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btrfs: subpage: make reader lock utilize bitmap 

Currently btrfs_subpage utilizes its atomic member @reader to manage the
reader counter.  However it is only utilized to prevent the page to be
released/unlocked when we still have reads underway.

In that use case, we don't really allow multiple readers on the same
subpage sector.  So here we can introduce a new locked bitmap to
represent exactly which subpage range is locked for read.

In theory we can remove btrfs_subpage::reader as it's just the set bits
of the new locked bitmap.  But unfortunately bitmap doesn't provide such
handy API yet, so we still keep the reader counter.

Signed-off-by: Qu Wenruo <wqu@suse.com>
Reviewed-by: David Sterba <dsterba@suse.com>
Signed-off-by: David Sterba <dsterba@suse.com>
This commit is contained in:
Qu Wenruo 2024-02-17 16:59:49 +10:30 committed by David Sterba
parent 621b9ff18c
commit 8e7e9c672f
2 changed files with 46 additions and 11 deletions
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45
fs/btrfs/subpage.c
View File

@ -111,6 +111,9 @@ void btrfs_init_subpage_info(struct btrfs_subpage_info *subpage_info, u32 sector
subpage_info->checked_offset = cur;
cur += nr_bits;
subpage_info->locked_offset = cur;
cur += nr_bits;
subpage_info->total_nr_bits = cur;
}
@ -237,28 +240,58 @@ static void btrfs_subpage_assert(const struct btrfs_fs_info *fs_info,
start + len <= folio_pos(folio) + PAGE_SIZE);
}
#define subpage_calc_start_bit(fs_info, folio, name, start, len) \
({ \
unsigned int start_bit; \
\
btrfs_subpage_assert(fs_info, folio, start, len); \
start_bit = offset_in_page(start) >> fs_info->sectorsize_bits; \
start_bit += fs_info->subpage_info->name##_offset; \
start_bit; \
})
void btrfs_subpage_start_reader(const struct btrfs_fs_info *fs_info,
struct folio *folio, u64 start, u32 len)
{
struct btrfs_subpage *subpage = folio_get_private(folio);
const int start_bit = subpage_calc_start_bit(fs_info, folio, locked, start, len);
const int nbits = len >> fs_info->sectorsize_bits;
unsigned long flags;
btrfs_subpage_assert(fs_info, folio, start, len);
spin_lock_irqsave(&subpage->lock, flags);
/*
* Even though it's just for reading the page, no one should have
* locked the subpage range.
*/
ASSERT(bitmap_test_range_all_zero(subpage->bitmaps, start_bit, nbits));
bitmap_set(subpage->bitmaps, start_bit, nbits);
atomic_add(nbits, &subpage->readers);
spin_unlock_irqrestore(&subpage->lock, flags);
}
void btrfs_subpage_end_reader(const struct btrfs_fs_info *fs_info,
struct folio *folio, u64 start, u32 len)
{
struct btrfs_subpage *subpage = folio_get_private(folio);
const int start_bit = subpage_calc_start_bit(fs_info, folio, locked, start, len);
const int nbits = len >> fs_info->sectorsize_bits;
unsigned long flags;
bool is_data;
bool last;
btrfs_subpage_assert(fs_info, folio, start, len);
is_data = is_data_inode(folio->mapping->host);
spin_lock_irqsave(&subpage->lock, flags);
/* The range should have already been locked. */
ASSERT(bitmap_test_range_all_set(subpage->bitmaps, start_bit, nbits));
ASSERT(atomic_read(&subpage->readers) >= nbits);
bitmap_clear(subpage->bitmaps, start_bit, nbits);
last = atomic_sub_and_test(nbits, &subpage->readers);
/*
@ -270,6 +303,7 @@ void btrfs_subpage_end_reader(const struct btrfs_fs_info *fs_info,
*/
if (is_data && last)
folio_unlock(folio);
spin_unlock_irqrestore(&subpage->lock, flags);
}
static void btrfs_subpage_clamp_range(struct folio *folio, u64 *start, u32 *len)
@ -365,16 +399,6 @@ void btrfs_folio_end_writer_lock(const struct btrfs_fs_info *fs_info,
folio_unlock(folio);
}
#define subpage_calc_start_bit(fs_info, folio, name, start, len) \
({ \
unsigned int start_bit; \
\
btrfs_subpage_assert(fs_info, folio, start, len); \
start_bit = offset_in_page(start) >> fs_info->sectorsize_bits; \
start_bit += fs_info->subpage_info->name##_offset; \
start_bit; \
})
#define subpage_test_bitmap_all_set(fs_info, subpage, name) \
bitmap_test_range_all_set(subpage->bitmaps, \
fs_info->subpage_info->name##_offset, \
@ -751,6 +775,7 @@ void __cold btrfs_subpage_dump_bitmap(const struct btrfs_fs_info *fs_info,
GET_SUBPAGE_BITMAP(subpage, subpage_info, writeback, &writeback_bitmap);
GET_SUBPAGE_BITMAP(subpage, subpage_info, ordered, &ordered_bitmap);
GET_SUBPAGE_BITMAP(subpage, subpage_info, checked, &checked_bitmap);
GET_SUBPAGE_BITMAP(subpage, subpage_info, locked, &checked_bitmap);
spin_unlock_irqrestore(&subpage->lock, flags);
dump_page(folio_page(folio, 0), "btrfs subpage dump");

12
fs/btrfs/subpage.h
View File

@ -33,7 +33,7 @@ struct btrfs_subpage_info {
unsigned int total_nr_bits;
/*
* *_start indicates where the bitmap starts, the length is always
* *_offset indicates where the bitmap starts, the length is always
* @bitmap_size, which is calculated from PAGE_SIZE / sectorsize.
*/
unsigned int uptodate_offset;
@ -41,6 +41,16 @@ struct btrfs_subpage_info {
unsigned int writeback_offset;
unsigned int ordered_offset;
unsigned int checked_offset;
/*
* For locked bitmaps, normally it's subpage representation for folio
* Locked flag, but metadata is different:
*
* - Metadata doesn't really lock the folio
* It's just to prevent page::private get cleared before the last
* end_page_read().
*/
unsigned int locked_offset;
};
/*