diff mbox series

[v3,2/2] btrfs: allocate page arrays using bulk page allocator

Message ID ede1d39f7878ee2ed12c1526cc2ec358a2d862cf.1648669832.git.sweettea-kernel@dorminy.me (mailing list archive)
State New, archived
Headers show
Series btrfs: allocate page arrays more efficiently | expand

Commit Message

Sweet Tea Dorminy March 30, 2022, 8:11 p.m. UTC 
While calling alloc_page() in a loop is an effective way to populate an
array of pages, the kernel provides a method to allocate pages in bulk.
alloc_pages_bulk_array() populates the NULL slots in a page array, trying to
grab more than one page at a time.

Unfortunately, it doesn't guarantee allocating all slots in the array,
but it's easy to call it in a loop and return an error if no progress
occurs. Similar code can be found in xfs/xfs_buf.c:xfs_buf_alloc_pages().

Signed-off-by: Sweet Tea Dorminy <sweettea-kernel@dorminy.me>
Reviewed-by: Nikolay Borisov <nborisov@suse.com>
---
Changes in v3:
 - Added a newline after variable declaration
Changes in v2:
 - Moved from ctree.c to extent_io.c
---
 fs/btrfs/extent_io.c | 24 +++++++++++++++---------
 1 file changed, 15 insertions(+), 9 deletions(-)

Comments

David Sterba March 31, 2022, 5:35 p.m. UTC | #1 
On Wed, Mar 30, 2022 at 04:11:23PM -0400, Sweet Tea Dorminy wrote:
> While calling alloc_page() in a loop is an effective way to populate an
> array of pages, the kernel provides a method to allocate pages in bulk.
> alloc_pages_bulk_array() populates the NULL slots in a page array, trying to
> grab more than one page at a time.
> 
> Unfortunately, it doesn't guarantee allocating all slots in the array,
> but it's easy to call it in a loop and return an error if no progress
> occurs. Similar code can be found in xfs/xfs_buf.c:xfs_buf_alloc_pages().
> 
> Signed-off-by: Sweet Tea Dorminy <sweettea-kernel@dorminy.me>
> Reviewed-by: Nikolay Borisov <nborisov@suse.com>
> ---
> Changes in v3:
>  - Added a newline after variable declaration
> Changes in v2:
>  - Moved from ctree.c to extent_io.c
> ---
>  fs/btrfs/extent_io.c | 24 +++++++++++++++---------
>  1 file changed, 15 insertions(+), 9 deletions(-)
> 
> diff --git a/fs/btrfs/extent_io.c b/fs/btrfs/extent_io.c
> index ab4c1c4d1b59..b268e47aa2b7 100644
> --- a/fs/btrfs/extent_io.c
> +++ b/fs/btrfs/extent_io.c
> @@ -3144,19 +3144,25 @@ static void end_bio_extent_readpage(struct bio *bio)
>   */
>  int btrfs_alloc_page_array(unsigned long nr_pages, struct page **page_array)
>  {
> -	int i;
> +	long allocated = 0;
> +
> +	for (;;) {
> +		long last = allocated;
>  
> -	for (i = 0; i < nr_pages; i++) {
> -		struct page *page;
> +		allocated = alloc_pages_bulk_array(GFP_NOFS, nr_pages,
> +						   page_array);
> +		if (allocated == nr_pages)
> +			return 0;
>  
> -		if (page_array[i])
> +		if (allocated != last)
>  			continue;
> -		page = alloc_page(GFP_NOFS);
> -		if (!page)
> -			return -ENOMEM;
> -		page_array[i] = page;
> +		/*
> +		 * During this iteration, no page could be allocated, even
> +		 * though alloc_pages_bulk_array() falls back to alloc_page()
> +		 * if  it could not bulk-allocate. So we must be out of memory.
> +		 */
> +		return -ENOMEM;
>  	}

I find the way the loop is structured a bit cumbersome so I'd suggest to
rewrite it as:

int btrfs_alloc_page_array(unsigned int nr_pages, struct page **page_array)
{
        unsigned int allocated;

        for (allocated = 0; allocated < nr_pages;) {
                unsigned int last = allocated;

                allocated = alloc_pages_bulk_array(GFP_NOFS, nr_pages, page_array);

                /*
                 * During this iteration, no page could be allocated, even
                 * though alloc_pages_bulk_array() falls back to alloc_page()
                 * if  it could not bulk-allocate. So we must be out of memory.
                 */
                if (allocated == last)
                        return -ENOMEM;
        }
        return 0;
}

Also in the xfs code there's memalloc_retry_wait() which is supposed to be
called when repeated memory allocation is retried. What was the reason
you removed it?
Sweet Tea Dorminy March 31, 2022, 6:19 p.m. UTC | #2 
On 3/31/22 13:35, David Sterba wrote:
> On Wed, Mar 30, 2022 at 04:11:23PM -0400, Sweet Tea Dorminy wrote:
>> While calling alloc_page() in a loop is an effective way to populate an
>> array of pages, the kernel provides a method to allocate pages in bulk.
>> alloc_pages_bulk_array() populates the NULL slots in a page array, trying to
>> grab more than one page at a time.
>>
>> Unfortunately, it doesn't guarantee allocating all slots in the array,
>> but it's easy to call it in a loop and return an error if no progress
>> occurs. Similar code can be found in xfs/xfs_buf.c:xfs_buf_alloc_pages().
>>
>> Signed-off-by: Sweet Tea Dorminy <sweettea-kernel@dorminy.me>
>> Reviewed-by: Nikolay Borisov <nborisov@suse.com>
>> ---
>> Changes in v3:
>>   - Added a newline after variable declaration
>> Changes in v2:
>>   - Moved from ctree.c to extent_io.c
>> ---
>>   fs/btrfs/extent_io.c | 24 +++++++++++++++---------
>>   1 file changed, 15 insertions(+), 9 deletions(-)
>>
>> diff --git a/fs/btrfs/extent_io.c b/fs/btrfs/extent_io.c
>> index ab4c1c4d1b59..b268e47aa2b7 100644
>> --- a/fs/btrfs/extent_io.c
>> +++ b/fs/btrfs/extent_io.c
>> @@ -3144,19 +3144,25 @@ static void end_bio_extent_readpage(struct bio *bio)
>>    */
>>   int btrfs_alloc_page_array(unsigned long nr_pages, struct page **page_array)
>>   {
>> -	int i;
>> +	long allocated = 0;
>> +
>> +	for (;;) {
>> +		long last = allocated;
>>   
>> -	for (i = 0; i < nr_pages; i++) {
>> -		struct page *page;
>> +		allocated = alloc_pages_bulk_array(GFP_NOFS, nr_pages,
>> +						   page_array);
>> +		if (allocated == nr_pages)
>> +			return 0;
>>   
>> -		if (page_array[i])
>> +		if (allocated != last)
>>   			continue;
>> -		page = alloc_page(GFP_NOFS);
>> -		if (!page)
>> -			return -ENOMEM;
>> -		page_array[i] = page;
>> +		/*
>> +		 * During this iteration, no page could be allocated, even
>> +		 * though alloc_pages_bulk_array() falls back to alloc_page()
>> +		 * if  it could not bulk-allocate. So we must be out of memory.
>> +		 */
>> +		return -ENOMEM;
>>   	}
> 
> I find the way the loop is structured a bit cumbersome so I'd suggest to
> rewrite it as:
> 
> int btrfs_alloc_page_array(unsigned int nr_pages, struct page **page_array)
> {
>          unsigned int allocated;
> 
>          for (allocated = 0; allocated < nr_pages;) {
>                  unsigned int last = allocated;
> 
>                  allocated = alloc_pages_bulk_array(GFP_NOFS, nr_pages, page_array);
> 
>                  /*
>                   * During this iteration, no page could be allocated, even
>                   * though alloc_pages_bulk_array() falls back to alloc_page()
>                   * if  it could not bulk-allocate. So we must be out of memory.
>                   */
>                  if (allocated == last)
>                          return -ENOMEM;
>          }
>          return 0;
> }
Sounds good, I'll amend it that way.

> 
> Also in the xfs code there's memalloc_retry_wait() which is supposed to be
> called when repeated memory allocation is retried. What was the reason
> you removed it?

Trying to keep the behavior as close as possible to the existing behavior.

The current behavior of each alloc_page loop is to fail if alloc_page() 
fails; in the worst case, alloc_pages_bulk_array() calls alloc_page() 
after trying to get a batch, so I figured the worst case is still 
basically a loop calling alloc_page() and failing if it ever fails.

Reading up on it, though, arguably the memalloc_retry_wait() should 
already be in all the callsites, so maybe I should insert a patch in the 
middle that just adds the memalloc_retry_wait() into 
btrfs_alloc_page_array()? Since it's an orthogonal fixup to either the 
refactoring or the conversion to alloc_pages_bulk_array()?

Thanks!

Sweet Tea
David Sterba April 1, 2022, 1:26 p.m. UTC | #3 
On Thu, Mar 31, 2022 at 02:19:07PM -0400, Sweet Tea Dorminy wrote:
> > Also in the xfs code there's memalloc_retry_wait() which is supposed to be
> > called when repeated memory allocation is retried. What was the reason
> > you removed it?
> 
> Trying to keep the behavior as close as possible to the existing behavior.

I see, makes sense.

> The current behavior of each alloc_page loop is to fail if alloc_page() 
> fails; in the worst case, alloc_pages_bulk_array() calls alloc_page() 
> after trying to get a batch, so I figured the worst case is still 
> basically a loop calling alloc_page() and failing if it ever fails.
> 
> Reading up on it, though, arguably the memalloc_retry_wait() should 
> already be in all the callsites, so maybe I should insert a patch in the 
> middle that just adds the memalloc_retry_wait() into 
> btrfs_alloc_page_array()? Since it's an orthogonal fixup to either the 
> refactoring or the conversion to alloc_pages_bulk_array()?

Yeah a separate patch with the reasonig about the potential effects is
better. The v3 is now in misc-next with the suggested loop refactoring,
so please send the memalloc_retry_wait() update on top of that. Thanks.
diff mbox series

Patch

diff --git a/fs/btrfs/extent_io.c b/fs/btrfs/extent_io.c
index ab4c1c4d1b59..b268e47aa2b7 100644
--- a/fs/btrfs/extent_io.c
+++ b/fs/btrfs/extent_io.c
@@ -3144,19 +3144,25 @@  static void end_bio_extent_readpage(struct bio *bio)
  */
 int btrfs_alloc_page_array(unsigned long nr_pages, struct page **page_array)
 {
-	int i;
+	long allocated = 0;
+
+	for (;;) {
+		long last = allocated;
 
-	for (i = 0; i < nr_pages; i++) {
-		struct page *page;
+		allocated = alloc_pages_bulk_array(GFP_NOFS, nr_pages,
+						   page_array);
+		if (allocated == nr_pages)
+			return 0;
 
-		if (page_array[i])
+		if (allocated != last)
 			continue;
-		page = alloc_page(GFP_NOFS);
-		if (!page)
-			return -ENOMEM;
-		page_array[i] = page;
+		/*
+		 * During this iteration, no page could be allocated, even
+		 * though alloc_pages_bulk_array() falls back to alloc_page()
+		 * if  it could not bulk-allocate. So we must be out of memory.
+		 */
+		return -ENOMEM;
 	}
-	return 0;
 }
 
 /*