When allocating an area from a chunk, pcpu_block_update_hint_alloc()
is called to update chunk metadata, including chunk's and global
nr_empty_pop_pages. However, if the allocation is not atomic, some
blocks may not be populated with pages yet, while we still account it
here. The number of pages will be subtracted with pcpu_chunk_populated()
when populating pages.

Adding code comment to make that more understandable.

Signed-off-by: Baoquan He <bhe@redhat.com>
---
 mm/percpu.c | 14 +++++++++++---
 1 file changed, 11 insertions(+), 3 deletions(-)

diff --git a/mm/percpu.c b/mm/percpu.c
index a8121302a79c..09e407338573 100644
--- a/mm/percpu.c
+++ b/mm/percpu.c
@@ -831,13 +831,15 @@ static void pcpu_block_update_hint_alloc(struct pcpu_chunk *chunk, int bit_off,
 
 	/*
 	 * Update s_block.
-	 * block->first_free must be updated if the allocation takes its place.
-	 * If the allocation breaks the contig_hint, a scan is required to
-	 * restore this hint.
 	 */
 	if (s_block->contig_hint == PCPU_BITMAP_BLOCK_BITS)
 		nr_empty_pages++;
 
+	/*
+	 * block->first_free must be updated if the allocation takes its place.
+	 * If the allocation breaks the contig_hint, a scan is required to
+	 * restore this hint.
+	 */
 	if (s_off == s_block->first_free)
 		s_block->first_free = find_next_zero_bit(
 					pcpu_index_alloc_map(chunk, s_index),
@@ -912,6 +914,12 @@ static void pcpu_block_update_hint_alloc(struct pcpu_chunk *chunk, int bit_off,
 		}
 	}
 
+	/*
+	 * If the allocation is not atomic, some blocks may not
+	 * be populated with pages, while we account it here.
+	 * The number of pages will be subtracted with
+	 * pcpu_chunk_populated() when populating pages.
+	 */
 	if (nr_empty_pages)
 		pcpu_update_empty_pages(chunk, -nr_empty_pages);
 
-- 
2.34.1

On Mon, Oct 24, 2022 at 04:14:31PM +0800, Baoquan He wrote:
> When allocating an area from a chunk, pcpu_block_update_hint_alloc()
> is called to update chunk metadata, including chunk's and global
> nr_empty_pop_pages. However, if the allocation is not atomic, some
> blocks may not be populated with pages yet, while we still account it
> here. The number of pages will be subtracted with pcpu_chunk_populated()
> when populating pages.
> 
> Adding code comment to make that more understandable.
> 
> Signed-off-by: Baoquan He <bhe@redhat.com>
> ---
>  mm/percpu.c | 14 +++++++++++---
>  1 file changed, 11 insertions(+), 3 deletions(-)
> 
> diff --git a/mm/percpu.c b/mm/percpu.c
> index a8121302a79c..09e407338573 100644
> --- a/mm/percpu.c
> +++ b/mm/percpu.c
> @@ -831,13 +831,15 @@ static void pcpu_block_update_hint_alloc(struct pcpu_chunk *chunk, int bit_off,
>  
>  	/*
>  	 * Update s_block.
> -	 * block->first_free must be updated if the allocation takes its place.
> -	 * If the allocation breaks the contig_hint, a scan is required to
> -	 * restore this hint.
>  	 */
>  	if (s_block->contig_hint == PCPU_BITMAP_BLOCK_BITS)
>  		nr_empty_pages++;
>  
> +	/*
> +	 * block->first_free must be updated if the allocation takes its place.
> +	 * If the allocation breaks the contig_hint, a scan is required to
> +	 * restore this hint.
> +	 */
>  	if (s_off == s_block->first_free)
>  		s_block->first_free = find_next_zero_bit(
>  					pcpu_index_alloc_map(chunk, s_index),
> @@ -912,6 +914,12 @@ static void pcpu_block_update_hint_alloc(struct pcpu_chunk *chunk, int bit_off,
>  		}
>  	}
>  
> +	/*
> +	 * If the allocation is not atomic, some blocks may not
> +	 * be populated with pages, while we account it here.
> +	 * The number of pages will be subtracted with
> +	 * pcpu_chunk_populated() when populating pages.
> +	 */
>  	if (nr_empty_pages)
>  		pcpu_update_empty_pages(chunk, -nr_empty_pages);
>  
> -- 
> 2.34.1
> 

Heh, that's a little more subtle than I remember it being.

Acked-by: Dennis Zhou <dennis@kernel.org>

Thanks,
Dennis

On 10/24/22 at 09:56am, Dennis Zhou wrote:
> On Mon, Oct 24, 2022 at 04:14:31PM +0800, Baoquan He wrote:
> > When allocating an area from a chunk, pcpu_block_update_hint_alloc()
> > is called to update chunk metadata, including chunk's and global
> > nr_empty_pop_pages. However, if the allocation is not atomic, some
> > blocks may not be populated with pages yet, while we still account it
> > here. The number of pages will be subtracted with pcpu_chunk_populated()
> > when populating pages.
> > 
> > Adding code comment to make that more understandable.
> > 
> > Signed-off-by: Baoquan He <bhe@redhat.com>
> > ---
> >  mm/percpu.c | 14 +++++++++++---
> >  1 file changed, 11 insertions(+), 3 deletions(-)
> > 
> > diff --git a/mm/percpu.c b/mm/percpu.c
> > index a8121302a79c..09e407338573 100644
> > --- a/mm/percpu.c
> > +++ b/mm/percpu.c
> > @@ -831,13 +831,15 @@ static void pcpu_block_update_hint_alloc(struct pcpu_chunk *chunk, int bit_off,
> >  
> >  	/*
> >  	 * Update s_block.
> > -	 * block->first_free must be updated if the allocation takes its place.
> > -	 * If the allocation breaks the contig_hint, a scan is required to
> > -	 * restore this hint.
> >  	 */
> >  	if (s_block->contig_hint == PCPU_BITMAP_BLOCK_BITS)
> >  		nr_empty_pages++;
> >  
> > +	/*
> > +	 * block->first_free must be updated if the allocation takes its place.
> > +	 * If the allocation breaks the contig_hint, a scan is required to
> > +	 * restore this hint.
> > +	 */
> >  	if (s_off == s_block->first_free)
> >  		s_block->first_free = find_next_zero_bit(
> >  					pcpu_index_alloc_map(chunk, s_index),
> > @@ -912,6 +914,12 @@ static void pcpu_block_update_hint_alloc(struct pcpu_chunk *chunk, int bit_off,
> >  		}
> >  	}
> >  
> > +	/*
> > +	 * If the allocation is not atomic, some blocks may not
> > +	 * be populated with pages, while we account it here.
> > +	 * The number of pages will be subtracted with
> > +	 * pcpu_chunk_populated() when populating pages.
> > +	 */
> >  	if (nr_empty_pages)
> >  		pcpu_update_empty_pages(chunk, -nr_empty_pages);
> >  
> > -- 
> > 2.34.1
> > 
> 
> Heh, that's a little more subtle than I remember it being.
> 
> Acked-by: Dennis Zhou <dennis@kernel.org>

Thanks for reviewing, Dennis.

When I rechecked the code comment, I realized I had said the opposite in
v1 about the newly added code comment. It subtracts the nr_empty_pop_pages
in pcpu_block_update_hint_alloc() whether it's atomic allocation or not.
If non atomic case, the number will be added back in pcpu_chunk_populated()
when populating pages.

I have updated and resent v2, sorry for the mistake.

Thanks
Baoquan

When allocating an area from a chunk, pcpu_block_update_hint_alloc()
is called to update chunk metadata, including chunk's and global
nr_empty_pop_pages. However, if the allocation is not atomic, some
blocks may not be populated with pages yet, while we still subtract 
the number here. The number of pages will be added back with
pcpu_chunk_populated() when populating pages.

Adding code comment to make that more understandable.

Signed-off-by: Baoquan He <bhe@redhat.com>
---
v1->v2:
- I said the opposite in the v1 code comment. It subtracts the nr_empty_pop_pages
  whether it's atomic allocation or not. In non atomic case, the number
  will be added back in pcpu_chunk_populated() when populating pages.

 mm/percpu.c | 14 +++++++++++---
 1 file changed, 11 insertions(+), 3 deletions(-)

diff --git a/mm/percpu.c b/mm/percpu.c
index 68d5ba61c935..786c7a2eb4f0 100644
--- a/mm/percpu.c
+++ b/mm/percpu.c
@@ -831,13 +831,15 @@ static void pcpu_block_update_hint_alloc(struct pcpu_chunk *chunk, int bit_off,
 
 	/*
 	 * Update s_block.
-	 * block->first_free must be updated if the allocation takes its place.
-	 * If the allocation breaks the contig_hint, a scan is required to
-	 * restore this hint.
 	 */
 	if (s_block->contig_hint == PCPU_BITMAP_BLOCK_BITS)
 		nr_empty_pages++;
 
+	/*
+	 * block->first_free must be updated if the allocation takes its place.
+	 * If the allocation breaks the contig_hint, a scan is required to
+	 * restore this hint.
+	 */
 	if (s_off == s_block->first_free)
 		s_block->first_free = find_next_zero_bit(
 					pcpu_index_alloc_map(chunk, s_index),
@@ -912,6 +914,12 @@ static void pcpu_block_update_hint_alloc(struct pcpu_chunk *chunk, int bit_off,
 		}
 	}
 
+	/*
+	 * If the allocation is not atomic, some blocks may not be
+	 * populated with pages, while we account it here. The number
+	 * of pages will be added back with pcpu_chunk_populated()
+	 * when populating pages.
+	 */
 	if (nr_empty_pages)
 		pcpu_update_empty_pages(chunk, -nr_empty_pages);
 
-- 
2.34.1