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Date: Thu, 29 Dec 2022 16:50:34 -0800
From: Ira Weiny <ira.weiny@...el.com>
To: "Fabio M. De Francesco" <fmdefrancesco@...il.com>
CC: Evgeniy Dushistov <dushistov@...l.ru>,
Al Viro <viro@...iv.linux.org.uk>,
<linux-kernel@...r.kernel.org>, <bpf@...r.kernel.org>,
<linux-fsdevel@...r.kernel.org>
Subject: Re: [PATCH v5 4/4] fs/ufs: Replace kmap() with kmap_local_page()
On Thu, Dec 29, 2022 at 11:51:00PM +0100, Fabio M. De Francesco wrote:
> kmap() is being deprecated in favor of kmap_local_page().
>
> There are two main problems with kmap(): (1) It comes with an overhead as
> the 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.
>
> The use of kmap_local_page() in fs/ufs is "safe" because (1) the kernel
> virtual addresses are exclusively re-used by the thread which
> established the mappings (i.e., thread locality is never violated) and (2)
> the nestings of mappings and un-mappings are always stack based (LIFO).
>
> Therefore, replace kmap() with kmap_local_page() in fs/ufs. kunmap_local()
> requires the mapping address, so return that address from ufs_get_page()
> and use it as parameter for the second argument of ufs_put_page().
>
> Suggested-by: Al Viro <viro@...iv.linux.org.uk>
> Suggested-by: Ira Weiny <ira.weiny@...el.com>
Reviewed-by: Ira Weiny <ira.weiny@...el.com>
> Signed-off-by: Fabio M. De Francesco <fmdefrancesco@...il.com>
> ---
> fs/ufs/dir.c | 72 +++++++++++++++++++++++++++++++++-----------------
> fs/ufs/namei.c | 8 +++---
> fs/ufs/ufs.h | 2 +-
> 3 files changed, 53 insertions(+), 29 deletions(-)
>
> diff --git a/fs/ufs/dir.c b/fs/ufs/dir.c
> index 0bfd563ab0c2..8676a144e589 100644
> --- a/fs/ufs/dir.c
> +++ b/fs/ufs/dir.c
> @@ -61,9 +61,9 @@ static int ufs_commit_chunk(struct page *page, loff_t pos, unsigned len)
> return err;
> }
>
> -inline void ufs_put_page(struct page *page)
> +inline void ufs_put_page(struct page *page, void *page_addr)
> {
> - kunmap(page);
> + kunmap_local(page_addr);
> put_page(page);
> }
>
> @@ -76,7 +76,7 @@ ino_t ufs_inode_by_name(struct inode *dir, const struct qstr *qstr)
> de = ufs_find_entry(dir, qstr, &page);
> if (de) {
> res = fs32_to_cpu(dir->i_sb, de->d_ino);
> - ufs_put_page(page);
> + ufs_put_page(page, de);
> }
> return res;
> }
> @@ -99,18 +99,17 @@ void ufs_set_link(struct inode *dir, struct ufs_dir_entry *de,
> ufs_set_de_type(dir->i_sb, de, inode->i_mode);
>
> err = ufs_commit_chunk(page, pos, len);
> - ufs_put_page(page);
> + ufs_put_page(page, de);
> if (update_times)
> dir->i_mtime = dir->i_ctime = current_time(dir);
> mark_inode_dirty(dir);
> }
>
>
> -static bool ufs_check_page(struct page *page)
> +static bool ufs_check_page(struct page *page, char *kaddr)
> {
> struct inode *dir = page->mapping->host;
> struct super_block *sb = dir->i_sb;
> - char *kaddr = page_address(page);
> unsigned offs, rec_len;
> unsigned limit = PAGE_SIZE;
> const unsigned chunk_mask = UFS_SB(sb)->s_uspi->s_dirblksize - 1;
> @@ -185,23 +184,32 @@ static bool ufs_check_page(struct page *page)
> return false;
> }
>
> +/*
> + * Calls to ufs_get_page()/ufs_put_page() must be nested according to the
> + * rules documented in kmap_local_page()/kunmap_local().
> + *
> + * NOTE: ufs_find_entry() and ufs_dotdot() act as calls to ufs_get_page()
> + * and must be treated accordingly for nesting purposes.
> + */
> static void *ufs_get_page(struct inode *dir, unsigned long n, struct page **p)
> {
> + char *kaddr;
> +
> struct address_space *mapping = dir->i_mapping;
> struct page *page = read_mapping_page(mapping, n, NULL);
> if (!IS_ERR(page)) {
> - kmap(page);
> + kaddr = kmap_local_page(page);
> if (unlikely(!PageChecked(page))) {
> - if (!ufs_check_page(page))
> + if (!ufs_check_page(page, kaddr))
> goto fail;
> }
> *p = page;
> - return page_address(page);
> + return kaddr;
> }
> return ERR_CAST(page);
>
> fail:
> - ufs_put_page(page);
> + ufs_put_page(page, kaddr);
> return ERR_PTR(-EIO);
> }
>
> @@ -227,6 +235,13 @@ ufs_next_entry(struct super_block *sb, struct ufs_dir_entry *p)
> fs16_to_cpu(sb, p->d_reclen));
> }
>
> +/*
> + * Calls to ufs_get_page()/ufs_put_page() must be nested according to the
> + * rules documented in kmap_local_page()/kunmap_local().
> + *
> + * ufs_dotdot() acts as a call to ufs_get_page() and must be treated
> + * accordingly for nesting purposes.
> + */
> struct ufs_dir_entry *ufs_dotdot(struct inode *dir, struct page **p)
> {
> struct ufs_dir_entry *de = ufs_get_page(dir, 0, p);
> @@ -244,6 +259,11 @@ struct ufs_dir_entry *ufs_dotdot(struct inode *dir, struct page **p)
> * returns the page in which the entry was found, and the entry itself
> * (as a parameter - res_dir). Page is returned mapped and unlocked.
> * Entry is guaranteed to be valid.
> + *
> + * On Success ufs_put_page() should be called on *res_page.
> + *
> + * ufs_find_entry() acts as a call to ufs_get_page() and must be treated
> + * accordingly for nesting purposes.
> */
> struct ufs_dir_entry *ufs_find_entry(struct inode *dir, const struct qstr *qstr,
> struct page **res_page)
> @@ -282,7 +302,7 @@ struct ufs_dir_entry *ufs_find_entry(struct inode *dir, const struct qstr *qstr,
> goto found;
> de = ufs_next_entry(sb, de);
> }
> - ufs_put_page(page);
> + ufs_put_page(page, kaddr);
> }
> if (++n >= npages)
> n = 0;
> @@ -360,7 +380,7 @@ int ufs_add_link(struct dentry *dentry, struct inode *inode)
> de = (struct ufs_dir_entry *) ((char *) de + rec_len);
> }
> unlock_page(page);
> - ufs_put_page(page);
> + ufs_put_page(page, kaddr);
> }
> BUG();
> return -EINVAL;
> @@ -390,7 +410,7 @@ int ufs_add_link(struct dentry *dentry, struct inode *inode)
> mark_inode_dirty(dir);
> /* OFFSET_CACHE */
> out_put:
> - ufs_put_page(page);
> + ufs_put_page(page, kaddr);
> return err;
> out_unlock:
> unlock_page(page);
> @@ -468,13 +488,13 @@ ufs_readdir(struct file *file, struct dir_context *ctx)
> ufs_get_de_namlen(sb, de),
> fs32_to_cpu(sb, de->d_ino),
> d_type)) {
> - ufs_put_page(page);
> + ufs_put_page(page, kaddr);
> return 0;
> }
> }
> ctx->pos += fs16_to_cpu(sb, de->d_reclen);
> }
> - ufs_put_page(page);
> + ufs_put_page(page, kaddr);
> }
> return 0;
> }
> @@ -485,10 +505,15 @@ ufs_readdir(struct file *file, struct dir_context *ctx)
> * previous entry.
> */
> int ufs_delete_entry(struct inode *inode, struct ufs_dir_entry *dir,
> - struct page * page)
> + struct page *page)
> {
> struct super_block *sb = inode->i_sb;
> - char *kaddr = page_address(page);
> + /*
> + * The "dir" dentry points somewhere in the same page whose we need the
> + * address of; therefore, we can simply get the base address "kaddr" by
> + * masking the previous with PAGE_MASK.
> + */
> + char *kaddr = (char *)((unsigned long)dir & PAGE_MASK);
> unsigned int from = offset_in_page(dir) & ~(UFS_SB(sb)->s_uspi->s_dirblksize - 1);
> unsigned int to = offset_in_page(dir) + fs16_to_cpu(sb, dir->d_reclen);
> loff_t pos;
> @@ -527,7 +552,7 @@ int ufs_delete_entry(struct inode *inode, struct ufs_dir_entry *dir,
> inode->i_ctime = inode->i_mtime = current_time(inode);
> mark_inode_dirty(inode);
> out:
> - ufs_put_page(page);
> + ufs_put_page(page, kaddr);
> UFSD("EXIT\n");
> return err;
> }
> @@ -551,8 +576,7 @@ int ufs_make_empty(struct inode * inode, struct inode *dir)
> goto fail;
> }
>
> - kmap(page);
> - base = (char*)page_address(page);
> + base = kmap_local_page(page);
> memset(base, 0, PAGE_SIZE);
>
> de = (struct ufs_dir_entry *) base;
> @@ -569,7 +593,7 @@ int ufs_make_empty(struct inode * inode, struct inode *dir)
> de->d_reclen = cpu_to_fs16(sb, chunk_size - UFS_DIR_REC_LEN(1));
> ufs_set_de_namlen(sb, de, 2);
> strcpy (de->d_name, "..");
> - kunmap(page);
> + kunmap_local(base);
>
> err = ufs_commit_chunk(page, 0, chunk_size);
> fail:
> @@ -585,9 +609,9 @@ int ufs_empty_dir(struct inode * inode)
> struct super_block *sb = inode->i_sb;
> struct page *page = NULL;
> unsigned long i, npages = dir_pages(inode);
> + char *kaddr;
>
> for (i = 0; i < npages; i++) {
> - char *kaddr;
> struct ufs_dir_entry *de;
>
> kaddr = ufs_get_page(inode, i, &page);
> @@ -620,12 +644,12 @@ int ufs_empty_dir(struct inode * inode)
> }
> de = ufs_next_entry(sb, de);
> }
> - ufs_put_page(page);
> + ufs_put_page(page, kaddr);
> }
> return 1;
>
> not_empty:
> - ufs_put_page(page);
> + ufs_put_page(page, kaddr);
> return 0;
> }
>
> diff --git a/fs/ufs/namei.c b/fs/ufs/namei.c
> index 486b0f2e8b7a..7175d45e704c 100644
> --- a/fs/ufs/namei.c
> +++ b/fs/ufs/namei.c
> @@ -250,7 +250,7 @@ static int ufs_rename(struct user_namespace *mnt_userns, struct inode *old_dir,
> struct inode *old_inode = d_inode(old_dentry);
> struct inode *new_inode = d_inode(new_dentry);
> struct page *dir_page = NULL;
> - struct ufs_dir_entry * dir_de = NULL;
> + struct ufs_dir_entry *dir_de = NULL;
> struct page *old_page;
> struct ufs_dir_entry *old_de;
> int err = -ENOENT;
> @@ -307,7 +307,7 @@ static int ufs_rename(struct user_namespace *mnt_userns, struct inode *old_dir,
> if (old_dir != new_dir)
> ufs_set_link(old_inode, dir_de, dir_page, new_dir, 0);
> else {
> - ufs_put_page(dir_page);
> + ufs_put_page(dir_page, dir_de);
> }
> inode_dec_link_count(old_dir);
> }
> @@ -316,10 +316,10 @@ static int ufs_rename(struct user_namespace *mnt_userns, struct inode *old_dir,
>
> out_dir:
> if (dir_de) {
> - ufs_put_page(dir_page);
> + ufs_put_page(dir_page, dir_de);
> }
> out_old:
> - ufs_put_page(old_page);
> + ufs_put_page(old_page, old_de);
> out:
> return err;
> }
> diff --git a/fs/ufs/ufs.h b/fs/ufs/ufs.h
> index f7ba8df25d03..942639e9a817 100644
> --- a/fs/ufs/ufs.h
> +++ b/fs/ufs/ufs.h
> @@ -98,7 +98,7 @@ extern struct ufs_cg_private_info * ufs_load_cylinder (struct super_block *, uns
> extern void ufs_put_cylinder (struct super_block *, unsigned);
>
> /* dir.c */
> -extern void ufs_put_page(struct page *page);
> +extern void ufs_put_page(struct page *page, void *vaddr);
> extern const struct inode_operations ufs_dir_inode_operations;
> extern int ufs_add_link (struct dentry *, struct inode *);
> extern ino_t ufs_inode_by_name(struct inode *, const struct qstr *);
> --
> 2.39.0
>
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