Re: [PATCH 05/13] btrfs: add btrfs_read_repair_ctrl to record corrupted sectors

[Qu Wenruo <quwenruo.btrfs@gmx.com>]

On 2022/5/3 23:06, Christoph Hellwig wrote:
> On Tue, May 03, 2022 at 02:49:49PM +0800, Qu Wenruo wrote:
>> - For the extra bitmaps
>>    We use btrfs_bio::read_repair_{cur|prev}_bitmap.
>>    This will add extra 16 bytes to btrfs_bio.
>>
>>    This is unavoidable, or we need to allocate extra memory at endio
>>    function which can be dead lock prone.
>>
>>    Furthermore, pre-allocating bitmap has a hidden benefit, if we're
>>    submitting a large read and failed to allocated enough memory for
>>    bitmap, we fail the bio, and VFS layer will automatically retry
>>    with smaller read range, giving us a higher chance to succeed in
>>    next try.
>
> This is a really bad idea.  Now every read needs to pay the quite
> large price for corner case of a read repair.  As I mentioned last time
> I think a mempool with a few entries that any read repair can dip into
> is a much better choice here.

The problem is, can mempool provide a variable length entry?

Thanks,
Qu