Re: bvec_iter rewind API

[Jens Axboe <axboe@kernel.dk>]

On 8/28/18 5:33 PM, Kent Overstreet wrote:
> I just came across bi_done and your patch that added it -
> f9df1cd99e bio: add bvec_iter rewind API
> 
> I invite you to think through what happens to a bio that gets split by something
> further down the stack, and then rewound after completion:
> 
> To create the first half of the split, you just truncate the iterator by setting
> bio->bi_iter.bi_size to be the length you want. So if the original bio becomes
> the first half of the split, we've lost the original value for bi_size. After
> completion, bio_rewind_iter will rewind the bio to the start position the
> original code expects, but the bio will _not_ have the same size as it did when
> that code submitted it.
> 
> So if bio_rewind_iter() is being used because we want to restore the bio to what
> it was when we submitted it, this is bogus.
> 
> To create the second half of the split, we just advance the bio's iterator up to
> where we want the split to start. So this shouldn't prevent us from restoring
> the bio to the state it was in when it was created by rewinding it.
> 
> Except if you look at bio_split(), it sets bi_done to 0, and git blame reveals
> an interesting commit message...
> 
>     block: reset bi_iter.bi_done after splitting bio
>     
>     After the bio has been updated to represent the remaining sectors, reset
>     bi_done so bio_rewind_iter() does not rewind further than it should.
>     
>     This resolves a bio_integrity_process() failure on reads where the
>     original request was split.
> 
> *sigh*
> 
> The original code was bogus, and so was the fix. If a bio is split _AFTER_ a
> chunk of code in the stack sees it - and wants to restore it to the state it was
> it when it saw it after it completes - then not touching bi_done gives us the
> result we want. But if the bio was split _BEFORE_ that chunk of code sees it,
> rewinding it without ever touching bi_done will rewind it to be _LARGER_ than
> the bio that chunk of code saw.
> 
> So what should bi_done be? There is no correct answer - fundamentally, bi_done
> _cannot_ do what you're trying to do with it. Advancing or truncating an
> iterator destroys information, and stacked layers can manipulate bio iterators
> in arbitrary ways.
> 
> The correct way, the ONLY way, to restore a bio's iterator to a previous state
> after it's been submitted and other code has messed with it, is to save the
> entire iterator state: before you submit the bio, you save a copy of
> bio->bi_iter in your driver's private state, and then you restore it after
> completion.
> 
> I know you want to "save memory" and avoid the hassle of having to allocate
> state for your layer of the stack, but fundamentally what you're trying to do
> _does not work_. You have to save the iterator, you cannot play games and assume
> you can restore it.
> 
> Jens, can you please make sure I get CC'd on patches that touch bio/bvec
> iterators in the future? This is not the first time I've seen people make this
> mistake...

Yeah, will do.

-- 
Jens Axboe