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Date: Tue, 9 Aug 2022 11:03:49 -0700
From: Viacheslav Dubeyko <slava@...eyko.com>
To: "Fabio M. De Francesco" <fmdefrancesco@...il.com>
Cc: "Matthew Wilcox (Oracle)" <willy@...radead.org>,
Andrew Morton <akpm@...ux-foundation.org>,
Damien Le Moal <damien.lemoal@...nsource.wdc.com>,
Jeff Layton <jlayton@...nel.org>,
Arnd Bergmann <arnd@...db.de>,
Muchun Song <songmuchun@...edance.com>,
Linux FS Devel <linux-fsdevel@...r.kernel.org>,
LKML <linux-kernel@...r.kernel.org>,
Ira Weiny <ira.weiny@...el.com>
Subject: Re: [PATCH 3/3] hfs: Replace kmap() with kmap_local_page() in btree.c
> On Aug 9, 2022, at 8:20 AM, Fabio M. De Francesco <fmdefrancesco@...il.com> wrote:
>
> kmap() is being deprecated in favor of kmap_local_page().
>
> Two main problems with kmap(): (1) It comes with an overhead as mapping
> space is restricted and protected by a global lock for synchronization and
> (2) it also requires global TLB invalidation when the kmap’s pool wraps
> and it might block when the mapping space is fully utilized until a slot
> becomes available.
>
> With kmap_local_page() the mappings are per thread, CPU local, can take
> page faults, and can be called from any context (including interrupts).
> It is faster than kmap() in kernels with HIGHMEM enabled. Furthermore,
> the tasks can be preempted and, when they are scheduled to run again, the
> kernel virtual addresses are restored and still valid.
>
> Since its use in btree.c is safe everywhere, it should be preferred.
>
> Therefore, replace kmap() with kmap_local_page() in btree.c. Where
> possible, use the suited standard helpers (memzero_page(), memcpy_page())
> instead of open coding kmap_local_page() plus memset() or memcpy().
>
> Tested in a QEMU/KVM x86_32 VM, 6GB RAM, booting a kernel with
> HIGHMEM64GB enabled.
>
> Suggested-by: Ira Weiny <ira.weiny@...el.com>
> Signed-off-by: Fabio M. De Francesco <fmdefrancesco@...il.com>
> ---
Looks good.
Reviewed-by: Viacheslav Dubeyko <slava@...eyko.com>
Thanks,
Slava.
> fs/hfs/btree.c | 30 ++++++++++++++++--------------
> 1 file changed, 16 insertions(+), 14 deletions(-)
>
> diff --git a/fs/hfs/btree.c b/fs/hfs/btree.c
> index 56c6782436e9..2fa4b1f8cc7f 100644
> --- a/fs/hfs/btree.c
> +++ b/fs/hfs/btree.c
> @@ -80,7 +80,8 @@ struct hfs_btree *hfs_btree_open(struct super_block *sb, u32 id, btree_keycmp ke
> goto free_inode;
>
> /* Load the header */
> - head = (struct hfs_btree_header_rec *)(kmap(page) + sizeof(struct hfs_bnode_desc));
> + head = (struct hfs_btree_header_rec *)(kmap_local_page(page) +
> + sizeof(struct hfs_bnode_desc));
> tree->root = be32_to_cpu(head->root);
> tree->leaf_count = be32_to_cpu(head->leaf_count);
> tree->leaf_head = be32_to_cpu(head->leaf_head);
> @@ -119,12 +120,12 @@ struct hfs_btree *hfs_btree_open(struct super_block *sb, u32 id, btree_keycmp ke
> tree->node_size_shift = ffs(size) - 1;
> tree->pages_per_bnode = (tree->node_size + PAGE_SIZE - 1) >> PAGE_SHIFT;
>
> - kunmap(page);
> + kunmap_local(head);
> put_page(page);
> return tree;
>
> fail_page:
> - kunmap(page);
> + kunmap_local(head);
> put_page(page);
> free_inode:
> tree->inode->i_mapping->a_ops = &hfs_aops;
> @@ -170,7 +171,8 @@ void hfs_btree_write(struct hfs_btree *tree)
> return;
> /* Load the header */
> page = node->page[0];
> - head = (struct hfs_btree_header_rec *)(kmap(page) + sizeof(struct hfs_bnode_desc));
> + head = (struct hfs_btree_header_rec *)(kmap_local_page(page) +
> + sizeof(struct hfs_bnode_desc));
>
> head->root = cpu_to_be32(tree->root);
> head->leaf_count = cpu_to_be32(tree->leaf_count);
> @@ -181,7 +183,7 @@ void hfs_btree_write(struct hfs_btree *tree)
> head->attributes = cpu_to_be32(tree->attributes);
> head->depth = cpu_to_be16(tree->depth);
>
> - kunmap(page);
> + kunmap_local(head);
> set_page_dirty(page);
> hfs_bnode_put(node);
> }
> @@ -269,7 +271,7 @@ struct hfs_bnode *hfs_bmap_alloc(struct hfs_btree *tree)
>
> off += node->page_offset;
> pagep = node->page + (off >> PAGE_SHIFT);
> - data = kmap(*pagep);
> + data = kmap_local_page(*pagep);
> off &= ~PAGE_MASK;
> idx = 0;
>
> @@ -282,7 +284,7 @@ struct hfs_bnode *hfs_bmap_alloc(struct hfs_btree *tree)
> idx += i;
> data[off] |= m;
> set_page_dirty(*pagep);
> - kunmap(*pagep);
> + kunmap_local(data);
> tree->free_nodes--;
> mark_inode_dirty(tree->inode);
> hfs_bnode_put(node);
> @@ -291,14 +293,14 @@ struct hfs_bnode *hfs_bmap_alloc(struct hfs_btree *tree)
> }
> }
> if (++off >= PAGE_SIZE) {
> - kunmap(*pagep);
> - data = kmap(*++pagep);
> + kunmap_local(data);
> + data = kmap_local_page(*++pagep);
> off = 0;
> }
> idx += 8;
> len--;
> }
> - kunmap(*pagep);
> + kunmap_local(data);
> nidx = node->next;
> if (!nidx) {
> printk(KERN_DEBUG "create new bmap node...\n");
> @@ -314,7 +316,7 @@ struct hfs_bnode *hfs_bmap_alloc(struct hfs_btree *tree)
> off = off16;
> off += node->page_offset;
> pagep = node->page + (off >> PAGE_SHIFT);
> - data = kmap(*pagep);
> + data = kmap_local_page(*pagep);
> off &= ~PAGE_MASK;
> }
> }
> @@ -361,20 +363,20 @@ void hfs_bmap_free(struct hfs_bnode *node)
> }
> off += node->page_offset + nidx / 8;
> page = node->page[off >> PAGE_SHIFT];
> - data = kmap(page);
> + data = kmap_local_page(page);
> off &= ~PAGE_MASK;
> m = 1 << (~nidx & 7);
> byte = data[off];
> if (!(byte & m)) {
> pr_crit("trying to free free bnode %u(%d)\n",
> node->this, node->type);
> - kunmap(page);
> + kunmap_local(data);
> hfs_bnode_put(node);
> return;
> }
> data[off] = byte & ~m;
> set_page_dirty(page);
> - kunmap(page);
> + kunmap_local(data);
> hfs_bnode_put(node);
> tree->free_nodes++;
> mark_inode_dirty(tree->inode);
> --
> 2.37.1
>
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