diff mbox series

[1/3] mm: Add folio_zero_tail() and use it in ext4

Message ID 20231107212643.3490372-2-willy@infradead.org
State Not Applicable
Headers show
Series Add folio_zero_tail() and folio_fill_tail() | expand

Commit Message

Matthew Wilcox Nov. 7, 2023, 9:26 p.m. UTC
Instead of unmapping the folio after copying the data to it, then mapping
it again to zero the tail, provide folio_zero_tail() to zero the tail
of an already-mapped folio.

Signed-off-by: Matthew Wilcox (Oracle) <willy@infradead.org>
---
 fs/ext4/inline.c        |  3 +--
 include/linux/highmem.h | 38 ++++++++++++++++++++++++++++++++++++++
 2 files changed, 39 insertions(+), 2 deletions(-)

Comments

Andrew Morton Nov. 8, 2023, 11:06 p.m. UTC | #1
On Tue,  7 Nov 2023 21:26:40 +0000 "Matthew Wilcox (Oracle)" <willy@infradead.org> wrote:

> Instead of unmapping the folio after copying the data to it, then mapping
> it again to zero the tail, provide folio_zero_tail() to zero the tail
> of an already-mapped folio.
> 
> ...
>
> --- a/include/linux/highmem.h
> +++ b/include/linux/highmem.h
> @@ -483,6 +483,44 @@ static inline void memcpy_to_folio(struct folio *folio, size_t offset,
>  	flush_dcache_folio(folio);
>  }
>  
> +/**
> + * folio_zero_tail - Zero the tail of a folio.
> + * @folio: The folio to zero.
> + * @kaddr: The address the folio is currently mapped to.
> + * @offset: The byte offset in the folio to start zeroing at.

That's the argument ordering I would expect.

> + * If you have already used kmap_local_folio() to map a folio, written
> + * some data to it and now need to zero the end of the folio (and flush
> + * the dcache), you can use this function.  If you do not have the
> + * folio kmapped (eg the folio has been partially populated by DMA),
> + * use folio_zero_range() or folio_zero_segment() instead.
> + *
> + * Return: An address which can be passed to kunmap_local().
> + */
> +static inline __must_check void *folio_zero_tail(struct folio *folio,
> +		size_t offset, void *kaddr)

While that is not.  addr,len is far more common that len,addr?

> +{
> +	size_t len = folio_size(folio) - offset;

Calling it `remaining' would be more clear.

> +
> +	if (folio_test_highmem(folio)) {
> +		size_t max = PAGE_SIZE - offset_in_page(offset);
> +
> +		while (len > max) {

Shouldn't this be `while (len)'?  AFAICT this code can fail to clear
the final page.

> +			memset(kaddr, 0, max);
> +			kunmap_local(kaddr);
> +			len -= max;
> +			offset += max;
> +			max = PAGE_SIZE;
> +			kaddr = kmap_local_folio(folio, offset);
> +		}
> +	}
> +
> +	memset(kaddr, 0, len);
> +	flush_dcache_folio(folio);
> +
> +	return kaddr;
> +}
> +
Andreas Grünbacher Nov. 9, 2023, 12:12 a.m. UTC | #2
Andrew,

Andrew Morton <akpm@linux-foundation.org> schrieb am Do., 9. Nov. 2023, 00:06:
> > +
> > +     if (folio_test_highmem(folio)) {
> > +             size_t max = PAGE_SIZE - offset_in_page(offset);
> > +
> > +             while (len > max) {
>
> Shouldn't this be `while (len)'?  AFAICT this code can fail to clear
> the final page.

not sure what you're seeing there, but this looks fine to me.

Thanks,
Andreas
Andrew Morton Nov. 9, 2023, 5:27 p.m. UTC | #3
On Thu, 9 Nov 2023 01:12:15 +0100 Andreas Grünbacher <andreas.gruenbacher@gmail.com> wrote:

> Andrew,
> 
> Andrew Morton <akpm@linux-foundation.org> schrieb am Do., 9. Nov. 2023, 00:06:
> > > +
> > > +     if (folio_test_highmem(folio)) {
> > > +             size_t max = PAGE_SIZE - offset_in_page(offset);
> > > +
> > > +             while (len > max) {
> >
> > Shouldn't this be `while (len)'?  AFAICT this code can fail to clear
> > the final page.
> 
> not sure what you're seeing there, but this looks fine to me.

I was right!  This code does fail to handle the final page.

: static inline void folio_fill_tail(struct folio *folio, size_t offset,
: 		const char *from, size_t len)
: {
: 	char *to = kmap_local_folio(folio, offset);
: 
: 	VM_BUG_ON(offset + len > folio_size(folio));
: 
: 	if (folio_test_highmem(folio)) {
: 		size_t max = PAGE_SIZE - offset_in_page(offset);
: 
: 		while (len > max) {
: 			memcpy(to, from, max);
: 			kunmap_local(to);
: 			len -= max;
: 			from += max;
: 			offset += max;
: 			max = PAGE_SIZE;
: 			to = kmap_local_folio(folio, offset);
: 		}
: 	}
: 
: 	memcpy(to, from, len);

This code down here handles it, doh.

: 	to = folio_zero_tail(folio, offset, to);
: 	kunmap_local(to);
: }

Implementation seems less straightforward than it might be?  Oh well.

Has it been runtime tested?

Anyway, let's please change the function argument ordering and remember
to cc linux-mm on v2?
Matthew Wilcox Nov. 9, 2023, 5:37 p.m. UTC | #4
On Wed, Nov 08, 2023 at 03:06:06PM -0800, Andrew Morton wrote:
> >  
> > +/**
> > + * folio_zero_tail - Zero the tail of a folio.
> > + * @folio: The folio to zero.
> > + * @kaddr: The address the folio is currently mapped to.
> > + * @offset: The byte offset in the folio to start zeroing at.
> 
> That's the argument ordering I would expect.
> 
> > + * If you have already used kmap_local_folio() to map a folio, written
> > + * some data to it and now need to zero the end of the folio (and flush
> > + * the dcache), you can use this function.  If you do not have the
> > + * folio kmapped (eg the folio has been partially populated by DMA),
> > + * use folio_zero_range() or folio_zero_segment() instead.
> > + *
> > + * Return: An address which can be passed to kunmap_local().
> > + */
> > +static inline __must_check void *folio_zero_tail(struct folio *folio,
> > +		size_t offset, void *kaddr)
> 
> While that is not.  addr,len is far more common that len,addr?

But that's not len!  That's offset-in-the-folio.  ie we're doing:

memset(folio_address(folio) + offset, 0, folio_size(folio) - offset);

If we were doing:

memset(folio_address(folio), 0, len);

then yes, your suggestion is the right order.

Indeed, having the arguments in the current order would hopefully make
filesystem authors realise that this _isn't_ "len".
Andreas Gruenbacher Nov. 9, 2023, 9:50 p.m. UTC | #5
Hi Willy,

On Tue, Nov 7, 2023 at 10:27 PM Matthew Wilcox (Oracle)
<willy@infradead.org> wrote:
> Instead of unmapping the folio after copying the data to it, then mapping
> it again to zero the tail, provide folio_zero_tail() to zero the tail
> of an already-mapped folio.
>
> Signed-off-by: Matthew Wilcox (Oracle) <willy@infradead.org>
> ---
>  fs/ext4/inline.c        |  3 +--
>  include/linux/highmem.h | 38 ++++++++++++++++++++++++++++++++++++++
>  2 files changed, 39 insertions(+), 2 deletions(-)
>
> diff --git a/fs/ext4/inline.c b/fs/ext4/inline.c
> index 9a84a5f9fef4..d5bd1e3a5d36 100644
> --- a/fs/ext4/inline.c
> +++ b/fs/ext4/inline.c
> @@ -502,9 +502,8 @@ static int ext4_read_inline_folio(struct inode *inode, struct folio *folio)
>         BUG_ON(len > PAGE_SIZE);
>         kaddr = kmap_local_folio(folio, 0);
>         ret = ext4_read_inline_data(inode, kaddr, len, &iloc);
> -       flush_dcache_folio(folio);
> +       kaddr = folio_zero_tail(folio, len, kaddr + len);
>         kunmap_local(kaddr);
> -       folio_zero_segment(folio, len, folio_size(folio));
>         folio_mark_uptodate(folio);
>         brelse(iloc.bh);
>
> diff --git a/include/linux/highmem.h b/include/linux/highmem.h
> index 4cacc0e43b51..1b81416196dd 100644
> --- a/include/linux/highmem.h
> +++ b/include/linux/highmem.h
> @@ -483,6 +483,44 @@ static inline void memcpy_to_folio(struct folio *folio, size_t offset,
>         flush_dcache_folio(folio);
>  }
>
> +/**
> + * folio_zero_tail - Zero the tail of a folio.
> + * @folio: The folio to zero.
> + * @kaddr: The address the folio is currently mapped to.
> + * @offset: The byte offset in the folio to start zeroing at.
> + *

As Andrew has pointed out, the order of the arguments in the
description doesn't match the order in the function definition. Other
than that, this patch looks good, so

Reviewed-by: Andreas Gruenbacher <agruenba@redhat.com>

> + * If you have already used kmap_local_folio() to map a folio, written
> + * some data to it and now need to zero the end of the folio (and flush
> + * the dcache), you can use this function.  If you do not have the
> + * folio kmapped (eg the folio has been partially populated by DMA),
> + * use folio_zero_range() or folio_zero_segment() instead.
> + *
> + * Return: An address which can be passed to kunmap_local().
> + */
> +static inline __must_check void *folio_zero_tail(struct folio *folio,
> +               size_t offset, void *kaddr)
> +{
> +       size_t len = folio_size(folio) - offset;
> +
> +       if (folio_test_highmem(folio)) {
> +               size_t max = PAGE_SIZE - offset_in_page(offset);
> +
> +               while (len > max) {
> +                       memset(kaddr, 0, max);
> +                       kunmap_local(kaddr);
> +                       len -= max;
> +                       offset += max;
> +                       max = PAGE_SIZE;
> +                       kaddr = kmap_local_folio(folio, offset);
> +               }
> +       }
> +
> +       memset(kaddr, 0, len);
> +       flush_dcache_folio(folio);
> +
> +       return kaddr;
> +}
> +
>  /**
>   * memcpy_from_file_folio - Copy some bytes from a file folio.
>   * @to: The destination buffer.
> --
> 2.42.0
>

Thanks,
Andreas
diff mbox series

Patch

diff --git a/fs/ext4/inline.c b/fs/ext4/inline.c
index 9a84a5f9fef4..d5bd1e3a5d36 100644
--- a/fs/ext4/inline.c
+++ b/fs/ext4/inline.c
@@ -502,9 +502,8 @@  static int ext4_read_inline_folio(struct inode *inode, struct folio *folio)
 	BUG_ON(len > PAGE_SIZE);
 	kaddr = kmap_local_folio(folio, 0);
 	ret = ext4_read_inline_data(inode, kaddr, len, &iloc);
-	flush_dcache_folio(folio);
+	kaddr = folio_zero_tail(folio, len, kaddr + len);
 	kunmap_local(kaddr);
-	folio_zero_segment(folio, len, folio_size(folio));
 	folio_mark_uptodate(folio);
 	brelse(iloc.bh);
 
diff --git a/include/linux/highmem.h b/include/linux/highmem.h
index 4cacc0e43b51..1b81416196dd 100644
--- a/include/linux/highmem.h
+++ b/include/linux/highmem.h
@@ -483,6 +483,44 @@  static inline void memcpy_to_folio(struct folio *folio, size_t offset,
 	flush_dcache_folio(folio);
 }
 
+/**
+ * folio_zero_tail - Zero the tail of a folio.
+ * @folio: The folio to zero.
+ * @kaddr: The address the folio is currently mapped to.
+ * @offset: The byte offset in the folio to start zeroing at.
+ *
+ * If you have already used kmap_local_folio() to map a folio, written
+ * some data to it and now need to zero the end of the folio (and flush
+ * the dcache), you can use this function.  If you do not have the
+ * folio kmapped (eg the folio has been partially populated by DMA),
+ * use folio_zero_range() or folio_zero_segment() instead.
+ *
+ * Return: An address which can be passed to kunmap_local().
+ */
+static inline __must_check void *folio_zero_tail(struct folio *folio,
+		size_t offset, void *kaddr)
+{
+	size_t len = folio_size(folio) - offset;
+
+	if (folio_test_highmem(folio)) {
+		size_t max = PAGE_SIZE - offset_in_page(offset);
+
+		while (len > max) {
+			memset(kaddr, 0, max);
+			kunmap_local(kaddr);
+			len -= max;
+			offset += max;
+			max = PAGE_SIZE;
+			kaddr = kmap_local_folio(folio, offset);
+		}
+	}
+
+	memset(kaddr, 0, len);
+	flush_dcache_folio(folio);
+
+	return kaddr;
+}
+
 /**
  * memcpy_from_file_folio - Copy some bytes from a file folio.
  * @to: The destination buffer.