md/raid5: let multiple devices of stripe_head share page
In current implementation, grow_buffers() uses alloc_page() to
allocate the buffers for each stripe_head, i.e. allocate a page
for each dev[i] in stripe_head.
After setting stripe_size as a configurable value by writing
sysfs entry, it means that we always allocate 64K buffers, but
just use 4K of them when stripe_size is 4K in 64KB arm64.
To avoid wasting memory, we try to let multiple sh->dev share
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:
A new 'pages' array will be added into stripe_head to record shared
page used by this stripe_head. Allocate them when grow_buffers()
and free them when shrink_buffers().
After trying to share page, the users of sh->dev[i].page need to take
care of the related page offset: page of issued bio and page passed
to xor compution functions. But thanks for previous different page offset
supported. Here, we just need to set correct dev[i].offset.
Signed-off-by: Yufen Yu <yuyufen@huawei.com> Signed-off-by: Song Liu <songliubraving@fb.com>