, Apr 29, 2022 at 8:41 AM Miaohe Lin <linmiaohe@...wei.com> wrote:
>>
>> Revert commit f1549cb5ab2b ("mm/z3fold.c: allow __GFP_HIGHMEM in
>> z3fold_alloc").
>>
>> z3fold can't support GFP_HIGHMEM page now. page_address is used
>> directly at all places. Moreover, z3fold_header is on per cpu
>> unbuddied list which could be access anytime. So we should rid
>> the support of GFP_HIGHMEM allocation for z3fold.
> 
> Could you please clarify how kmem_cache is affected here?

With this code changes, kmem_cache should be unaffected. HIGHMEM is still not supported for
kmem_cache just like before but caller ensures __GFP_HIGHMEM is not passed in now. The issue
I want to fix here is that if z3fold page is allocated from highmem, page_address can't be
used directly. Did I answer your question? Or don't I get your point?

Thanks!

> 
> Thanks,
> Vitaly

Many thanks for your time! :)

> 
>> Signed-off-by: Miaohe Lin <linmiaohe@...wei.com>
>> ---
>>  mm/z3fold.c | 8 +++-----
>>  1 file changed, 3 insertions(+), 5 deletions(-)
>>
>> diff --git a/mm/z3fold.c b/mm/z3fold.c
>> index b3b4e65c107f..5f5d5f1556be 100644
>> --- a/mm/z3fold.c
>> +++ b/mm/z3fold.c
>> @@ -212,10 +212,8 @@ static int size_to_chunks(size_t size)
>>  static inline struct z3fold_buddy_slots *alloc_slots(struct z3fold_pool *pool,
>>                                                         gfp_t gfp)
>>  {
>> -       struct z3fold_buddy_slots *slots;
>> -
>> -       slots = kmem_cache_zalloc(pool->c_handle,
>> -                                (gfp & ~(__GFP_HIGHMEM | __GFP_MOVABLE)));
>> +       struct z3fold_buddy_slots *slots = kmem_cache_zalloc(pool->c_handle,
>> +                                                            gfp);
>>
>>         if (slots) {
>>                 /* It will be freed separately in free_handle(). */
>> @@ -1075,7 +1073,7 @@ static int z3fold_alloc(struct z3fold_pool *pool, size_t size, gfp_t gfp,
>>         enum buddy bud;
>>         bool can_sleep = gfpflags_allow_blocking(gfp);
>>
>> -       if (!size)
>> +       if (!size || (gfp & __GFP_HIGHMEM))
>>                 return -EINVAL;
>>
>>         if (size > PAGE_SIZE)
>> --
>> 2.23.0
>>
> .
>