Re: [PATCH RFC 0/3] btrfs: refactor how we handle btrfs_io_context and slightly reduce memory usage for both btrfs_bio and btrfs_io_context

[Qu Wenruo <wqu@suse.com>]

On 2021/9/22 16:27, Qu Wenruo wrote:
> Currently btrfs_io_context is utilized as both bio->bi_private for
> mirrored stripes, and stripes mapping for RAID56.
> 
> This makes some members like btrfs_io_context::private to be there.
> 
> But in fact, since almost all bios in btrfs have btrfs_bio structure
> appended, we don't really need to reuse bio::bi_private for mirrored
> profiles.
> 
> So this patchset will:
> 
> - Introduce btrfs_bio::bioc member
>    So that btrfs_io_context don't need to hold @private.
>    This modification is still a net increase for memory usage, just
>    a trade-off between btrfs_io_context and btrfs_bio.
> 
> - Replace btrfs_bio::device with btrfs_bio::stripe_num
>    This reclaim the memory usage increase for btrfs_bio.
> 
>    But unfortunately, due to the short life time of btrfs_io_context,
>    we don't have as good device status accounting as the old code.
> 
>    Now for csum error, we can no longer distinguish source and target
>    device of dev-replace.
> 
>    This is the biggest blockage, and that's why it's RFC.

OK, here I'm over-thinking.

The truth is, dev-replace only affects two types of operations, WRITE 
and GET_READ_MIRRORS.

The former case won't need to bother csum mismatch, while the latter 
case won't result a bio used directly for read.

Thus it brings no behavior change.

Thanks,
Qu
> 
> The result of the patchset is:
> 
> btrfs_bio:		size: 240 -> 240
> btrfs_io_context:	size: 88 -> 80
> 
> Although to really reduce btrfs_bio, the main target should be
> csum_inline.
> 
> Qu Wenruo (3):
>    btrfs: add btrfs_bio::bioc pointer for further modification
>    btrfs: remove redundant parameters for submit_stripe_bio()
>    btrfs: replace btrfs_bio::device member with stripe_num
> 
>   fs/btrfs/compression.c |  6 ++----
>   fs/btrfs/ctree.h       |  2 ++
>   fs/btrfs/extent_io.c   |  2 --
>   fs/btrfs/inode.c       | 30 +++++++++++++++++++++++++++---
>   fs/btrfs/raid56.c      |  1 -
>   fs/btrfs/volumes.c     | 23 +++++++++++++----------
>   fs/btrfs/volumes.h     | 11 ++++++++---
>   7 files changed, 52 insertions(+), 23 deletions(-)
>