Re: [PATCH v4 3/3] fs/ufs: Replace kmap() with kmap_local_page()

From: Ira Weiny
Date: Thu Dec 22 2022 - 17:58:50 EST

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In reply to: Fabio M. De Francesco: "Re: [PATCH v4 3/3] fs/ufs: Replace kmap() with kmap_local_page()"
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On Thu, Dec 22, 2022 at 03:27:08PM +0100, Fabio M. De Francesco wrote:
> On giovedì 22 dicembre 2022 06:41:01 CET Ira Weiny wrote:
> > On Wed, Dec 21, 2022 at 06:28:02PM +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.
> > >
> > > 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().
> >
> > I don't see the calls to kunmap() in ufs_rename converted here?
> >
> > Did I miss them?
> >
>
> No, it's my fault.
> I must have used "grep" on all files in fs/ufs, but I forgot to run it :-(
>
> While at this... I'm wondering whether or not we could benefit from a WARNING
> about the use of kunmap(). I'm talking about adding this too to checkpatch.pl,
> exactly as we already have it for catching the deprecated use of kmap().

That would not have caught this issue. Any addition of kunmap() in a patch
would have to come with a call to kmap(). (Unless they are fixing some kmap
bug I suppose.) I'm not sure how the checkpatch.pl maintainers would feel
about this. You can always submit a patch and find out but I would not worry
about it.

>
> >
> > I think those calls need to be changed to ufs_put_page() calls in a
> precursor
> > patch to this one unless I'm missing something.
> >
>
> Again I think that you are not missing anything and that your suggestion
> sounds good.
>
> I'll replace the three kunmap() + put_page() with three calls to
> ufs_put_page() in ufs_rename(). I'll do these changes in patch 3/4. Instead
> the current 3/4 patch will move ahead and become 4/4.

Sounds good.

>
> >
> > > Suggested-by: Al Viro <viro@xxxxxxxxxxxxxxxxxx>
> > > Suggested-by: Ira Weiny <ira.weiny@xxxxxxxxx>
> > > Signed-off-by: Fabio M. De Francesco <fmdefrancesco@xxxxxxxxx>
> > > ---
> > >
> > > 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..ed3568da29a8 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_local((void *)((unsigned long)page_addr & PAGE_MASK));
> >
> > Any address in the page can be passed to kunmap_local() as this mask is done
> > internally.
> >
>
> I know that any address can be passed and that the bitwise and is performed
> internally in kunmap_local_indexed(). This is why I've never done something
> like this in any other of my precedent conversions.
>
> However, I thought that Al should have had reasons to suggest to call
> kunmap_local() this way. Copy-pasted from one of his message (https://
> lore.kernel.org/lkml/Y4E++JERgUMoqfjG@ZenIV/) while commenting the one patch
> old conversions:
>
> --- begin ---
>
> -static 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);
>
> Make that
> kunmap_local((void *)((unsigned long)page_addr & PAGE_MASK));
> and things become much easier.
>
> > put_page(page);
> > }
>
> --- end ---
>
> Did I misinterpret his words?
> However, it's my fault again because I should have asked why :-(

Perhaps Al did not know that kunmap_local() would take care of this for you?

>
> > > 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.
> >
> > I don't follow why this comment needed changing for this patch. It probably
> > warrants it's own patch.
> >
>
> Sure, the removal of the name of function is a different logical change, so
> I'll probably leave it as it was.
>
> > > + * 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.
> > >
> > > */
>
> But this last part should be still added. Am I wrong?

You are not wrong. Adding this is appropriate. Just not the rest which seemed
very minor changes anyway.

>
> > > 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);
> >
> > I feel like this deserves a comment to clarify that dir points somewhere in
> > the page we need the base address of.
>
> OK, it sounds reasonable.
>
> > > 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);
> >
> > NIT: I'd make this conversion a separate patch.
> >
> > Ira
> >
>
> We've always done multiple conversions at the same time if in the same file,
> even if they were unrelated.
>
> I don't understand why we want to change the usual procedure. Can you please
> elaborate a bit more on this topic?

The difference here is we are making a lot of changes to the
ufs_{get,put}_page() calls and all their callers and this is not part of those
changes. So reviewing those was hard enough without looking at this more
mundane change. But like I said it is a nit so feel free to leave it as the
change looks fine.

>
> Thanks so much for finding the missing conversions and for your other comments
> and advice on this patch.

NP!

Ira

>
> Fabio
>
> > > 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
>
>
>
>

Next message: Conor Dooley: "Re: [PATCH v2 04/13] riscv: hwcap: make ISA extension ids can be used in asm"
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In reply to: Fabio M. De Francesco: "Re: [PATCH v4 3/3] fs/ufs: Replace kmap() with kmap_local_page()"
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