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Message-ID: <CAPjX3FdJGfHarYMjVyR7qzKowbiauepcOMDJjDRt6Rs-5w0pHA@mail.gmail.com>
Date: Tue, 13 May 2025 10:05:21 +0200
From: Daniel Vacek <neelx@...e.com>
To: Qu Wenruo <quwenruo.btrfs@....com>
Cc: Chris Mason <clm@...com>, Josef Bacik <josef@...icpanda.com>, David Sterba <dsterba@...e.com>,
linux-btrfs@...r.kernel.org, linux-kernel@...r.kernel.org
Subject: Re: [PATCH] btrfs: index buffer_tree using node size
On Tue, 13 May 2025 at 00:44, Qu Wenruo <quwenruo.btrfs@....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@...e.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.
> 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;
>
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