On 2021/11/23 15:43, Christoph Hellwig wrote:
On Tue, Nov 23, 2021 at 02:44:32PM +0800, Qu Wenruo wrote:
Hi,
Although there are some out-of-date comments mentions other
bio_clone_*() variants, but there isn't really any other bio clone
variants other than __bio_clone_fast(), which shares bi_io_vec with the
source bio.
This limits means we can't free the source bio before the cloned one.
Is there any bio_clone variant which do a deep clone, including bi_io_vec?
There is no use case for that, unless the actual data changes like in
the bounce buffering code.
That's why the bio_clone thing is involved, there is still some corner
cases that we don't want to fail the whole large bio if there is only
one stripe failed (mostly for read bio, that we want to salvage as much
data as possible)
Thus regular bio_split() + bio_chain() solution is not that good here.
Any idea why no such bio_clone_slow() or bio_split_slow() provided in
block layer?
Or really bio_split() + bio_chain() is the only recommended solution?
You can use bio_split witout bio_chain. You just need your own
bi_end_io handler that first performs the action you want and then
contains code equivalent to __bio_chain_endio. As a bonus you can
pint bi_private to whatever you want, it does not have to be the parent
bio, just something that allows you to find it.
Without bio_chain() sounds pretty good, as we can still utilize
bi_end_io and bi_private.
But this also means, we're now responsible not to release the source bio
since it has the real bi_io_vec.
Let me explore this and hopefully to align btrfs with dm/md code more.
Thanks,
Qu
