linux-kernel - Re: [PATCH mm-unstable v1 1/4] mm/swap: stop using page->private on tail pages for THP

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Date:   Wed, 23 Aug 2023 17:26:41 +0200
From:   David Hildenbrand <david@...hat.com>
To:     Yosry Ahmed <yosryahmed@...gle.com>
Cc:     linux-kernel@...r.kernel.org, linux-mm@...ck.org,
        linux-arm-kernel@...ts.infradead.org,
        Andrew Morton <akpm@...ux-foundation.org>,
        Matthew Wilcox <willy@...radead.org>,
        Peter Xu <peterx@...hat.com>,
        Catalin Marinas <catalin.marinas@....com>,
        Will Deacon <will@...nel.org>, Hugh Dickins <hughd@...gle.com>,
        Seth Jennings <sjenning@...hat.com>,
        Dan Streetman <ddstreet@...e.org>,
        Vitaly Wool <vitaly.wool@...sulko.com>
Subject: Re: [PATCH mm-unstable v1 1/4] mm/swap: stop using page->private on
 tail pages for THP_SWAP

On 23.08.23 17:21, Yosry Ahmed wrote:
> On Wed, Aug 23, 2023 at 8:17 AM David Hildenbrand <david@...hat.com> wrote:
>>
>> On 23.08.23 17:12, Yosry Ahmed wrote:
>>> On Mon, Aug 21, 2023 at 9:09 AM David Hildenbrand <david@...hat.com> wrote:
>>>>
>>>> Let's stop using page->private on tail pages, making it possible to
>>>> just unconditionally reuse that field in the tail pages of large folios.
>>>>
>>>> The remaining usage of the private field for THP_SWAP is in the THP
>>>> splitting code (mm/huge_memory.c), that we'll handle separately later.
>>>>
>>>> Update the THP_SWAP documentation and sanity checks in mm_types.h and
>>>> __split_huge_page_tail().
>>>>
>>>> Signed-off-by: David Hildenbrand <david@...hat.com>
>>>
>>> The mm part looks good to me (with the added fixup):
>>>
>>> Reviewed-by: Yosry Ahmed <yosryahmed@...gle.com>
>>
>> Thanks!
>>
>>>>    /**
>>>> diff --git a/include/linux/swap.h b/include/linux/swap.h
>>>> index bb5adc604144..84fe0e94f5cd 100644
>>>> --- a/include/linux/swap.h
>>>> +++ b/include/linux/swap.h
>>>> @@ -339,6 +339,15 @@ static inline swp_entry_t folio_swap_entry(struct folio *folio)
>>>>           return entry;
>>>>    }
>>>>
>>>> +static inline swp_entry_t page_swap_entry(struct page *page)
>>>> +{
>>>> +       struct folio *folio = page_folio(page);
>>>> +       swp_entry_t entry = folio_swap_entry(folio);
>>>> +
>>>> +       entry.val += page - &folio->page;
>>>> +       return entry;
>>>> +}
>>>> +
>>>>    static inline void folio_set_swap_entry(struct folio *folio, swp_entry_t entry)
>>>>    {
>>>>           folio->private = (void *)entry.val;
>>>> diff --git a/mm/huge_memory.c b/mm/huge_memory.c
>>>> index cc2f65f8cc62..c04702ae71d2 100644
>>>> --- a/mm/huge_memory.c
>>>> +++ b/mm/huge_memory.c
>>>> @@ -2446,18 +2446,15 @@ static void __split_huge_page_tail(struct page *head, int tail,
>>>>           page_tail->index = head->index + tail;
>>>>
>>>>           /*
>>>> -        * page->private should not be set in tail pages with the exception
>>>> -        * of swap cache pages that store the swp_entry_t in tail pages.
>>>> -        * Fix up and warn once if private is unexpectedly set.
>>>> -        *
>>>> -        * What of 32-bit systems, on which folio->_pincount overlays
>>>> -        * head[1].private?  No problem: THP_SWAP is not enabled on 32-bit, and
>>>> -        * pincount must be 0 for folio_ref_freeze() to have succeeded.
>>>> +        * page->private should not be set in tail pages. Fix up and warn once
>>>> +        * if private is unexpectedly set.
>>>>            */
>>>> -       if (!folio_test_swapcache(page_folio(head))) {
>>>> -               VM_WARN_ON_ONCE_PAGE(page_tail->private != 0, page_tail);
>>>> +       if (unlikely(page_tail->private)) {
>>>> +               VM_WARN_ON_ONCE_PAGE(true, page_tail);
>>>>                   page_tail->private = 0;
>>>>           }
>>>
>>> Could probably save a couple of lines here:
>>>
>>> if (VM_WARN_ON_ONCE_PAGE(page_tail->private != 0, page_tail))
>>>
>>>          page_tail->private = 0;
>>>
>>
>> That would mean that we eventually compile out the runtime check
>>
>> #define VM_WARN_ON_ONCE_PAGE(cond, page)  BUILD_BUG_ON_INVALID(cond)
> 
> I thought the warning would be compiled out but not the check, my bad.

I even remembered that VM_WARN_ON_ONCE and friends could/should not be
used in conditionals.

But we do seem to have two users now:

  $ git grep "if (VM_WARN_ON"
mm/mmap.c:              if (VM_WARN_ON_ONCE_MM(vma->vm_end != vmi_end, mm))
mm/mmap.c:              if (VM_WARN_ON_ONCE_MM(vma->vm_start != vmi_start, mm))

But they only do warning-related action, to dump the stack, the vma, ...

So if the warnings get compiled out, also all the other stuff gets compiled out as well,
which makes sense here.

-- 
Cheers,

David / dhildenb