2014-08-15 오전 6:22, Andrew Morton 쓴 글:

On Thu, 14 Aug 2014 14:15:40 +0900 Gioh Kim [off-list ref] wrote:

quoted

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.

quoted

--- 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.

quoted

  {
         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.

quoted

         /*
          * 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

quoted

  {
         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

quoted

  {
         /* 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

quoted

                 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.

quoted

         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.

quoted

@@ -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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