On Fri 28 Feb 2025 at 15:46, Xiao Ni <xni@xxxxxxxxxx> wrote:
On Fri, Feb 28, 2025 at 10:07 AM Yu Kuai
<yukuai1@xxxxxxxxxxxxxxx> wrote:
Hi,
在 2025/02/25 23:32, Nigel Croxon 写道:
> - md_super_write(mddev, rdev, sboff + ps, (int) size,
> page);
> + md_super_write(mddev, rdev, sboff + ps,
> (int)min(size,
> bitmap_limit), page);
> return 0;
>
> This patch still will attempt to send writes greater than a
> page using
> only a single page pointer for multi-page bitmaps. The bitmap
> uses an
> array (the filemap) of pages when the bitmap cannot fit in a
> single
> page. These pages are allocated separately and not guaranteed
> to be
> contiguous. So this patch will keep writes in a multi-page
> bitmap from
> trashing data beyond the bitmap, but can create writes which
> corrupt
> other parts of the bitmap with random memory.
Is this problem introduced by:
8745faa95611 ("md: Use optimal I/O size for last bitmap page")
I think so.
>
> The opt using logic in this function is fundamentally flawed
> as
> __write_sb_page should never send a write bigger than a page
> at a time.
> It would need to use a new interface which can build
> multi-page bio and
> not md_super_write() if it wanted to send multi-page I/Os.
I argree. And I don't understand that patch yet, it said:
If the bitmap space has enough room, size the I/O for the last
bitmap
page write to the optimal I/O size for the storage device.
Does this mean, for example, if bitmap space is 128k, while
there is
only one page, means 124k is not used. In this case, if device
opt io
size is 128k, this patch will choose to issue 128k IO instead
of just
4k IO? And how can this improve performance ...
It looks like it does as you mentioned above. Through the commit
8745faa95611 message, the io size(3584 bytes, 7 sectors) is
smaller
than 4K. Without the patch 8745faa95611, the io size is round
up with
bdev_logical_block_size(bdev). If we round up io size with
PAGE_SIZE,
can it fix the performance problem? Because bitmap space is
4K/64K/128K, if it doesn't affect performance only changing the
round
up value to PAGE_SIZE, it'll easy to fix this problem.
I'm afiraid that the perf will drop if rounding up io size to 4K
for devices
optimal I/O size smaller than 4K. IMO better version of
md_super_write to is
necessary to handle bitmap pages as Nigel said.
--
Su
Best Regards
Xiao
Thanks,
Kuai
