[PATCH 1/2] fs/buffer.c: allocate buffer cache from non-movable area

[PATCH 1/2] fs/buffer.c: allocate buffer cache from non-movable area

[Gioh Kim]

A buffer cache is allocated from movable area
because it is referred for a while and released soon.
But some filesystems are taking buffer cache for a long time
and it can disturb page migration.

A new API should be introduced to allocate buffer cache from
non-movable area.

Signed-off-by: Gioh Kim <gioh.kim@lge.com>
---
 fs/buffer.c                 |   39 ++++++++++++++++++++++++++++++++-------
 include/linux/buffer_head.h |    8 ++++++++
 2 files changed, 40 insertions(+), 7 deletions(-)

diff --git a/fs/buffer.c b/fs/buffer.c
index 8f05111..8c7ed02 100644
--- a/fs/buffer.c
+++ b/fs/buffer.c
@@ -993,7 +993,7 @@ init_page_buffers(struct page *page, struct block_device *bdev,
  */
 static int
 grow_dev_page(struct block_device *bdev, sector_t block,
-               pgoff_t index, int size, int sizebits)
+             pgoff_t index, int size, int sizebits, gfp_t movable_mask)
 {
        struct inode *inode = bdev->bd_inode;
        struct page *page;
@@ -1003,7 +1003,8 @@ grow_dev_page(struct block_device *bdev, sector_t block,
        gfp_t gfp_mask;

        gfp_mask = mapping_gfp_mask(inode->i_mapping) & ~__GFP_FS;
-       gfp_mask |= __GFP_MOVABLE;
+       if (movable_mask & __GFP_MOVABLE)
+               gfp_mask |= __GFP_MOVABLE;
        /*
         * XXX: __getblk_slow() can not really deal with failure and
         * will endlessly loop on improvised global reclaim.  Prefer
@@ -1058,7 +1059,8 @@ failed:
  * that page was dirty, the buffers are set dirty also.
  */
 static int
-grow_buffers(struct block_device *bdev, sector_t block, int size)
+grow_buffers(struct block_device *bdev, sector_t block,
+            int size, gfp_t movable_mask)
 {
        pgoff_t index;
        int sizebits;
@@ -1085,11 +1087,12 @@ grow_buffers(struct block_device *bdev, sector_t block, int size)
        }

        /* Create a page with the proper size buffers.. */
-       return grow_dev_page(bdev, block, index, size, sizebits);
+       return grow_dev_page(bdev, block, index, size, sizebits, movable_mask);
 }

 static struct buffer_head *
-__getblk_slow(struct block_device *bdev, sector_t block, int size)
+__getblk_slow(struct block_device *bdev, sector_t block,
+             int size, gfp_t movable_mask)
 {
        /* Size must be multiple of hard sectorsize */
        if (unlikely(size & (bdev_logical_block_size(bdev)-1) ||
@@ -1111,7 +1114,7 @@ __getblk_slow(struct block_device *bdev, sector_t block, int size)
                if (bh)
                        return bh;

-               ret = grow_buffers(bdev, block, size);
+               ret = grow_buffers(bdev, block, size, movable_mask);
                if (ret < 0)
                        return NULL;
                if (ret == 0)
@@ -1385,7 +1388,7 @@ __getblk(struct block_device *bdev, sector_t block, unsigned size)

        might_sleep();
        if (bh == NULL)
-               bh = __getblk_slow(bdev, block, size);
+               bh = __getblk_slow(bdev, block, size, __GFP_MOVABLE);
        return bh;
 }
 EXPORT_SYMBOL(__getblk);
@@ -1410,6 +1413,7 @@ EXPORT_SYMBOL(__breadahead);
  *  @size: size (in bytes) to read
  *
  *  Reads a specified block, and returns buffer head that contains it.
+ *  The page cache is allocated from movable area.
  *  It returns NULL if the block was unreadable.
  */
 struct buffer_head *
@@ -1423,6 +1427,27 @@ __bread(struct block_device *bdev, sector_t block, unsigned size)
 }
 EXPORT_SYMBOL(__bread);

+/**
+ *  __bread_nonmovable() - reads a specified block and returns the bh
+ *  @bdev: the block_device to read from
+ *  @block: number of block
+ *  @size: size (in bytes) to read
+ *
+ *  Reads a specified block, and returns buffer head that contains it.
+ *  The page cache is allocated from non-movable area.
+ *  It returns NULL if the block was unreadable.
+ */
+struct buffer_head *
+__bread_nonmovable(struct block_device *bdev, sector_t block, unsigned size)
+{
+       struct buffer_head *bh = __getblk_slow(bdev, block, size, 0);
+
+       if (likely(bh) && !buffer_uptodate(bh))
+               bh = __bread_slow(bh);
+       return bh;
+}
+EXPORT_SYMBOL(__bread_nonmovable);
+
 /*
  * invalidate_bh_lrus() is called rarely - but not only at unmount.
  * This doesn't race because it runs in each cpu either in irq
diff --git a/include/linux/buffer_head.h b/include/linux/buffer_head.h
index 324329c..cf8def2 100644
--- a/include/linux/buffer_head.h
+++ b/include/linux/buffer_head.h
@@ -181,6 +181,8 @@ void __brelse(struct buffer_head *);
 void __bforget(struct buffer_head *);
 void __breadahead(struct block_device *, sector_t block, unsigned int size);
 struct buffer_head *__bread(struct block_device *, sector_t block, unsigned size);
+struct buffer_head *__bread_nonmovable(struct block_device *,
+                                      sector_t block, unsigned size);
 void invalidate_bh_lrus(void);
 struct buffer_head *alloc_buffer_head(gfp_t gfp_flags);
 void free_buffer_head(struct buffer_head * bh);
@@ -298,6 +300,12 @@ sb_bread(struct super_block *sb, sector_t block)
        return __bread(sb->s_bdev, block, sb->s_blocksize);
 }

+static inline struct buffer_head *
+sb_bread_nonmovable(struct super_block *sb, sector_t block)
+{
+       return __bread_nonmovable(sb->s_bdev, block, sb->s_blocksize);
+}
+
 static inline void
 sb_breadahead(struct super_block *sb, sector_t block)
 {
--
1.7.9.5

[PATCH 1/2] fs/buffer.c: allocate buffer cache from non-movable area

[Gioh Kim]

A buffer cache is allocated from movable area
because it is referred for a while and released soon.
But some filesystems are taking buffer cache for a long time
and it can disturb page migration.

A new API should be introduced to allocate buffer cache from
non-movable area.

Signed-off-by: Gioh Kim <gioh.kim@lge.com>
---
 fs/buffer.c                 |   63 ++++++++++++++++++++++++++++++++++++++-----
 include/linux/buffer_head.h |   10 +++++++
 2 files changed, 66 insertions(+), 7 deletions(-)

diff --git a/fs/buffer.c b/fs/buffer.c
index 8f05111..7ef658f 100644
--- a/fs/buffer.c
+++ b/fs/buffer.c
@@ -993,7 +993,7 @@ init_page_buffers(struct page *page, struct block_device *bdev,
  */
 static int
 grow_dev_page(struct block_device *bdev, sector_t block,
-               pgoff_t index, int size, int sizebits)
+             pgoff_t index, int size, int sizebits, gfp_t movable_mask)
 {
        struct inode *inode = bdev->bd_inode;
        struct page *page;
@@ -1003,7 +1003,8 @@ grow_dev_page(struct block_device *bdev, sector_t block,
        gfp_t gfp_mask;

        gfp_mask = mapping_gfp_mask(inode->i_mapping) & ~__GFP_FS;
-       gfp_mask |= __GFP_MOVABLE;
+       if (movable_mask & __GFP_MOVABLE)
+               gfp_mask |= __GFP_MOVABLE;
        /*
         * XXX: __getblk_slow() can not really deal with failure and
         * will endlessly loop on improvised global reclaim.  Prefer
@@ -1058,7 +1059,8 @@ failed:
  * that page was dirty, the buffers are set dirty also.
  */
 static int
-grow_buffers(struct block_device *bdev, sector_t block, int size)
+grow_buffers(struct block_device *bdev, sector_t block,
+            int size, gfp_t movable_mask)
 {
        pgoff_t index;
        int sizebits;
@@ -1085,11 +1087,12 @@ grow_buffers(struct block_device *bdev, sector_t block, int size)
        }

        /* Create a page with the proper size buffers.. */
-       return grow_dev_page(bdev, block, index, size, sizebits);
+       return grow_dev_page(bdev, block, index, size, sizebits, movable_mask);
 }

 static struct buffer_head *
-__getblk_slow(struct block_device *bdev, sector_t block, int size)
+__getblk_slow(struct block_device *bdev, sector_t block,
+             int size, gfp_t movable_mask)
 {
        /* Size must be multiple of hard sectorsize */
        if (unlikely(size & (bdev_logical_block_size(bdev)-1) ||
@@ -1111,7 +1114,7 @@ __getblk_slow(struct block_device *bdev, sector_t block, int size)
                if (bh)
                        return bh;

-               ret = grow_buffers(bdev, block, size);
+               ret = grow_buffers(bdev, block, size, movable_mask);
                if (ret < 0)
                        return NULL;
                if (ret == 0)
@@ -1385,11 +1388,34 @@ __getblk(struct block_device *bdev, sector_t block, unsigned size)

        might_sleep();
        if (bh == NULL)
-               bh = __getblk_slow(bdev, block, size);
+               bh = __getblk_slow(bdev, block, size, __GFP_MOVABLE);
        return bh;
 }
 EXPORT_SYMBOL(__getblk);

+ /*
+ * __getblk_nonmovable will locate (and, if necessary, create) the buffer_head
+ * which corresponds to the passed block_device, block and size. The
+ * returned buffer has its reference count incremented.
+ *
+ * The page cache is allocated from non-movable area
+ * not to prevent page migration.
+ *
+ * __getblk()_nonmovable will lock up the machine
+ * if grow_dev_page's try_to_free_buffers() attempt is failing. FIXME, perhaps?
+ */
+struct buffer_head *
+__getblk_nonmovable(struct block_device *bdev, sector_t block, unsigned size)
+{
+       struct buffer_head *bh = __find_get_block(bdev, block, size);
+
+       might_sleep();
+       if (bh == NULL)
+               bh = __getblk_slow(bdev, block, size, 0);
+       return bh;
+}
+EXPORT_SYMBOL(__getblk_nonmovable);
+
 /*
  * Do async read-ahead on a buffer..
  */
@@ -1410,6 +1436,7 @@ EXPORT_SYMBOL(__breadahead);
  *  @size: size (in bytes) to read
  *
  *  Reads a specified block, and returns buffer head that contains it.
+ *  The page cache is allocated from movable area so that it can be migrated.
  *  It returns NULL if the block was unreadable.
  */
 struct buffer_head *
@@ -1423,6 +1450,28 @@ __bread(struct block_device *bdev, sector_t block, unsigned size)
 }
 EXPORT_SYMBOL(__bread);

+/**
+ *  __bread_nonmovable() - reads a specified block and returns the bh
+ *  @bdev: the block_device to read from
+ *  @block: number of block
+ *  @size: size (in bytes) to read
+ *
+ *  Reads a specified block, and returns buffer head that contains it.
+ *  The page cache is allocated from non-movable area
+ *  not to prevent page migration.
+ *  It returns NULL if the block was unreadable.
+ */
+struct buffer_head *
+__bread_nonmovable(struct block_device *bdev, sector_t block, unsigned size)
+{
+       struct buffer_head *bh = __getblk_slow(bdev, block, size, 0);
+
+       if (likely(bh) && !buffer_uptodate(bh))
+               bh = __bread_slow(bh);
+       return bh;
+}
+EXPORT_SYMBOL(__bread_nonmovable);
+
 /*
  * invalidate_bh_lrus() is called rarely - but not only at unmount.
  * This doesn't race because it runs in each cpu either in irq
diff --git a/include/linux/buffer_head.h b/include/linux/buffer_head.h
index 324329c..3f52370 100644
--- a/include/linux/buffer_head.h
+++ b/include/linux/buffer_head.h
@@ -177,10 +177,14 @@ struct buffer_head *__find_get_block(struct block_device *bdev, sector_t block,
                        unsigned size);
 struct buffer_head *__getblk(struct block_device *bdev, sector_t block,
                        unsigned size);
+struct buffer_head *__getblk_nonmovable(struct block_device *bdev,
+                                       sector_t block, unsigned size);
 void __brelse(struct buffer_head *);
 void __bforget(struct buffer_head *);
 void __breadahead(struct block_device *, sector_t block, unsigned int size);
 struct buffer_head *__bread(struct block_device *, sector_t block, unsigned size);
+struct buffer_head *__bread_nonmovable(struct block_device *,
+                                      sector_t block, unsigned size);
 void invalidate_bh_lrus(void);
 struct buffer_head *alloc_buffer_head(gfp_t gfp_flags);
 void free_buffer_head(struct buffer_head * bh);
@@ -298,6 +302,12 @@ sb_bread(struct super_block *sb, sector_t block)
        return __bread(sb->s_bdev, block, sb->s_blocksize);
 }

+static inline struct buffer_head *
+sb_bread_nonmovable(struct super_block *sb, sector_t block)
+{
+       return __bread_nonmovable(sb->s_bdev, block, sb->s_blocksize);
+}
+
 static inline void
 sb_breadahead(struct super_block *sb, sector_t block)
 {
--
1.7.9.5

Re: [PATCH 1/2] fs/buffer.c: allocate buffer cache from non-movable area

[Gioh Kim]

I'm sorry for a typo at the title.
It is 1/3, not 1/2.

Re: [PATCH 1/2] fs/buffer.c: allocate buffer cache from non-movable area

[Andrew Morton]

On Thu, 14 Aug 2014 14:15:40 +0900 Gioh Kim <gioh.kim@lge.com> wrote:

> A buffer cache is allocated from movable area
> because it is referred for a while and released soon.
> But some filesystems are taking buffer cache for a long time
> and it can disturb page migration.
> 
> A new API should be introduced to allocate buffer cache from
> non-movable area.

I think the API could and should be more flexible than this.

Rather than making the API be "movable or not movable", let's permit
callers to specify the gfp_t and leave it at that.  That way, if
someone later wants to allocate a buffer head with, I dunno,
__GFP_NOTRACK then they can do so.

So the word "movable" shouldn't appear in buffer.c at all, except in a
single place.

> --- a/fs/buffer.c
> +++ b/fs/buffer.c
> @@ -993,7 +993,7 @@ init_page_buffers(struct page *page, struct block_device *bdev,
>   */
>  static int
>  grow_dev_page(struct block_device *bdev, sector_t block,
> -               pgoff_t index, int size, int sizebits)
> +             pgoff_t index, int size, int sizebits, gfp_t movable_mask)

s/movable_mask/gfp/

>  {
>         struct inode *inode = bdev->bd_inode;
>         struct page *page;
> @@ -1003,7 +1003,8 @@ grow_dev_page(struct block_device *bdev, sector_t block,
>         gfp_t gfp_mask;
> 
>         gfp_mask = mapping_gfp_mask(inode->i_mapping) & ~__GFP_FS;
> -       gfp_mask |= __GFP_MOVABLE;
> +       if (movable_mask & __GFP_MOVABLE)
> +               gfp_mask |= __GFP_MOVABLE;

This becomes

	gfp_mask |= gfp;

>         /*
>          * XXX: __getblk_slow() can not really deal with failure and
>          * will endlessly loop on improvised global reclaim.  Prefer
> @@ -1058,7 +1059,8 @@ failed:
>   * that page was dirty, the buffers are set dirty also.
>   */
>  static int
> -grow_buffers(struct block_device *bdev, sector_t block, int size)
> +grow_buffers(struct block_device *bdev, sector_t block,
> +            int size, gfp_t movable_mask)

gfp

>  {
>         pgoff_t index;
>         int sizebits;
> @@ -1085,11 +1087,12 @@ grow_buffers(struct block_device *bdev, sector_t block, int size)
>         }
> 
>         /* Create a page with the proper size buffers.. */
> -       return grow_dev_page(bdev, block, index, size, sizebits);
> +       return grow_dev_page(bdev, block, index, size, sizebits, movable_mask);
>  }
> 
>  static struct buffer_head *
> -__getblk_slow(struct block_device *bdev, sector_t block, int size)
> +__getblk_slow(struct block_device *bdev, sector_t block,
> +             int size, gfp_t movable_mask)

gfp

>  {
>         /* Size must be multiple of hard sectorsize */
>         if (unlikely(size & (bdev_logical_block_size(bdev)-1) ||
> @@ -1111,7 +1114,7 @@ __getblk_slow(struct block_device *bdev, sector_t block, int size)
>                 if (bh)
>                         return bh;
> 
> -               ret = grow_buffers(bdev, block, size);
> +               ret = grow_buffers(bdev, block, size, movable_mask);

gfp

>                 if (ret < 0)
>                         return NULL;
>                 if (ret == 0)
> @@ -1385,11 +1388,34 @@ __getblk(struct block_device *bdev, sector_t block, unsigned size)
> 
>         might_sleep();
>         if (bh == NULL)
> -               bh = __getblk_slow(bdev, block, size);
> +               bh = __getblk_slow(bdev, block, size, __GFP_MOVABLE);

Here is the place where buffer.c. mentions "movable".

>         return bh;
>  }
>  EXPORT_SYMBOL(__getblk);
> 
> + /*
> + * __getblk_nonmovable will locate (and, if necessary, create) the buffer_head
> + * which corresponds to the passed block_device, block and size. The
> + * returned buffer has its reference count incremented.
> + *
> + * The page cache is allocated from non-movable area
> + * not to prevent page migration.
> + *
> + * __getblk()_nonmovable will lock up the machine
> + * if grow_dev_page's try_to_free_buffers() attempt is failing. FIXME, perhaps?
> + */
> +struct buffer_head *
> +__getblk_nonmovable(struct block_device *bdev, sector_t block, unsigned size)
> +{
> +       struct buffer_head *bh = __find_get_block(bdev, block, size);
> +
> +       might_sleep();
> +       if (bh == NULL)
> +               bh = __getblk_slow(bdev, block, size, 0);
> +       return bh;
> +}
> +EXPORT_SYMBOL(__getblk_nonmovable);

Suggest this be called __getblk_gfp(bdev, block, size, gfp) and then
__getblk() be changed to call __getblk_gfp(..., __GFP_MOVABLE).

We could then write a __getblk_nonmovable() which calls __getblk_gfp()
(a static inlined one-line function) or we can just call
__getblk_gfp(..., 0) directly from filesystems.

> @@ -1423,6 +1450,28 @@ __bread(struct block_device *bdev, sector_t block, unsigned size)
>  }
>  EXPORT_SYMBOL(__bread);
> 
> +/**
> + *  __bread_nonmovable() - reads a specified block and returns the bh
> + *  @bdev: the block_device to read from
> + *  @block: number of block
> + *  @size: size (in bytes) to read
> + *
> + *  Reads a specified block, and returns buffer head that contains it.
> + *  The page cache is allocated from non-movable area
> + *  not to prevent page migration.
> + *  It returns NULL if the block was unreadable.
> + */
> +struct buffer_head *
> +__bread_nonmovable(struct block_device *bdev, sector_t block, unsigned size)
> +{
> +       struct buffer_head *bh = __getblk_slow(bdev, block, size, 0);
> +
> +       if (likely(bh) && !buffer_uptodate(bh))
> +               bh = __bread_slow(bh);
> +       return bh;
> +}
> +EXPORT_SYMBOL(__bread_nonmovable);

Treat this in the same fashion as __getblk_nonmovable().

Re: [PATCH 1/2] fs/buffer.c: allocate buffer cache from non-movable area

[Gioh Kim]

2014-08-15 오전 6:22, Andrew Morton 쓴 글:
> On Thu, 14 Aug 2014 14:15:40 +0900 Gioh Kim <gioh.kim@lge.com> wrote:
>
>> A buffer cache is allocated from movable area
>> because it is referred for a while and released soon.
>> But some filesystems are taking buffer cache for a long time
>> and it can disturb page migration.
>>
>> A new API should be introduced to allocate buffer cache from
>> non-movable area.
>
> I think the API could and should be more flexible than this.
>
> Rather than making the API be "movable or not movable", let's permit
> callers to specify the gfp_t and leave it at that.  That way, if
> someone later wants to allocate a buffer head with, I dunno,
> __GFP_NOTRACK then they can do so.
>
> So the word "movable" shouldn't appear in buffer.c at all, except in a
> single place.

Absolutely I agree with you.
If filesystem developers agree this patch I will send 2nd patch that applies your ideas.

Thank you for your advices.

>
>> --- a/fs/buffer.c
>> +++ b/fs/buffer.c
>> @@ -993,7 +993,7 @@ init_page_buffers(struct page *page, struct block_device *bdev,
>>    */
>>   static int
>>   grow_dev_page(struct block_device *bdev, sector_t block,
>> -               pgoff_t index, int size, int sizebits)
>> +             pgoff_t index, int size, int sizebits, gfp_t movable_mask)
>
> s/movable_mask/gfp/

I got it.

>
>>   {
>>          struct inode *inode = bdev->bd_inode;
>>          struct page *page;
>> @@ -1003,7 +1003,8 @@ grow_dev_page(struct block_device *bdev, sector_t block,
>>          gfp_t gfp_mask;
>>
>>          gfp_mask = mapping_gfp_mask(inode->i_mapping) & ~__GFP_FS;
>> -       gfp_mask |= __GFP_MOVABLE;
>> +       if (movable_mask & __GFP_MOVABLE)
>> +               gfp_mask |= __GFP_MOVABLE;
>
> This becomes
>
> 	gfp_mask |= gfp;

I got it.

>
>>          /*
>>           * XXX: __getblk_slow() can not really deal with failure and
>>           * will endlessly loop on improvised global reclaim.  Prefer
>> @@ -1058,7 +1059,8 @@ failed:
>>    * that page was dirty, the buffers are set dirty also.
>>    */
>>   static int
>> -grow_buffers(struct block_device *bdev, sector_t block, int size)
>> +grow_buffers(struct block_device *bdev, sector_t block,
>> +            int size, gfp_t movable_mask)
>
> gfp
>
>>   {
>>          pgoff_t index;
>>          int sizebits;
>> @@ -1085,11 +1087,12 @@ grow_buffers(struct block_device *bdev, sector_t block, int size)
>>          }
>>
>>          /* Create a page with the proper size buffers.. */
>> -       return grow_dev_page(bdev, block, index, size, sizebits);
>> +       return grow_dev_page(bdev, block, index, size, sizebits, movable_mask);
>>   }
>>
>>   static struct buffer_head *
>> -__getblk_slow(struct block_device *bdev, sector_t block, int size)
>> +__getblk_slow(struct block_device *bdev, sector_t block,
>> +             int size, gfp_t movable_mask)
>
> gfp
>
>>   {
>>          /* Size must be multiple of hard sectorsize */
>>          if (unlikely(size & (bdev_logical_block_size(bdev)-1) ||
>> @@ -1111,7 +1114,7 @@ __getblk_slow(struct block_device *bdev, sector_t block, int size)
>>                  if (bh)
>>                          return bh;
>>
>> -               ret = grow_buffers(bdev, block, size);
>> +               ret = grow_buffers(bdev, block, size, movable_mask);
>
> gfp
>
>>                  if (ret < 0)
>>                          return NULL;
>>                  if (ret == 0)
>> @@ -1385,11 +1388,34 @@ __getblk(struct block_device *bdev, sector_t block, unsigned size)
>>
>>          might_sleep();
>>          if (bh == NULL)
>> -               bh = __getblk_slow(bdev, block, size);
>> +               bh = __getblk_slow(bdev, block, size, __GFP_MOVABLE);
>
> Here is the place where buffer.c. mentions "movable".

I got it.

>
>>          return bh;
>>   }
>>   EXPORT_SYMBOL(__getblk);
>>
>> + /*
>> + * __getblk_nonmovable will locate (and, if necessary, create) the buffer_head
>> + * which corresponds to the passed block_device, block and size. The
>> + * returned buffer has its reference count incremented.
>> + *
>> + * The page cache is allocated from non-movable area
>> + * not to prevent page migration.
>> + *
>> + * __getblk()_nonmovable will lock up the machine
>> + * if grow_dev_page's try_to_free_buffers() attempt is failing. FIXME, perhaps?
>> + */
>> +struct buffer_head *
>> +__getblk_nonmovable(struct block_device *bdev, sector_t block, unsigned size)
>> +{
>> +       struct buffer_head *bh = __find_get_block(bdev, block, size);
>> +
>> +       might_sleep();
>> +       if (bh == NULL)
>> +               bh = __getblk_slow(bdev, block, size, 0);
>> +       return bh;
>> +}
>> +EXPORT_SYMBOL(__getblk_nonmovable);
>
> Suggest this be called __getblk_gfp(bdev, block, size, gfp) and then
> __getblk() be changed to call __getblk_gfp(..., __GFP_MOVABLE).
>
> We could then write a __getblk_nonmovable() which calls __getblk_gfp()
> (a static inlined one-line function) or we can just call
> __getblk_gfp(..., 0) directly from filesystems.

I got it.


>
>> @@ -1423,6 +1450,28 @@ __bread(struct block_device *bdev, sector_t block, unsigned size)
>>   }
>>   EXPORT_SYMBOL(__bread);
>>
>> +/**
>> + *  __bread_nonmovable() - reads a specified block and returns the bh
>> + *  @bdev: the block_device to read from
>> + *  @block: number of block
>> + *  @size: size (in bytes) to read
>> + *
>> + *  Reads a specified block, and returns buffer head that contains it.
>> + *  The page cache is allocated from non-movable area
>> + *  not to prevent page migration.
>> + *  It returns NULL if the block was unreadable.
>> + */
>> +struct buffer_head *
>> +__bread_nonmovable(struct block_device *bdev, sector_t block, unsigned size)
>> +{
>> +       struct buffer_head *bh = __getblk_slow(bdev, block, size, 0);
>> +
>> +       if (likely(bh) && !buffer_uptodate(bh))
>> +               bh = __bread_slow(bh);
>> +       return bh;
>> +}
>> +EXPORT_SYMBOL(__bread_nonmovable);
>
> Treat this in the same fashion as __getblk_nonmovable().

I got it.

>
>
>
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