[v8] iov_iter: Improve page extraction (pin or just list)

[PATCH v8 04/10] iomap: don't get an reference on ZERO_PAGE for direct I/O block zeroing

Posted by David Howells 3 years, 2 months ago

From: Christoph Hellwig <hch@lst.de>

ZERO_PAGE can't go away, no need to hold an extra reference.

Signed-off-by: Christoph Hellwig <hch@lst.de>
Signed-off-by: David Howells <dhowells@redhat.com>
---
 fs/iomap/direct-io.c | 2 +-
 1 file changed, 1 insertion(+), 1 deletion(-)

diff --git a/fs/iomap/direct-io.c b/fs/iomap/direct-io.c
index 9804714b1751..47db4ead1e74 100644
--- a/fs/iomap/direct-io.c
+++ b/fs/iomap/direct-io.c
@@ -202,7 +202,7 @@ static void iomap_dio_zero(const struct iomap_iter *iter, struct iomap_dio *dio,
 	bio->bi_private = dio;
 	bio->bi_end_io = iomap_dio_bio_end_io;
 
-	get_page(page);
+	bio_set_flag(bio, BIO_NO_PAGE_REF);
 	__bio_add_page(bio, page, len, 0);
 	iomap_dio_submit_bio(iter, dio, bio, pos);
 }

Re: [PATCH v8 04/10] iomap: don't get an reference on ZERO_PAGE for direct I/O block zeroing

Posted by David Hildenbrand 3 years, 2 months ago

On 23.01.23 18:30, David Howells wrote:
> From: Christoph Hellwig <hch@lst.de>
> 
> ZERO_PAGE can't go away, no need to hold an extra reference.
> 
> Signed-off-by: Christoph Hellwig <hch@lst.de>
> Signed-off-by: David Howells <dhowells@redhat.com>
> ---
>   fs/iomap/direct-io.c | 2 +-
>   1 file changed, 1 insertion(+), 1 deletion(-)
> 
> diff --git a/fs/iomap/direct-io.c b/fs/iomap/direct-io.c
> index 9804714b1751..47db4ead1e74 100644
> --- a/fs/iomap/direct-io.c
> +++ b/fs/iomap/direct-io.c
> @@ -202,7 +202,7 @@ static void iomap_dio_zero(const struct iomap_iter *iter, struct iomap_dio *dio,
>   	bio->bi_private = dio;
>   	bio->bi_end_io = iomap_dio_bio_end_io;
>   
> -	get_page(page);
> +	bio_set_flag(bio, BIO_NO_PAGE_REF);
>   	__bio_add_page(bio, page, len, 0);
>   	iomap_dio_submit_bio(iter, dio, bio, pos);
>   }
> 

Reviewed-by: David Hildenbrand <david@redhat.com>

-- 
Thanks,

David / dhildenb

Re: [PATCH v8 04/10] iomap: don't get an reference on ZERO_PAGE for direct I/O block zeroing

Posted by John Hubbard 3 years, 2 months ago

On 1/23/23 09:30, David Howells wrote:
> From: Christoph Hellwig <hch@lst.de>
> 
> ZERO_PAGE can't go away, no need to hold an extra reference.

That statement is true, but...

> 
> Signed-off-by: Christoph Hellwig <hch@lst.de>
> Signed-off-by: David Howells <dhowells@redhat.com>
> ---
>   fs/iomap/direct-io.c | 2 +-
>   1 file changed, 1 insertion(+), 1 deletion(-)
> 
> diff --git a/fs/iomap/direct-io.c b/fs/iomap/direct-io.c
> index 9804714b1751..47db4ead1e74 100644
> --- a/fs/iomap/direct-io.c
> +++ b/fs/iomap/direct-io.c
> @@ -202,7 +202,7 @@ static void iomap_dio_zero(const struct iomap_iter *iter, struct iomap_dio *dio,
>   	bio->bi_private = dio;
>   	bio->bi_end_io = iomap_dio_bio_end_io;
>   
> -	get_page(page);
> +	bio_set_flag(bio, BIO_NO_PAGE_REF);

...is it accurate to assume that the entire bio is pointing to the zero
page? I recall working through this area earlier last year, and ended up
just letting the zero page get pinned, and then unpinning upon release,
which is harmless.

I like your approach, if it is possible. I'm just not sure that it's
correct given that bio's can have more than one page.

thanks,
-- 
John Hubbard
NVIDIA

Re: [PATCH v8 04/10] iomap: don't get an reference on ZERO_PAGE for direct I/O block zeroing

Posted by Christoph Hellwig 3 years, 2 months ago

On Mon, Jan 23, 2023 at 06:42:28PM -0800, John Hubbard wrote:
> > @@ -202,7 +202,7 @@ static void iomap_dio_zero(const struct iomap_iter *iter, struct iomap_dio *dio,
> >   	bio->bi_private = dio;
> >   	bio->bi_end_io = iomap_dio_bio_end_io;
> > -	get_page(page);
> > +	bio_set_flag(bio, BIO_NO_PAGE_REF);
> 
> ...is it accurate to assume that the entire bio is pointing to the zero
> page? I recall working through this area earlier last year, and ended up
> just letting the zero page get pinned, and then unpinning upon release,
> which is harmless.

Yes, the bio is built 4 lines above what is quoted here, and submitted
right after it.  It only contains the ZERO_PAGE.

Re: [PATCH v8 04/10] iomap: don't get an reference on ZERO_PAGE for direct I/O block zeroing

Posted by John Hubbard 3 years, 2 months ago

On 1/23/23 21:59, Christoph Hellwig wrote:
> On Mon, Jan 23, 2023 at 06:42:28PM -0800, John Hubbard wrote:
>>> @@ -202,7 +202,7 @@ static void iomap_dio_zero(const struct iomap_iter *iter, struct iomap_dio *dio,
>>>   	bio->bi_private = dio;
>>>   	bio->bi_end_io = iomap_dio_bio_end_io;
>>> -	get_page(page);
>>> +	bio_set_flag(bio, BIO_NO_PAGE_REF);
>>
>> ...is it accurate to assume that the entire bio is pointing to the zero
>> page? I recall working through this area earlier last year, and ended up
>> just letting the zero page get pinned, and then unpinning upon release,
>> which is harmless.
> 
> Yes, the bio is built 4 lines above what is quoted here, and submitted
> right after it.  It only contains the ZERO_PAGE.

OK, yes. All good, then.

thanks,
-- 
John Hubbard
NVIDIA

Re: [PATCH v8 04/10] iomap: don't get an reference on ZERO_PAGE for direct I/O block zeroing

Posted by Christoph Hellwig 3 years, 2 months ago

On Mon, Jan 23, 2023 at 05:30:01PM +0000, David Howells wrote:
> From: Christoph Hellwig <hch@lst.de>
> 
> ZERO_PAGE can't go away, no need to hold an extra reference.
> 
> Signed-off-by: Christoph Hellwig <hch@lst.de>
> Signed-off-by: David Howells <dhowells@redhat.com>

I did originally attribute this to you as it's just extracted as
an intermedia step from your patch.  But I can live with it being
credited to me if you want.