Re: [PATCH for-next v7 4/5] block: add helper to map bvec iterator for passthrough

[Kanchan Joshi <joshi.k@samsung.com>]

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On Fri, Sep 23, 2022 at 05:29:41PM +0200, Christoph Hellwig wrote:
>On Thu, Sep 22, 2022 at 08:53:31PM +0530, Kanchan Joshi wrote:
>>> blk_rq_map_user_iov really should be able to detect that it is called
>>> on a bvec iter and just do the right thing rather than needing different
>>> helpers.
>>
>> I too explored that possibility, but found that it does not. It maps the
>> user-pages into bio either directly or by doing that copy (in certain odd
>> conditions) but does not know how to deal with existing bvec.
>
>What do you mean with existing bvec?  We allocate a brand new bio here
>that we want to map the next chunk of the iov_iter to, and that
>is exactly what blk_rq_map_user_iov does.  What blk_rq_map_user_iov
>currently does not do is to implement this mapping efficiently
>for ITER_BVEC iters

It is clear that it was not written for ITER_BVEC iters.
Otherwise that WARN_ON would not have hit.

And efficency is the concern as we are moving to more heavyweight
helper that 'handles' weird conditions rather than just 'bails out'.
These alignment checks end up adding a loop that traverses
the entire ITER_BVEC.
Also blk_rq_map_user_iov uses bio_iter_advance which also seems
cycle-consuming given below code-comment in io_import_fixed():

if (offset) {
        /*
         * Don't use iov_iter_advance() here, as it's really slow for
         * using the latter parts of a big fixed buffer - it iterates
         * over each segment manually. We can cheat a bit here, because
         * we know that:

So if at all I could move the code inside blk_rq_map_user_iov, I will
need to see that I skip doing iov_iter_advance.

I still think it would be better to take this route only when there are
other usecases/callers of this. And that is a future thing. For the current
requirement, it seems better to prioritze efficency.

>, but that is something that could and should
>be fixed.
>
>> And it really felt cleaner to me write a new function rather than
>> overloading the blk_rq_map_user_iov with multiple if/else canals.
>
>No.  The whole point of the iov_iter is to support this "overload".

Even if I try taking that route, WARN_ON is a blocker that  prevents 
me to put this code inside blk_rq_map_user_iov.

>> But iov_iter_gap_alignment does not work on bvec iters. Line #1274 below
>
>So we'll need to fix it.

Do you see good way to trigger this virt-alignment condition? I have
not seen this hitting (the SG gap checks) when running with fixebufs.

>> 1264 unsigned long iov_iter_gap_alignment(const struct iov_iter *i)
>> 1265 {
>> 1266         unsigned long res = 0;
>> 1267         unsigned long v = 0;
>> 1268         size_t size = i->count;
>> 1269         unsigned k;
>> 1270
>> 1271         if (iter_is_ubuf(i))
>> 1272                 return 0;
>> 1273
>> 1274         if (WARN_ON(!iter_is_iovec(i)))
>> 1275                 return ~0U;
>>
>> Do you see a way to overcome this. Or maybe this can be revisted as we
>> are not missing a lot?
>
>We just need to implement the equivalent functionality for bvecs.  It
>isn't really hard, it just wasn't required so far.

Can the virt-boundary alignment gap exist for ITER_BVEC iter in first
place? Two reasons to ask this question:

1. Commit description of this code (from Al viro) says -

"iov_iter_gap_alignment(): get rid of iterate_all_kinds()

For one thing, it's only used for iovec (and makes sense only for
those)."

2. I did not hit it so far as I mentioned above.

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