Re: [PATCH] btrfs: index buffer_tree using node size

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

在 2025/5/13 17:35, Daniel Vacek 写道:
> On Tue, 13 May 2025 at 00:44, Qu Wenruo <quwenruo.btrfs@gmx.com> wrote:
>>
>>
>>
>> 在 2025/5/13 02:53, Daniel Vacek 写道:
>>> So far we are deriving the buffer tree index using the sector size. But each
>>> extent buffer covers multiple sectors. This makes the buffer tree rather sparse.
>>>
>>> For example the typical and quite common configuration uses sector size of 4KiB
>>> and node size of 16KiB. In this case it means the buffer tree is using up to
>>> the maximum of 25% of it's slots. Or in other words at least 75% of the tree
>>> slots are wasted as never used.
>>>
>>> We can score significant memory savings on the required tree nodes by indexing
>>> the tree using the node size instead. As a result far less slots are wasted
>>> and the tree can now use up to all 100% of it's slots this way.
>>>
>>> Signed-off-by: Daniel Vacek <neelx@suse.com>
>>
>> I'm fine with this change, but I still believe, we should address
>> unaligned tree blocks before doing this.
> 
> Yeah, we discussed it on chat. But giving it another thought I
> realized that the ebs don't overlap each other. Well, I think they do
> not, right?
> 
> That means they are always nodesize apart. So no matter what
> alignment/offset each one always falls into a dedicated tree slot. It
> can never happen that two neighboring ebs would fall into the same
> slot. Hence I think we're safe here with this regard.

Oh, this is indeed a brilliant trick/hack.

Yes, even with unaligned tree blocks, we can still get unique index by 
doing eb->start >> nodesize_shift.

Mind to add this part into the commit message?

In that case, it looks good to me.

Reviewed-by: Qu Wenruo <wqu@suse.com>

Thanks,
Qu

> 
>> As this requires all tree blocks are nodesize aligned.
> 
> Does it? I don't think that's correct. Am I missing something?
> 
>> If we have some metadata chunks which starts at a bytenr that is not
>> node size aligned, all tree blocks inside that chunk will not be
>> nodesize aligned and causing problems.
> 
> As explained above, I don't really see any problems. But then again, I
> may be missing something. Let me know, please.
> 
>> Thanks,
>> Qu
>>
>>> ---
>>>    fs/btrfs/disk-io.c   |  1 +
>>>    fs/btrfs/extent_io.c | 30 +++++++++++++++---------------
>>>    fs/btrfs/fs.h        |  3 ++-
>>>    3 files changed, 18 insertions(+), 16 deletions(-)
>>>
>>> diff --git a/fs/btrfs/disk-io.c b/fs/btrfs/disk-io.c
>>> index 5bcf11246ba66..dcea5b0a2db50 100644
>>> --- a/fs/btrfs/disk-io.c
>>> +++ b/fs/btrfs/disk-io.c
>>> @@ -3395,6 +3395,7 @@ int __cold open_ctree(struct super_block *sb, struct btrfs_fs_devices *fs_device
>>>        fs_info->delalloc_batch = sectorsize * 512 * (1 + ilog2(nr_cpu_ids));
>>>
>>>        fs_info->nodesize = nodesize;
>>> +     fs_info->node_bits = ilog2(nodesize);
>>>        fs_info->sectorsize = sectorsize;
>>>        fs_info->sectorsize_bits = ilog2(sectorsize);
>>>        fs_info->csums_per_leaf = BTRFS_MAX_ITEM_SIZE(fs_info) / fs_info->csum_size;
>>> diff --git a/fs/btrfs/extent_io.c b/fs/btrfs/extent_io.c
>>> index 4d3584790cf7f..80a8563a25add 100644
>>> --- a/fs/btrfs/extent_io.c
>>> +++ b/fs/btrfs/extent_io.c
>>> @@ -1774,7 +1774,7 @@ static noinline_for_stack bool lock_extent_buffer_for_io(struct extent_buffer *e
>>>         */
>>>        spin_lock(&eb->refs_lock);
>>>        if (test_and_clear_bit(EXTENT_BUFFER_DIRTY, &eb->bflags)) {
>>> -             XA_STATE(xas, &fs_info->buffer_tree, eb->start >> fs_info->sectorsize_bits);
>>> +             XA_STATE(xas, &fs_info->buffer_tree, eb->start >> fs_info->node_bits);
>>>                unsigned long flags;
>>>
>>>                set_bit(EXTENT_BUFFER_WRITEBACK, &eb->bflags);
>>> @@ -1874,7 +1874,7 @@ static void set_btree_ioerr(struct extent_buffer *eb)
>>>    static void buffer_tree_set_mark(const struct extent_buffer *eb, xa_mark_t mark)
>>>    {
>>>        struct btrfs_fs_info *fs_info = eb->fs_info;
>>> -     XA_STATE(xas, &fs_info->buffer_tree, eb->start >> fs_info->sectorsize_bits);
>>> +     XA_STATE(xas, &fs_info->buffer_tree, eb->start >> fs_info->node_bits);
>>>        unsigned long flags;
>>>
>>>        xas_lock_irqsave(&xas, flags);
>>> @@ -1886,7 +1886,7 @@ static void buffer_tree_set_mark(const struct extent_buffer *eb, xa_mark_t mark)
>>>    static void buffer_tree_clear_mark(const struct extent_buffer *eb, xa_mark_t mark)
>>>    {
>>>        struct btrfs_fs_info *fs_info = eb->fs_info;
>>> -     XA_STATE(xas, &fs_info->buffer_tree, eb->start >> fs_info->sectorsize_bits);
>>> +     XA_STATE(xas, &fs_info->buffer_tree, eb->start >> fs_info->node_bits);
>>>        unsigned long flags;
>>>
>>>        xas_lock_irqsave(&xas, flags);
>>> @@ -1986,7 +1986,7 @@ static unsigned int buffer_tree_get_ebs_tag(struct btrfs_fs_info *fs_info,
>>>        rcu_read_lock();
>>>        while ((eb = find_get_eb(&xas, end, tag)) != NULL) {
>>>                if (!eb_batch_add(batch, eb)) {
>>> -                     *start = (eb->start + eb->len) >> fs_info->sectorsize_bits;
>>> +                     *start = (eb->start + eb->len) >> fs_info->node_bits;
>>>                        goto out;
>>>                }
>>>        }
>>> @@ -2008,7 +2008,7 @@ static struct extent_buffer *find_extent_buffer_nolock(
>>>                struct btrfs_fs_info *fs_info, u64 start)
>>>    {
>>>        struct extent_buffer *eb;
>>> -     unsigned long index = start >> fs_info->sectorsize_bits;
>>> +     unsigned long index = start >> fs_info->node_bits;
>>>
>>>        rcu_read_lock();
>>>        eb = xa_load(&fs_info->buffer_tree, index);
>>> @@ -2114,8 +2114,8 @@ void btrfs_btree_wait_writeback_range(struct btrfs_fs_info *fs_info, u64 start,
>>>                                      u64 end)
>>>    {
>>>        struct eb_batch batch;
>>> -     unsigned long start_index = start >> fs_info->sectorsize_bits;
>>> -     unsigned long end_index = end >> fs_info->sectorsize_bits;
>>> +     unsigned long start_index = start >> fs_info->node_bits;
>>> +     unsigned long end_index = end >> fs_info->node_bits;
>>>
>>>        eb_batch_init(&batch);
>>>        while (start_index <= end_index) {
>>> @@ -2151,7 +2151,7 @@ int btree_write_cache_pages(struct address_space *mapping,
>>>
>>>        eb_batch_init(&batch);
>>>        if (wbc->range_cyclic) {
>>> -             index = (mapping->writeback_index << PAGE_SHIFT) >> fs_info->sectorsize_bits;
>>> +             index = (mapping->writeback_index << PAGE_SHIFT) >> fs_info->node_bits;
>>>                end = -1;
>>>
>>>                /*
>>> @@ -2160,8 +2160,8 @@ int btree_write_cache_pages(struct address_space *mapping,
>>>                 */
>>>                scanned = (index == 0);
>>>        } else {
>>> -             index = wbc->range_start >> fs_info->sectorsize_bits;
>>> -             end = wbc->range_end >> fs_info->sectorsize_bits;
>>> +             index = wbc->range_start >> fs_info->node_bits;
>>> +             end = wbc->range_end >> fs_info->node_bits;
>>>
>>>                scanned = 1;
>>>        }
>>> @@ -3037,7 +3037,7 @@ struct extent_buffer *alloc_test_extent_buffer(struct btrfs_fs_info *fs_info,
>>>        eb->fs_info = fs_info;
>>>    again:
>>>        xa_lock_irq(&fs_info->buffer_tree);
>>> -     exists = __xa_cmpxchg(&fs_info->buffer_tree, start >> fs_info->sectorsize_bits,
>>> +     exists = __xa_cmpxchg(&fs_info->buffer_tree, start >> fs_info->node_bits,
>>>                              NULL, eb, GFP_NOFS);
>>>        if (xa_is_err(exists)) {
>>>                ret = xa_err(exists);
>>> @@ -3353,7 +3353,7 @@ struct extent_buffer *alloc_extent_buffer(struct btrfs_fs_info *fs_info,
>>>    again:
>>>        xa_lock_irq(&fs_info->buffer_tree);
>>>        existing_eb = __xa_cmpxchg(&fs_info->buffer_tree,
>>> -                                start >> fs_info->sectorsize_bits, NULL, eb,
>>> +                                start >> fs_info->node_bits, NULL, eb,
>>>                                   GFP_NOFS);
>>>        if (xa_is_err(existing_eb)) {
>>>                ret = xa_err(existing_eb);
>>> @@ -3456,7 +3456,7 @@ static int release_extent_buffer(struct extent_buffer *eb)
>>>                 * in this case.
>>>                 */
>>>                xa_cmpxchg_irq(&fs_info->buffer_tree,
>>> -                            eb->start >> fs_info->sectorsize_bits, eb, NULL,
>>> +                            eb->start >> fs_info->node_bits, eb, NULL,
>>>                               GFP_ATOMIC);
>>>
>>>                btrfs_leak_debug_del_eb(eb);
>>> @@ -4294,9 +4294,9 @@ static int try_release_subpage_extent_buffer(struct folio *folio)
>>>    {
>>>        struct btrfs_fs_info *fs_info = folio_to_fs_info(folio);
>>>        struct extent_buffer *eb;
>>> -     unsigned long start = folio_pos(folio) >> fs_info->sectorsize_bits;
>>> +     unsigned long start = folio_pos(folio) >> fs_info->node_bits;
>>>        unsigned long index = start;
>>> -     unsigned long end = index + (PAGE_SIZE >> fs_info->sectorsize_bits) - 1;
>>> +     unsigned long end = index + (PAGE_SIZE >> fs_info->node_bits) - 1;
>>>        int ret;
>>>
>>>        xa_lock_irq(&fs_info->buffer_tree);
>>> diff --git a/fs/btrfs/fs.h b/fs/btrfs/fs.h
>>> index cf805b4032af3..8c9113304fabe 100644
>>> --- a/fs/btrfs/fs.h
>>> +++ b/fs/btrfs/fs.h
>>> @@ -778,8 +778,9 @@ struct btrfs_fs_info {
>>>
>>>        struct btrfs_delayed_root *delayed_root;
>>>
>>> -     /* Entries are eb->start / sectorsize */
>>> +     /* Entries are eb->start >> node_bits */
>>>        struct xarray buffer_tree;
>>> +     int node_bits;
>>>
>>>        /* Next backup root to be overwritten */
>>>        int backup_root_index;
>>
>