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.
Best Regards
Xiao
> Thanks,
> Kuai
>
>
