[PATCH v3 0/3] fs/ufs: replace kmap() with kmap_local_page

[PATCH v3 0/3] fs/ufs: replace kmap() with kmap_local_page

[Fabio M. De Francesco]

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/ufs is safe everywhere, it should be preferred.

Therefore, replace kmap() with kmap_local_page() in fs/ufs. kunmap_local()
requires the mapping address, so return that address from ufs_get_page()
to be used in ufs_put_page().

This series could have not been ever made because nothing prevented the
previous patch from working properly but Al Viro made a long series of
very appreciated comments about how many unnecessary and redundant lines
of code I could have removed. He could see things I was entirely unable
to notice. Furthermore, he also provided solutions and details about how
I could decompose a single patch into a small series of three
independent units.[1][2][3]

I want to thank him so much for the patience, kindness and the time he
decided to spend to provide those analysis and write three messages full
of interesting insights.[1][2][3]

Changes from v1:
	1/3: No changes.
	2/3: Restore the return of "err" that was mistakenly deleted
	     together with the removal of the "out" label in
	     ufs_add_link(). Thanks to Al Viro.[4]
	     Return the address of the kmap()'ed page instead of a
	     pointer to a pointer to the mapped page; a page_address()
	     had been overlooked in ufs_get_page(). Thanks to Al
	     Viro.[5]
	3/3: Return the kernel virtual address got from the call to
	     kmap_local_page() after conversion from kmap(). Again
	     thanks to Al Viro.[6]

Changes from v2:
	1/3: No changes.
	2/3: Rework ufs_get_page() because the previous version had two
	     errors: (1) It could return an invalid pages with the out
	     argument "page" and (2) it could return "page_address(page)"
	     also in cases where read_mapping_page() returned an error
	     and the page is never kmap()'ed. Thanks to Al Viro.[7]
	3/3: Rework ufs_get_page() after conversion to
	     kmap_local_page(), in accordance to the last changes in 2/3.

[1] https://lore.kernel.org/lkml/Y4E++JERgUMoqfjG@ZenIV/
[2] https://lore.kernel.org/lkml/Y4FG0O7VWTTng5yh@ZenIV/
[3] https://lore.kernel.org/lkml/Y4ONIFJatIGsVNpf@ZenIV/
[4] https://lore.kernel.org/lkml/Y5Zc0qZ3+zsI74OZ@ZenIV/
[5] https://lore.kernel.org/lkml/Y5ZZy23FFAnQDR3C@ZenIV/
[6] https://lore.kernel.org/lkml/Y5ZcMPzPG9h6C9eh@ZenIV/
[7] https://lore.kernel.org/lkml/Y5glgpD7fFifC4Fi@ZenIV/#t

The cover letter of the v1 series is at
https://lore.kernel.org/lkml/20221211213111.30085-1-fmdefrancesco@gmail.com/
The cover letter of the v2 series is at
https://lore.kernel.org/lkml/20221212231906.19424-1-fmdefrancesco@gmail.com/

Fabio M. De Francesco (3):
  fs/ufs: Use the offset_in_page() helper
  fs/ufs: Change the signature of ufs_get_page()
  fs/ufs: Replace kmap() with kmap_local_page()

 fs/ufs/dir.c | 134 +++++++++++++++++++++++++++------------------------
 1 file changed, 71 insertions(+), 63 deletions(-)

-- 
2.39.0

[PATCH v3 1/3] fs/ufs: Use the offset_in_page() helper

[Fabio M. De Francesco]

Use the offset_in_page() helper because it is more suitable than doing
explicit subtractions between pointers to directory entries and kernel
virtual addresses of mapped pages.

Cc: Ira Weiny <ira.weiny@intel.com>
Suggested-by: Al Viro <viro@zeniv.linux.org.uk>
Signed-off-by: Fabio M. De Francesco <fmdefrancesco@gmail.com>
---
 fs/ufs/dir.c | 12 +++++-------
 1 file changed, 5 insertions(+), 7 deletions(-)

diff --git a/fs/ufs/dir.c b/fs/ufs/dir.c
index 391efaf1d528..69f78583c9c1 100644
--- a/fs/ufs/dir.c
+++ b/fs/ufs/dir.c
@@ -87,8 +87,7 @@ void ufs_set_link(struct inode *dir, struct ufs_dir_entry *de,
 		  struct page *page, struct inode *inode,
 		  bool update_times)
 {
-	loff_t pos = page_offset(page) +
-			(char *) de - (char *) page_address(page);
+	loff_t pos = page_offset(page) + offset_in_page(de);
 	unsigned len = fs16_to_cpu(dir->i_sb, de->d_reclen);
 	int err;
 
@@ -371,8 +370,7 @@ int ufs_add_link(struct dentry *dentry, struct inode *inode)
 	return -EINVAL;
 
 got_it:
-	pos = page_offset(page) +
-			(char*)de - (char*)page_address(page);
+	pos = page_offset(page) + offset_in_page(de);
 	err = ufs_prepare_chunk(page, pos, rec_len);
 	if (err)
 		goto out_unlock;
@@ -497,8 +495,8 @@ int ufs_delete_entry(struct inode *inode, struct ufs_dir_entry *dir,
 {
 	struct super_block *sb = inode->i_sb;
 	char *kaddr = page_address(page);
-	unsigned from = ((char*)dir - kaddr) & ~(UFS_SB(sb)->s_uspi->s_dirblksize - 1);
-	unsigned to = ((char*)dir - kaddr) + fs16_to_cpu(sb, dir->d_reclen);
+	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;
 	struct ufs_dir_entry *pde = NULL;
 	struct ufs_dir_entry *de = (struct ufs_dir_entry *) (kaddr + from);
@@ -522,7 +520,7 @@ int ufs_delete_entry(struct inode *inode, struct ufs_dir_entry *dir,
 		de = ufs_next_entry(sb, de);
 	}
 	if (pde)
-		from = (char*)pde - (char*)page_address(page);
+		from = offset_in_page(pde);
 
 	pos = page_offset(page) + from;
 	lock_page(page);
-- 
2.39.0

[PATCH v3 2/3] fs/ufs: Change the signature of ufs_get_page()

[Fabio M. De Francesco]

Change the signature of ufs_get_page() in order to prepare this function
to the conversion to the use of kmap_local_page(). Change also those call
sites which are required to conform its invocations to the new
signature.

Cc: Ira Weiny <ira.weiny@intel.com>
Suggested-by: Al Viro <viro@zeniv.linux.org.uk>
Signed-off-by: Fabio M. De Francesco <fmdefrancesco@gmail.com>
---
 fs/ufs/dir.c | 49 +++++++++++++++++++++----------------------------
 1 file changed, 21 insertions(+), 28 deletions(-)

diff --git a/fs/ufs/dir.c b/fs/ufs/dir.c
index 69f78583c9c1..9fa86614d2d1 100644
--- a/fs/ufs/dir.c
+++ b/fs/ufs/dir.c
@@ -185,7 +185,7 @@ static bool ufs_check_page(struct page *page)
 	return false;
 }
 
-static struct page *ufs_get_page(struct inode *dir, unsigned long n)
+static void *ufs_get_page(struct inode *dir, unsigned long n, struct page **p)
 {
 	struct address_space *mapping = dir->i_mapping;
 	struct page *page = read_mapping_page(mapping, n, NULL);
@@ -195,8 +195,10 @@ static struct page *ufs_get_page(struct inode *dir, unsigned long n)
 			if (!ufs_check_page(page))
 				goto fail;
 		}
+		*p = page;
+		return page_address(page);
 	}
-	return page;
+	return ERR_CAST(page);
 
 fail:
 	ufs_put_page(page);
@@ -227,15 +229,12 @@ ufs_next_entry(struct super_block *sb, struct ufs_dir_entry *p)
 
 struct ufs_dir_entry *ufs_dotdot(struct inode *dir, struct page **p)
 {
-	struct page *page = ufs_get_page(dir, 0);
-	struct ufs_dir_entry *de = NULL;
+	struct ufs_dir_entry *de = ufs_get_page(dir, 0, p);
 
-	if (!IS_ERR(page)) {
-		de = ufs_next_entry(dir->i_sb,
-				    (struct ufs_dir_entry *)page_address(page));
-		*p = page;
-	}
-	return de;
+	if (!IS_ERR(de))
+		return ufs_next_entry(dir->i_sb, de);
+	else
+		return NULL;
 }
 
 /*
@@ -273,11 +272,10 @@ struct ufs_dir_entry *ufs_find_entry(struct inode *dir, const struct qstr *qstr,
 		start = 0;
 	n = start;
 	do {
-		char *kaddr;
-		page = ufs_get_page(dir, n);
-		if (!IS_ERR(page)) {
-			kaddr = page_address(page);
-			de = (struct ufs_dir_entry *) kaddr;
+		char *kaddr = ufs_get_page(dir, n, &page);
+
+		if (!IS_ERR(kaddr)) {
+			de = (struct ufs_dir_entry *)kaddr;
 			kaddr += ufs_last_byte(dir, n) - reclen;
 			while ((char *) de <= kaddr) {
 				if (ufs_match(sb, namelen, name, de))
@@ -328,12 +326,10 @@ int ufs_add_link(struct dentry *dentry, struct inode *inode)
 	for (n = 0; n <= npages; n++) {
 		char *dir_end;
 
-		page = ufs_get_page(dir, n);
-		err = PTR_ERR(page);
-		if (IS_ERR(page))
-			goto out;
+		kaddr = ufs_get_page(dir, n, &page);
+		if (IS_ERR(kaddr))
+			return PTR_ERR(kaddr);
 		lock_page(page);
-		kaddr = page_address(page);
 		dir_end = kaddr + ufs_last_byte(dir, n);
 		de = (struct ufs_dir_entry *)kaddr;
 		kaddr += PAGE_SIZE - reclen;
@@ -395,7 +391,6 @@ int ufs_add_link(struct dentry *dentry, struct inode *inode)
 	/* OFFSET_CACHE */
 out_put:
 	ufs_put_page(page);
-out:
 	return err;
 out_unlock:
 	unlock_page(page);
@@ -429,6 +424,7 @@ ufs_readdir(struct file *file, struct dir_context *ctx)
 	unsigned chunk_mask = ~(UFS_SB(sb)->s_uspi->s_dirblksize - 1);
 	bool need_revalidate = !inode_eq_iversion(inode, file->f_version);
 	unsigned flags = UFS_SB(sb)->s_flags;
+	struct page *page;
 
 	UFSD("BEGIN\n");
 
@@ -439,16 +435,14 @@ ufs_readdir(struct file *file, struct dir_context *ctx)
 		char *kaddr, *limit;
 		struct ufs_dir_entry *de;
 
-		struct page *page = ufs_get_page(inode, n);
-
-		if (IS_ERR(page)) {
+		kaddr = ufs_get_page(inode, n, &page);
+		if (IS_ERR(kaddr)) {
 			ufs_error(sb, __func__,
 				  "bad page in #%lu",
 				  inode->i_ino);
 			ctx->pos += PAGE_SIZE - offset;
 			return -EIO;
 		}
-		kaddr = page_address(page);
 		if (unlikely(need_revalidate)) {
 			if (offset) {
 				offset = ufs_validate_entry(sb, kaddr, offset, chunk_mask);
@@ -595,12 +589,11 @@ int ufs_empty_dir(struct inode * inode)
 	for (i = 0; i < npages; i++) {
 		char *kaddr;
 		struct ufs_dir_entry *de;
-		page = ufs_get_page(inode, i);
 
-		if (IS_ERR(page))
+		kaddr = ufs_get_page(inode, i, &page);
+		if (IS_ERR(kaddr))
 			continue;
 
-		kaddr = page_address(page);
 		de = (struct ufs_dir_entry *)kaddr;
 		kaddr += ufs_last_byte(inode, i) - UFS_DIR_REC_LEN(1);
 
-- 
2.39.0

[PATCH v3 3/3] fs/ufs: Replace kmap() with kmap_local_page()

[Fabio M. De Francesco]

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/ufs is safe everywhere, it should be preferred.

Therefore, replace kmap() with kmap_local_page() in fs/ufs. kunmap_local()
requires the mapping address, so return that address from ufs_get_page()
to be used in ufs_put_page().

Suggested-by: Al Viro <viro@zeniv.linux.org.uk>
Suggested-by: Ira Weiny <ira.weiny@intel.com>
Signed-off-by: Fabio M. De Francesco <fmdefrancesco@gmail.com>
---
 fs/ufs/dir.c | 75 ++++++++++++++++++++++++++++++++--------------------
 1 file changed, 46 insertions(+), 29 deletions(-)

diff --git a/fs/ufs/dir.c b/fs/ufs/dir.c
index 9fa86614d2d1..a9dd5023b604 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;
 }
 
-static inline void ufs_put_page(struct page *page)
+static inline void ufs_put_page(struct page *page, void *page_addr)
 {
-	kunmap(page);
+	kunmap((void *)((unsigned long)page_addr & PAGE_MASK));
 	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);
@@ -238,12 +253,15 @@ struct ufs_dir_entry *ufs_dotdot(struct inode *dir, struct page **p)
 }
 
 /*
- *	ufs_find_entry()
+ * Finds an entry in the specified directory with the wanted name. It returns a
+ * pointer to the directory's entry. The page in which the entry was found is
+ * in the res_page out parameter. The page is returned mapped and unlocked.
+ * The entry is guaranteed to be valid.
  *
- * finds an entry in the specified directory with the wanted name. It
- * 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 +300,7 @@ 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 +378,7 @@ 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 +408,7 @@ 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 +486,13 @@ 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 +503,10 @@ 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);
+	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 +545,7 @@ 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 +569,7 @@ 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 +586,7 @@ 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 +602,9 @@ 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 +637,12 @@ 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;
 }
 
-- 
2.39.0

Re: [PATCH v3 3/3] fs/ufs: Replace kmap() with kmap_local_page()

[Fabio M. De Francesco]

On sabato 17 dicembre 2022 19:47:49 CET 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/ufs is safe everywhere, it should be preferred.
> 
> Therefore, replace kmap() with kmap_local_page() in fs/ufs. kunmap_local()
> requires the mapping address, so return that address from ufs_get_page()
> to be used in ufs_put_page().
> 
> Suggested-by: Al Viro <viro@zeniv.linux.org.uk>
> Suggested-by: Ira Weiny <ira.weiny@intel.com>
> Signed-off-by: Fabio M. De Francesco <fmdefrancesco@gmail.com>
> ---
>  fs/ufs/dir.c | 75 ++++++++++++++++++++++++++++++++--------------------
>  1 file changed, 46 insertions(+), 29 deletions(-)
> 
> diff --git a/fs/ufs/dir.c b/fs/ufs/dir.c
> index 9fa86614d2d1..a9dd5023b604 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;
>  }
> 
> -static inline void ufs_put_page(struct page *page)
> +static inline void ufs_put_page(struct page *page, void *page_addr)
>  {
> -	kunmap(page);
> +	kunmap((void *)((unsigned long)page_addr & PAGE_MASK));

While working on patch 3/3 of a similar series for fs/sysv a few minutes ago, 
I noticed this call to kunmap() instead of kunmap_local().

I will wait a day or two before sending the next version, to leave additional 
time for those who want to comment / review.

Fabio

>  	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);
> @@ -238,12 +253,15 @@ struct ufs_dir_entry *ufs_dotdot(struct inode *dir,
> struct page **p) }
> 
>  /*
> - *	ufs_find_entry()
> + * Finds an entry in the specified directory with the wanted name. It 
returns
> a + * pointer to the directory's entry. The page in which the entry was 
found
> is + * in the res_page out parameter. The page is returned mapped and
> unlocked. + * The entry is guaranteed to be valid.
>   *
> - * finds an entry in the specified directory with the wanted name. It
> - * 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 +300,7 @@ 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 +378,7 @@ 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 +408,7 @@ 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 +486,13 @@ 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 +503,10 @@ 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);
> +	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 +545,7 @@ 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 +569,7 @@ 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 +586,7 @@ 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 +602,9 @@ 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 +637,12 @@ 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;
>  }
> 
> --
> 2.39.0

Re: [PATCH v3 0/3] fs/ufs: replace kmap() with kmap_local_page

[Fabio M. De Francesco]

On sabato 17 dicembre 2022 19:47:46 CET 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/ufs is safe everywhere, it should be preferred.
> 
> Therefore, replace kmap() with kmap_local_page() in fs/ufs. kunmap_local()
> requires the mapping address, so return that address from ufs_get_page()
> to be used in ufs_put_page().
> 
> This series could have not been ever made because nothing prevented the
> previous patch from working properly but Al Viro made a long series of
> very appreciated comments about how many unnecessary and redundant lines
> of code I could have removed. He could see things I was entirely unable
> to notice. Furthermore, he also provided solutions and details about how
> I could decompose a single patch into a small series of three
> independent units.[1][2][3]
> 
> I want to thank him so much for the patience, kindness and the time he
> decided to spend to provide those analysis and write three messages full
> of interesting insights.[1][2][3]
> 
> Changes from v1:
> 	1/3: No changes.
> 	2/3: Restore the return of "err" that was mistakenly deleted
> 	     together with the removal of the "out" label in
> 	     ufs_add_link(). Thanks to Al Viro.[4]
> 	     Return the address of the kmap()'ed page instead of a
> 	     pointer to a pointer to the mapped page; a page_address()
> 	     had been overlooked in ufs_get_page(). Thanks to Al
> 	     Viro.[5]
> 	3/3: Return the kernel virtual address got from the call to
> 	     kmap_local_page() after conversion from kmap(). Again
> 	     thanks to Al Viro.[6]
> 
> Changes from v2:
> 	1/3: No changes.
> 	2/3: Rework ufs_get_page() because the previous version had two
> 	     errors: (1) It could return an invalid pages with the out
> 	     argument "page" and (2) it could return "page_address(page)"
> 	     also in cases where read_mapping_page() returned an error
> 	     and the page is never kmap()'ed. Thanks to Al Viro.[7]
> 	3/3: Rework ufs_get_page() after conversion to
> 	     kmap_local_page(), in accordance to the last changes in 2/3.
> 
> [1] https://lore.kernel.org/lkml/Y4E++JERgUMoqfjG@ZenIV/
> [2] https://lore.kernel.org/lkml/Y4FG0O7VWTTng5yh@ZenIV/
> [3] https://lore.kernel.org/lkml/Y4ONIFJatIGsVNpf@ZenIV/
> [4] https://lore.kernel.org/lkml/Y5Zc0qZ3+zsI74OZ@ZenIV/
> [5] https://lore.kernel.org/lkml/Y5ZZy23FFAnQDR3C@ZenIV/
> [6] https://lore.kernel.org/lkml/Y5ZcMPzPG9h6C9eh@ZenIV/
> [7] https://lore.kernel.org/lkml/Y5glgpD7fFifC4Fi@ZenIV/#t
> 
> The cover letter of the v1 series is at
> https://lore.kernel.org/lkml/20221211213111.30085-1-fmdefrancesco@gmail.com/
> The cover letter of the v2 series is at
> https://lore.kernel.org/lkml/20221212231906.19424-1-fmdefrancesco@gmail.com/
> 
> Fabio M. De Francesco (3):
>   fs/ufs: Use the offset_in_page() helper
>   fs/ufs: Change the signature of ufs_get_page()
>   fs/ufs: Replace kmap() with kmap_local_page()
> 
>  fs/ufs/dir.c | 134 +++++++++++++++++++++++++++------------------------
>  1 file changed, 71 insertions(+), 63 deletions(-)
> 
> --
> 2.39.0

Please drop this v3 because of a mistake in patch 3/3.
The updated v4 series is at 
https://lore.kernel.org/lkml/20221221172802.18743-1-fmdefrancesco@gmail.com/T/

Thanks,

Fabio