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Message-ID: <20210319041621.GB1431129@xiangao.remote.csb>
Date: Fri, 19 Mar 2021 12:16:21 +0800
From: Gao Xiang <hsiangkao@...hat.com>
To: Chao Yu <yuchao0@...wei.com>
Cc: Gao Xiang <hsiangkao@....com>, linux-erofs@...ts.ozlabs.org,
Chao Yu <chao@...nel.org>, linux-kernel@...r.kernel.org,
Martin DEVERA <devik@...labs.cz>
Subject: Re: [PATCH v2] erofs: fix bio->bi_max_vecs behavior change
Hi Chao,
On Fri, Mar 19, 2021 at 10:15:18AM +0800, Chao Yu wrote:
> On 2021/3/6 12:04, Gao Xiang wrote:
...
> > + (*last_block + 1 != current_block || !*eblks)) {
>
> Xiang,
>
> I found below function during checking bi_max_vecs usage in f2fs:
>
> /**
> * bio_full - check if the bio is full
> * @bio: bio to check
> * @len: length of one segment to be added
> *
> * Return true if @bio is full and one segment with @len bytes can't be
> * added to the bio, otherwise return false
> */
> static inline bool bio_full(struct bio *bio, unsigned len)
> {
> if (bio->bi_vcnt >= bio->bi_max_vecs)
> return true;
>
> if (bio->bi_iter.bi_size > UINT_MAX - len)
> return true;
>
> return false;
> }
>
> Could you please check that whether it will be better to use bio_full()
> rather than using left-space-in-bio maintained by erofs itself? something
> like:
>
> if (bio && (bio_full(bio, PAGE_SIZE) ||
> /* not continuous */
> (*last_block + 1 != current_block))
>
> I'm thinking we need to decouple bio detail implementation as much as
> possible, to avoid regression whenever bio used/max size definition
> updates, though I've no idea how to fix f2fs case.
Thanks for your suggestion.
Not quite sure I understand the idea... The original problem was that
when EROFS bio_alloc, the number of requested bvec also partially stood
for remaining blocks of the current on-disk extent to limit the read
length. but after that bio behavior change, bi_max_vec could be increased
internally by block layer (e.g. 1 --> 4), so bi_max_vecs is no longer
as what we expect (I mean passed-in). so could cause read request
out-of-bound or hung. That's why I decided to record it manually (never
rely on bio statistics anymore...)
Also btw, AFAIK, Jianan is still investigating to use iomap instead
(mainly resolve tail-packing inline path). And I'm also busy in big
pcluster and LZMA new features for the next cycle. So I think we might
leave it just as is and it would be replaced with iomap in the future.
Thanks,
Gao Xiang
>
> Let me know if you have other concern.
>
> Thanks,
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