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SubjectRe: [PATCH RFC v3] mm: Proper document tail pages fields for folio
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On 17.08.23 00:05, Matthew Wilcox wrote:
> On Wed, Aug 16, 2023 at 08:51:49PM +0200, David Hildenbrand wrote:
>> On 16.08.23 20:41, Matthew Wilcox wrote:
>>> On Wed, Aug 16, 2023 at 03:33:30PM +0200, David Hildenbrand wrote:
>>>> My simple tests passed so far. If there isn't something obvious missing,
>>>> I can do more testing and send this as an official patch.
>>>
>>> I think you missed one:
>>>
>>> +++ b/mm/swapfile.c
>>> @@ -1490,7 +1490,7 @@ int swp_swapcount(swp_entry_t entry)
>>>
>>> page = vmalloc_to_page(p->swap_map + offset);
>>> offset &= ~PAGE_MASK;
>>> - VM_BUG_ON(page_private(page) != SWP_CONTINUED);
>>> + VM_BUG_ON(page_swap_entry(page).val != SWP_CONTINUED);
>>
>> That falls under the "weird handling of SWP_CONTINUED using vmalloced
>> pages". So different user of page_private().
>>
>> Note that we don't even store swap entries in there but extended swap
>> counts.
>
> Ah, right. I see now.
>
>
> Not necessarily as part of this patch, but it got me wondering ...
> should we do this? And then maybe we could remove folio_swap_entry()
> and folio_set_swap_entry() and just use folio->swap directly.
>
>
> diff --git a/include/linux/mm_types.h b/include/linux/mm_types.h
> index 3880b3f2e321..e23d1356e504 100644
> --- a/include/linux/mm_types.h
> +++ b/include/linux/mm_types.h
> @@ -266,6 +266,14 @@ static inline struct page *encoded_page_ptr(struct encoded_page *page)
> return (struct page *)(~ENCODE_PAGE_BITS & (unsigned long)page);
> }
>
> +/*
> + * A swap entry has to fit into a "unsigned long", as the entry is hidden
> + * in the "index" field of the swapper address space.
> + */
> +typedef struct {
> + unsigned long val;
> +} swp_entry_t;
> +
> /**
> * struct folio - Represents a contiguous set of bytes.
> * @flags: Identical to the page flags.
> @@ -276,7 +284,7 @@ static inline struct page *encoded_page_ptr(struct encoded_page *page)
> * @index: Offset within the file, in units of pages. For anonymous memory,
> * this is the index from the beginning of the mmap.
> * @private: Filesystem per-folio data (see folio_attach_private()).
> - * Used for swp_entry_t if folio_test_swapcache().
> + * @swap: Used for swp_entry_t if folio_test_swapcache().
> * @_mapcount: Do not access this member directly. Use folio_mapcount() to
> * find out how many times this folio is mapped by userspace.
> * @_refcount: Do not access this member directly. Use folio_ref_count()
> @@ -319,7 +327,10 @@ struct folio {
> };
> struct address_space *mapping;
> pgoff_t index;
> - void *private;
> + union {
> + void *private;
> + swp_entry_t swap;

Probably with "/* for anon and shm pages only */

> + };
> atomic_t _mapcount;
> atomic_t _refcount;
> #ifdef CONFIG_MEMCG
> @@ -1158,14 +1169,6 @@ enum tlb_flush_reason {
> NR_TLB_FLUSH_REASONS,
> };
>
> - /*
> - * A swap entry has to fit into a "unsigned long", as the entry is hidden
> - * in the "index" field of the swapper address space.
> - */
> -typedef struct {
> - unsigned long val;
> -} swp_entry_t;
> -
> /**
> * enum fault_flag - Fault flag definitions.
> * @FAULT_FLAG_WRITE: Fault was a write fault.
> diff --git a/include/linux/swap.h b/include/linux/swap.h
> index bb5adc604144..59b0f37eae5b 100644
> --- a/include/linux/swap.h
> +++ b/include/linux/swap.h
> @@ -335,13 +335,12 @@ struct swap_info_struct {
>
> static inline swp_entry_t folio_swap_entry(struct folio *folio)
> {
> - swp_entry_t entry = { .val = page_private(&folio->page) };
> - return entry;
> + return folio->swap;
> }
>
> static inline void folio_set_swap_entry(struct folio *folio, swp_entry_t entry)
> {
> - folio->private = (void *)entry.val;
> + folio->swap = entry;
> }
>
> /* linux/mm/workingset.c */
>

Sound reasonable, but maybe we should even get rid of the getter/setter
completely then?

--
Cheers,

David / dhildenb

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