Re: [PATCH] fs/sysv: Replace kmap() with kmap_local_page()

From: Ira Weiny
Date: Thu Sep 22 2022 - 16:31:54 EST

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On Fri, Aug 26, 2022 at 07:50:41PM +0200, 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.
>
> Since its use in fs/sysv is safe everywhere, it should be preferred.
>
> Therefore, replace kmap() with kmap_local_page() in fs/sysv. kunmap_local()
> requires the mapping address, so return that address from dir_get_page()
> to be used in dir_put_page().
>
> Cc: "Venkataramanan, Anirudh" <anirudh.venkataramanan@xxxxxxxxx>
> Suggested-by: Ira Weiny <ira.weiny@xxxxxxxxx>

Reviewed-by: Ira Weiny <ira.weiny@xxxxxxxxx>

> Signed-off-by: Fabio M. De Francesco <fmdefrancesco@xxxxxxxxx>
> ---
>
> This code is not tested. I have no means to create an SysV filesystem.
> Despite nothing here seems to break the strict rules about the use of
> kmap_local_page(), any help with testing will be much appreciated :-)
>
> fs/sysv/dir.c | 83 ++++++++++++++++++++++---------------------------
> fs/sysv/namei.c | 26 +++++++++-------
> fs/sysv/sysv.h | 19 ++++++++---
> 3 files changed, 65 insertions(+), 63 deletions(-)
>
> diff --git a/fs/sysv/dir.c b/fs/sysv/dir.c
> index 88e38cd8f5c9..130350fde106 100644
> --- a/fs/sysv/dir.c
> +++ b/fs/sysv/dir.c
> @@ -28,12 +28,6 @@ const struct file_operations sysv_dir_operations = {
> .fsync = generic_file_fsync,
> };
>
> -static inline void dir_put_page(struct page *page)
> -{
> - kunmap(page);
> - put_page(page);
> -}
> -
> static int dir_commit_chunk(struct page *page, loff_t pos, unsigned len)
> {
> struct address_space *mapping = page->mapping;
> @@ -52,12 +46,12 @@ static int dir_commit_chunk(struct page *page, loff_t pos, unsigned len)
> return err;
> }
>
> -static struct page * dir_get_page(struct inode *dir, unsigned long n)
> +static struct page *dir_get_page(struct inode *dir, unsigned long n, void **page_addr)
> {
> struct address_space *mapping = dir->i_mapping;
> struct page *page = read_mapping_page(mapping, n, NULL);
> if (!IS_ERR(page))
> - kmap(page);
> + *page_addr = kmap_local_page(page);
> return page;
> }
>
> @@ -80,11 +74,10 @@ static int sysv_readdir(struct file *file, struct dir_context *ctx)
> for ( ; n < npages; n++, offset = 0) {
> char *kaddr, *limit;
> struct sysv_dir_entry *de;
> - struct page *page = dir_get_page(inode, n);
> + struct page *page = dir_get_page(inode, n, (void **)&kaddr);
>
> if (IS_ERR(page))
> continue;
> - kaddr = (char *)page_address(page);
> de = (struct sysv_dir_entry *)(kaddr+offset);
> limit = kaddr + PAGE_SIZE - SYSV_DIRSIZE;
> for ( ;(char*)de <= limit; de++, ctx->pos += sizeof(*de)) {
> @@ -96,11 +89,11 @@ static int sysv_readdir(struct file *file, struct dir_context *ctx)
> if (!dir_emit(ctx, name, strnlen(name,SYSV_NAMELEN),
> fs16_to_cpu(SYSV_SB(sb), de->inode),
> DT_UNKNOWN)) {
> - dir_put_page(page);
> + dir_put_page(page, kaddr);
> return 0;
> }
> }
> - dir_put_page(page);
> + dir_put_page(page, kaddr);
> }
> return 0;
> }
> @@ -124,7 +117,8 @@ static inline int namecompare(int len, int maxlen,
> * itself (as a parameter - res_dir). It does NOT read the inode of the
> * entry - you'll have to do that yourself if you want to.
> */
> -struct sysv_dir_entry *sysv_find_entry(struct dentry *dentry, struct page **res_page)
> +struct sysv_dir_entry *sysv_find_entry(struct dentry *dentry,
> + struct page **res_page, void **res_page_addr)
> {
> const char * name = dentry->d_name.name;
> int namelen = dentry->d_name.len;
> @@ -133,8 +127,10 @@ struct sysv_dir_entry *sysv_find_entry(struct dentry *dentry, struct page **res_
> unsigned long npages = dir_pages(dir);
> struct page *page = NULL;
> struct sysv_dir_entry *de;
> + char *kaddr;
>
> *res_page = NULL;
> + *res_page_addr = NULL;
>
> start = SYSV_I(dir)->i_dir_start_lookup;
> if (start >= npages)
> @@ -142,10 +138,8 @@ struct sysv_dir_entry *sysv_find_entry(struct dentry *dentry, struct page **res_
> n = start;
>
> do {
> - char *kaddr;
> - page = dir_get_page(dir, n);
> + page = dir_get_page(dir, n, (void **)&kaddr);
> if (!IS_ERR(page)) {
> - kaddr = (char*)page_address(page);
> de = (struct sysv_dir_entry *) kaddr;
> kaddr += PAGE_SIZE - SYSV_DIRSIZE;
> for ( ; (char *) de <= kaddr ; de++) {
> @@ -155,7 +149,7 @@ struct sysv_dir_entry *sysv_find_entry(struct dentry *dentry, struct page **res_
> name, de->name))
> goto found;
> }
> - dir_put_page(page);
> + dir_put_page(page, kaddr);
> }
>
> if (++n >= npages)
> @@ -167,6 +161,7 @@ struct sysv_dir_entry *sysv_find_entry(struct dentry *dentry, struct page **res_
> found:
> SYSV_I(dir)->i_dir_start_lookup = n;
> *res_page = page;
> + *res_page_addr = kaddr;
> return de;
> }
>
> @@ -176,6 +171,7 @@ int sysv_add_link(struct dentry *dentry, struct inode *inode)
> const char * name = dentry->d_name.name;
> int namelen = dentry->d_name.len;
> struct page *page = NULL;
> + void *page_addr = NULL;
> struct sysv_dir_entry * de;
> unsigned long npages = dir_pages(dir);
> unsigned long n;
> @@ -185,11 +181,11 @@ int sysv_add_link(struct dentry *dentry, struct inode *inode)
>
> /* We take care of directory expansion in the same loop */
> for (n = 0; n <= npages; n++) {
> - page = dir_get_page(dir, n);
> + page = dir_get_page(dir, n, &page_addr);
> err = PTR_ERR(page);
> if (IS_ERR(page))
> goto out;
> - kaddr = (char*)page_address(page);
> + kaddr = page_addr;
> de = (struct sysv_dir_entry *)kaddr;
> kaddr += PAGE_SIZE - SYSV_DIRSIZE;
> while ((char *)de <= kaddr) {
> @@ -200,14 +196,13 @@ int sysv_add_link(struct dentry *dentry, struct inode *inode)
> goto out_page;
> de++;
> }
> - dir_put_page(page);
> + dir_put_page(page, page_addr);
> }
> BUG();
> return -EINVAL;
>
> got_it:
> - pos = page_offset(page) +
> - (char*)de - (char*)page_address(page);
> + pos = page_offset(page) + (char *)de - (char *)page_addr;
> lock_page(page);
> err = sysv_prepare_chunk(page, pos, SYSV_DIRSIZE);
> if (err)
> @@ -219,7 +214,7 @@ int sysv_add_link(struct dentry *dentry, struct inode *inode)
> dir->i_mtime = dir->i_ctime = current_time(dir);
> mark_inode_dirty(dir);
> out_page:
> - dir_put_page(page);
> + dir_put_page(page, page_addr);
> out:
> return err;
> out_unlock:
> @@ -227,10 +222,9 @@ int sysv_add_link(struct dentry *dentry, struct inode *inode)
> goto out_page;
> }
>
> -int sysv_delete_entry(struct sysv_dir_entry *de, struct page *page)
> +int sysv_delete_entry(struct sysv_dir_entry *de, struct page *page, char *kaddr)
> {
> struct inode *inode = page->mapping->host;
> - char *kaddr = (char*)page_address(page);
> loff_t pos = page_offset(page) + (char *)de - kaddr;
> int err;
>
> @@ -239,7 +233,7 @@ int sysv_delete_entry(struct sysv_dir_entry *de, struct page *page)
> BUG_ON(err);
> de->inode = 0;
> err = dir_commit_chunk(page, pos, SYSV_DIRSIZE);
> - dir_put_page(page);
> + dir_put_page(page, kaddr);
> inode->i_ctime = inode->i_mtime = current_time(inode);
> mark_inode_dirty(inode);
> return err;
> @@ -259,19 +253,15 @@ int sysv_make_empty(struct inode *inode, struct inode *dir)
> unlock_page(page);
> goto fail;
> }
> - kmap(page);
> -
> - base = (char*)page_address(page);
> + base = kmap_local_page(page);
> memset(base, 0, PAGE_SIZE);
> -
> de = (struct sysv_dir_entry *) base;
> de->inode = cpu_to_fs16(SYSV_SB(inode->i_sb), inode->i_ino);
> strcpy(de->name,".");
> de++;
> de->inode = cpu_to_fs16(SYSV_SB(inode->i_sb), dir->i_ino);
> strcpy(de->name,"..");
> -
> - kunmap(page);
> + kunmap_local(base);
> err = dir_commit_chunk(page, 0, 2 * SYSV_DIRSIZE);
> fail:
> put_page(page);
> @@ -286,16 +276,15 @@ int sysv_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 sysv_dir_entry * de;
> - page = dir_get_page(inode, i);
> + page = dir_get_page(inode, i, (void **)&kaddr);
>
> if (IS_ERR(page))
> continue;
>
> - kaddr = (char *)page_address(page);
> de = (struct sysv_dir_entry *)kaddr;
> kaddr += PAGE_SIZE-SYSV_DIRSIZE;
>
> @@ -314,22 +303,21 @@ int sysv_empty_dir(struct inode * inode)
> if (de->name[1] != '.' || de->name[2])
> goto not_empty;
> }
> - dir_put_page(page);
> + dir_put_page(page, kaddr);
> }
> return 1;
>
> not_empty:
> - dir_put_page(page);
> + dir_put_page(page, kaddr);
> return 0;
> }
>
> /* Releases the page */
> void sysv_set_link(struct sysv_dir_entry *de, struct page *page,
> - struct inode *inode)
> + void *page_addr, struct inode *inode)
> {
> struct inode *dir = page->mapping->host;
> - loff_t pos = page_offset(page) +
> - (char *)de-(char*)page_address(page);
> + loff_t pos = page_offset(page) + (char *)de - (char *)page_addr;
> int err;
>
> lock_page(page);
> @@ -337,19 +325,21 @@ void sysv_set_link(struct sysv_dir_entry *de, struct page *page,
> BUG_ON(err);
> de->inode = cpu_to_fs16(SYSV_SB(inode->i_sb), inode->i_ino);
> err = dir_commit_chunk(page, pos, SYSV_DIRSIZE);
> - dir_put_page(page);
> + dir_put_page(page, page_addr);
> dir->i_mtime = dir->i_ctime = current_time(dir);
> mark_inode_dirty(dir);
> }
>
> -struct sysv_dir_entry * sysv_dotdot (struct inode *dir, struct page **p)
> +struct sysv_dir_entry *sysv_dotdot(struct inode *dir, struct page **p, void **pa)
> {
> - struct page *page = dir_get_page(dir, 0);
> + void *page_addr;
> + struct page *page = dir_get_page(dir, 0, &page_addr);
> struct sysv_dir_entry *de = NULL;
>
> if (!IS_ERR(page)) {
> - de = (struct sysv_dir_entry*) page_address(page) + 1;
> + de = (struct sysv_dir_entry *)page_addr + 1;
> *p = page;
> + *pa = page_addr;
> }
> return de;
> }
> @@ -357,12 +347,13 @@ struct sysv_dir_entry * sysv_dotdot (struct inode *dir, struct page **p)
> ino_t sysv_inode_by_name(struct dentry *dentry)
> {
> struct page *page;
> - struct sysv_dir_entry *de = sysv_find_entry (dentry, &page);
> + void *page_addr;
> + struct sysv_dir_entry *de = sysv_find_entry(dentry, &page, &page_addr);
> ino_t res = 0;
>
> if (de) {
> res = fs16_to_cpu(SYSV_SB(dentry->d_sb), de->inode);
> - dir_put_page(page);
> + dir_put_page(page, page_addr);
> }
> return res;
> }
> diff --git a/fs/sysv/namei.c b/fs/sysv/namei.c
> index b2e6abc06a2d..1371980ec5fb 100644
> --- a/fs/sysv/namei.c
> +++ b/fs/sysv/namei.c
> @@ -152,14 +152,15 @@ static int sysv_unlink(struct inode * dir, struct dentry * dentry)
> {
> struct inode * inode = d_inode(dentry);
> struct page * page;
> + void *page_addr;
> struct sysv_dir_entry * de;
> int err = -ENOENT;
>
> - de = sysv_find_entry(dentry, &page);
> + de = sysv_find_entry(dentry, &page, &page_addr);
> if (!de)
> goto out;
>
> - err = sysv_delete_entry (de, page);
> + err = sysv_delete_entry(de, page, page_addr);
> if (err)
> goto out;
>
> @@ -196,26 +197,29 @@ static int sysv_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;
> + void *dir_page_addr;
> struct sysv_dir_entry * dir_de = NULL;
> struct page * old_page;
> + void *old_page_addr;
> struct sysv_dir_entry * old_de;
> int err = -ENOENT;
>
> if (flags & ~RENAME_NOREPLACE)
> return -EINVAL;
>
> - old_de = sysv_find_entry(old_dentry, &old_page);
> + old_de = sysv_find_entry(old_dentry, &old_page, &old_page_addr);
> if (!old_de)
> goto out;
>
> if (S_ISDIR(old_inode->i_mode)) {
> err = -EIO;
> - dir_de = sysv_dotdot(old_inode, &dir_page);
> + dir_de = sysv_dotdot(old_inode, &dir_page, &dir_page_addr);
> if (!dir_de)
> goto out_old;
> }
>
> if (new_inode) {
> + void *new_page_addr;
> struct page * new_page;
> struct sysv_dir_entry * new_de;
>
> @@ -224,10 +228,10 @@ static int sysv_rename(struct user_namespace *mnt_userns, struct inode *old_dir,
> goto out_dir;
>
> err = -ENOENT;
> - new_de = sysv_find_entry(new_dentry, &new_page);
> + new_de = sysv_find_entry(new_dentry, &new_page, &new_page_addr);
> if (!new_de)
> goto out_dir;
> - sysv_set_link(new_de, new_page, old_inode);
> + sysv_set_link(new_de, new_page, new_page_addr, old_inode);
> new_inode->i_ctime = current_time(new_inode);
> if (dir_de)
> drop_nlink(new_inode);
> @@ -240,23 +244,21 @@ static int sysv_rename(struct user_namespace *mnt_userns, struct inode *old_dir,
> inode_inc_link_count(new_dir);
> }
>
> - sysv_delete_entry(old_de, old_page);
> + sysv_delete_entry(old_de, old_page, old_page_addr);
> mark_inode_dirty(old_inode);
>
> if (dir_de) {
> - sysv_set_link(dir_de, dir_page, new_dir);
> + sysv_set_link(dir_de, dir_page, dir_page_addr, new_dir);
> inode_dec_link_count(old_dir);
> }
> return 0;
>
> out_dir:
> if (dir_de) {
> - kunmap(dir_page);
> - put_page(dir_page);
> + dir_put_page(dir_page, dir_page_addr);
> }
> out_old:
> - kunmap(old_page);
> - put_page(old_page);
> + dir_put_page(old_page, old_page_addr);
> out:
> return err;
> }
> diff --git a/fs/sysv/sysv.h b/fs/sysv/sysv.h
> index 99ddf033da4f..b0631ea6b506 100644
> --- a/fs/sysv/sysv.h
> +++ b/fs/sysv/sysv.h
> @@ -119,6 +119,11 @@ static inline void dirty_sb(struct super_block *sb)
> mark_buffer_dirty(sbi->s_bh2);
> }
>
> +static inline void dir_put_page(struct page *page, void *page_addr)
> +{
> + kunmap_local(page_addr);
> + put_page(page);
> +}
>
> /* ialloc.c */
> extern struct sysv_inode *sysv_raw_inode(struct super_block *, unsigned,
> @@ -148,14 +153,18 @@ extern void sysv_destroy_icache(void);
>
>
> /* dir.c */
> -extern struct sysv_dir_entry *sysv_find_entry(struct dentry *, struct page **);
> +extern struct sysv_dir_entry *sysv_find_entry(struct dentry *dir,
> + struct page **res_page,
> + void **res_page_addr);
> extern int sysv_add_link(struct dentry *, struct inode *);
> -extern int sysv_delete_entry(struct sysv_dir_entry *, struct page *);
> +extern int sysv_delete_entry(struct sysv_dir_entry *dir, struct page *page,
> + char *kaddr);
> extern int sysv_make_empty(struct inode *, struct inode *);
> extern int sysv_empty_dir(struct inode *);
> -extern void sysv_set_link(struct sysv_dir_entry *, struct page *,
> - struct inode *);
> -extern struct sysv_dir_entry *sysv_dotdot(struct inode *, struct page **);
> +extern void sysv_set_link(struct sysv_dir_entry *de, struct page *page,
> + void *page_addr, struct inode *inode);
> +extern struct sysv_dir_entry *sysv_dotdot(struct inode *inode,
> + struct page **page, void **page_addr);
> extern ino_t sysv_inode_by_name(struct dentry *);
>
>
> --
> 2.37.2
>

Next message: Will Deacon: "Re: [PATCH 1/1] perf/arm-cmn: Add more bits to child node address offset field"
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