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

[Andrew Morton <akpm@linux-foundation.org>]

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().