Re: [PATCH v4 08/18] btrfs: introduce helper for subpage uptodate status

[Qu Wenruo <quwenruo.btrfs@gmx.com>]

On 2021/1/21 上午9:28, Josef Bacik wrote:
> On 1/20/21 7:49 PM, Qu Wenruo wrote:
>>
>>
>> On 2021/1/20 下午11:00, Josef Bacik wrote:
>>> On 1/16/21 2:15 AM, Qu Wenruo wrote:
>>>> This patch introduce the following functions to handle btrfs subpage
>>>> uptodate status:
>>>> - btrfs_subpage_set_uptodate()
>>>> - btrfs_subpage_clear_uptodate()
>>>> - btrfs_subpage_test_uptodate()
>>>>    Those helpers can only be called when the range is ensured to be
>>>>    inside the page.
>>>>
>>>> - btrfs_page_set_uptodate()
>>>> - btrfs_page_clear_uptodate()
>>>> - btrfs_page_test_uptodate()
>>>>    Those helpers can handle both regular sector size and subpage 
>>>> without
>>>>    problem.
>>>>    Although caller should still ensure that the range is inside the 
>>>> page.
>>>>
>>>> Signed-off-by: Qu Wenruo <wqu@suse.com>
>>>> ---
>>>>   fs/btrfs/subpage.h | 115 
>>>> +++++++++++++++++++++++++++++++++++++++++++++
>>>>   1 file changed, 115 insertions(+)
>>>>
>>>> diff --git a/fs/btrfs/subpage.h b/fs/btrfs/subpage.h
>>>> index d8b34879368d..3373ef4ffec1 100644
>>>> --- a/fs/btrfs/subpage.h
>>>> +++ b/fs/btrfs/subpage.h
>>>> @@ -23,6 +23,7 @@
>>>>   struct btrfs_subpage {
>>>>       /* Common members for both data and metadata pages */
>>>>       spinlock_t lock;
>>>> +    u16 uptodate_bitmap;
>>>>       union {
>>>>           /* Structures only used by metadata */
>>>>           bool under_alloc;
>>>> @@ -78,4 +79,118 @@ static inline void 
>>>> btrfs_page_end_meta_alloc(struct btrfs_fs_info *fs_info,
>>>>   int btrfs_attach_subpage(struct btrfs_fs_info *fs_info, struct 
>>>> page *page);
>>>>   void btrfs_detach_subpage(struct btrfs_fs_info *fs_info, struct 
>>>> page *page);
>>>> +/*
>>>> + * Convert the [start, start + len) range into a u16 bitmap
>>>> + *
>>>> + * E.g. if start == page_offset() + 16K, len = 16K, we get 0x00f0.
>>>> + */
>>>> +static inline u16 btrfs_subpage_calc_bitmap(struct btrfs_fs_info 
>>>> *fs_info,
>>>> +            struct page *page, u64 start, u32 len)
>>>> +{
>>>> +    int bit_start = offset_in_page(start) >> fs_info->sectorsize_bits;
>>>> +    int nbits = len >> fs_info->sectorsize_bits;
>>>> +
>>>> +    /* Basic checks */
>>>> +    ASSERT(PagePrivate(page) && page->private);
>>>> +    ASSERT(IS_ALIGNED(start, fs_info->sectorsize) &&
>>>> +           IS_ALIGNED(len, fs_info->sectorsize));
>>>> +
>>>> +    /*
>>>> +     * The range check only works for mapped page, we can
>>>> +     * still have unampped page like dummy extent buffer pages.
>>>> +     */
>>>> +    if (page->mapping)
>>>> +        ASSERT(page_offset(page) <= start &&
>>>> +            start + len <= page_offset(page) + PAGE_SIZE);
>>>> +    /*
>>>> +     * Here nbits can be 16, thus can go beyond u16 range. Here we 
>>>> make the
>>>> +     * first left shift to be calculated in unsigned long (u32), then
>>>> +     * truncate the result to u16.
>>>> +     */
>>>> +    return (u16)(((1UL << nbits) - 1) << bit_start);
>>>> +}
>>>> +
>>>> +static inline void btrfs_subpage_set_uptodate(struct btrfs_fs_info 
>>>> *fs_info,
>>>> +            struct page *page, u64 start, u32 len)
>>>> +{
>>>> +    struct btrfs_subpage *subpage = (struct btrfs_subpage 
>>>> *)page->private;
>>>> +    u16 tmp = btrfs_subpage_calc_bitmap(fs_info, page, start, len);
>>>> +    unsigned long flags;
>>>> +
>>>> +    spin_lock_irqsave(&subpage->lock, flags);
>>>> +    subpage->uptodate_bitmap |= tmp;
>>>> +    if (subpage->uptodate_bitmap == U16_MAX)
>>>> +        SetPageUptodate(page);
>>>> +    spin_unlock_irqrestore(&subpage->lock, flags);
>>>> +}
>>>> +
>>>> +static inline void btrfs_subpage_clear_uptodate(struct 
>>>> btrfs_fs_info *fs_info,
>>>> +            struct page *page, u64 start, u32 len)
>>>> +{
>>>> +    struct btrfs_subpage *subpage = (struct btrfs_subpage 
>>>> *)page->private;
>>>> +    u16 tmp = btrfs_subpage_calc_bitmap(fs_info, page, start, len);
>>>> +    unsigned long flags;
>>>> +
>>>> +    spin_lock_irqsave(&subpage->lock, flags);
>>>> +    subpage->uptodate_bitmap &= ~tmp;
>>>> +    ClearPageUptodate(page);
>>>> +    spin_unlock_irqrestore(&subpage->lock, flags);
>>>> +}
>>>> +
>>>> +/*
>>>> + * Unlike set/clear which is dependent on each page status, for 
>>>> test all bits
>>>> + * are tested in the same way.
>>>> + */
>>>> +#define DECLARE_BTRFS_SUBPAGE_TEST_OP(name)                \
>>>> +static inline bool btrfs_subpage_test_##name(struct btrfs_fs_info 
>>>> *fs_info, \
>>>> +            struct page *page, u64 start, u32 len)        \
>>>> +{                                    \
>>>> +    struct btrfs_subpage *subpage = (struct btrfs_subpage 
>>>> *)page->private; \
>>>> +    u16 tmp = btrfs_subpage_calc_bitmap(fs_info, page, start, len); \
>>>> +    unsigned long flags;                        \
>>>> +    bool ret;                            \
>>>> +                                    \
>>>> +    spin_lock_irqsave(&subpage->lock, flags);            \
>>>> +    ret = ((subpage->name##_bitmap & tmp) == tmp);            \
>>>> +    spin_unlock_irqrestore(&subpage->lock, flags);            \
>>>> +    return ret;                            \
>>>> +}
>>>> +DECLARE_BTRFS_SUBPAGE_TEST_OP(uptodate);
>>>> +
>>>> +/*
>>>> + * Note that, in selftest, especially extent-io-tests, we can have 
>>>> empty
>>>> + * fs_info passed in.
>>>> + * Thankfully in selftest, we only test sectorsize == PAGE_SIZE 
>>>> cases so far,
>>>> + * thus we can fall back to regular sectorsize branch.
>>>> + */
>>>> +#define DECLARE_BTRFS_PAGE_OPS(name, set_page_func, 
>>>> clear_page_func,    \
>>>> +                   test_page_func)                \
>>>> +static inline void btrfs_page_set_##name(struct btrfs_fs_info 
>>>> *fs_info,    \
>>>> +            struct page *page, u64 start, u32 len)        \
>>>> +{                                    \
>>>> +    if (unlikely(!fs_info) || fs_info->sectorsize == PAGE_SIZE) {    \
>>>> +        set_page_func(page);                    \
>>>> +        return;                            \
>>>> +    }                                \
>>>> +    btrfs_subpage_set_##name(fs_info, page, start, len);        \
>>>> +}                                    \
>>>> +static inline void btrfs_page_clear_##name(struct btrfs_fs_info 
>>>> *fs_info, \
>>>> +            struct page *page, u64 start, u32 len)        \
>>>> +{                                    \
>>>> +    if (unlikely(!fs_info) || fs_info->sectorsize == PAGE_SIZE) {    \
>>>> +        clear_page_func(page);                    \
>>>> +        return;                            \
>>>> +    }                                \
>>>> +    btrfs_subpage_clear_##name(fs_info, page, start, len);        \
>>>> +}                                    \
>>>> +static inline bool btrfs_page_test_##name(struct btrfs_fs_info 
>>>> *fs_info, \
>>>> +            struct page *page, u64 start, u32 len)        \
>>>> +{                                    \
>>>> +    if (unlikely(!fs_info) || fs_info->sectorsize == PAGE_SIZE)    \
>>>> +        return test_page_func(page);                \
>>>> +    return btrfs_subpage_test_##name(fs_info, page, start, len);    \
>>>> +}
>>>
>>> Another thing I just realized is you're doing this
>>>
>>> btrfs_page_set_uptodate(fs_info, page, eb->start, eb->len);
>>>
>>> but we default to a nodesize > PAGE_SIZE on x86.  This is fine, 
>>> because you're checking fs_info->sectorsize == PAGE_SIZE, which will 
>>> mean we do the right thing.
>>>
>>> But what happens if fs_info->nodesize < PAGE_SIZE && 
>>> fs_info->sectorsize == PAGE_SIZE?  We by default have fs'es that 
>>> ->nodesize != ->sectorsize, so really what we should be doing is 
>>> checking if len == PAGE_SIZE here, but then you need to take into 
>>> account the case that eb->len > PAGE_SIZE.  Fix this to do the right 
>>> thing in either of those cases. Thanks,
>>
>> Impossible.
>>
>> Nodesize must be >= sectorsize.
>>
>> As sectorsize is currently the minimal access unit for both data and 
>> metadata.
>>
> 
> Ok the consider the alternative, we have PAGE_SIZE == 64k, nodesize == 
> 64k and sectorsize == 4k, something that's actually allowed.  You're now 
> doing the subpage operations on something that won't/shouldn't have the 
> subpage private attached to the page.  We need to key off of the right 
> thing, so for metadata we need to check ->nodesize, and data we check 
> ->sectorsize, and for these accessors you can simply do len >= 
> PAGE_SIZE.  Thanks.

For nodesize == 64K and sectorsize == 4K, subpage way of handling it 
still works.

eb->len == 64K now, btrfs_page_set_uptodate() for subpage will just mark 
the full page uptodate.

I tend not to make the nodesize == 64K a special case, or the check 
condition will be a mess.

Now we unify the check to just sectorsize == PAGE_SIZE, nothing else to 
bother.

Thanks,
Qu

> 
> Josef