在 2020/7/3 6:56, Song Liu 写道:

On Thu, Jul 2, 2020 at 5:05 AM Yufen Yu [off-list ref] wrote:

quoted

Since grow_buffers() uses alloc_page() to allocate the buffers for
each stripe_head(), means, it will allocate 64K buffers and just use
4K of them, after setting stripe_size as 4096.

To avoid wasting memory, we try to contain multiple 'page' of sh->dev
into one real page. That means, multiple sh->dev[i].page will point to
the only page with different offset. Example of 64K PAGE_SIZE and
4K stripe_size as following:

                     64K PAGE_SIZE
           +---+---+---+---+------------------------------+
           |   |   |   |   |
           |   |   |   |   |
           +-+-+-+-+-+-+-+-+------------------------------+
             ^   ^   ^   ^
             |   |   |   +----------------------------+
             |   |   |                                |
             |   |   +-------------------+            |
             |   |                       |            |
             |   +----------+            |            |
             |              |            |            |
             +-+            |            |            |
               |            |            |            |
         +-----+-----+------+-----+------+-----+------+------+
sh      | offset(0) | offset(4K) | offset(8K) | offset(12K) |
  +      +-----------+------------+------------+-------------+
  +----> dev[0].page  dev[1].page  dev[2].page  dev[3].page

After trying to share one page, the users of sh->dev[i].page need to
take care:

   1) When issue bio into stripe_head, bi_io_vec.bv_page will point to
      the page directly. So, we should make sure bv_offset to been set
      with correct offset.

   2) When compute xor, the page will be passed to computer function.
      So, we also need to pass offset of that page to computer. Let it
      compute correct location of each sh->dev[i].page.

This patch will add a new member of r5pages into stripe_head to manage
all pages needed by each sh->dev[i]. We also add 'offset' for each r5dev
so that users can get related page offset easily. And add helper function
to get page and it's index in r5pages array by disk index.

Signed-off-by: Yufen Yu <redacted>
---
  drivers/md/raid5.h | 61 ++++++++++++++++++++++++++++++++++++++++++++++
  1 file changed, 61 insertions(+)

[...]

quoted

+/*
+ * Return corresponding page index of r5pages array.
+ */
+static inline int raid5_get_page_index(struct stripe_head *sh, int disk_idx)
+{
+       struct r5conf *conf = sh->raid_conf;
+       int cnt;
+
+       WARN_ON(!sh->pages.page);
+       BUG_ON(conf->stripe_size > PAGE_SIZE);

We have too many of these WARN_ON() and BUG_ON().

Yes. Yufen, please avoid using BUG_ON() becuase we are reducing the 
usage of it and Linus hate it:

https://lkml.org/lkml/2013/5/17/254
https://linuxinsider.com/story/torvalds-blows-stack-over-buggy-new-kernel-83975.html
https://lore.kernel.org/patchwork/patch/568291/


Thanks,
Jason