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From: Linus Torvalds <[email protected]>
To: Jens Axboe <[email protected]>
Cc: "[email protected]" <[email protected]>,
	[email protected]
Subject: Re: [GIT PULL] Fixes for 5.4-rc8/final
Date: Fri, 15 Nov 2019 13:17:25 -0800	[thread overview]
Message-ID: <CAHk-=wjm-UQpZS2T03z2iVxCQUUQN5BGvzVguDStQw2WddM46Q@mail.gmail.com> (raw)
In-Reply-To: <[email protected]>

On Fri, Nov 15, 2019 at 11:40 AM Jens Axboe <[email protected]> wrote:
>
> - Fix impossible-to-hit overflow merge condition, that still hit some
>   folks very rarely (Junichi)

Hmm. This sounded intriguing, so I looked at it.

It sounds like the 32-bit "bi_size" overflowed, which is one of the
things that bio_full() checks for.

However.

Looking at the *users* of bio_full(), it's not obvious that everything
is ok. For example, in __bio_add_pc_page(), the code does that

        if (((bio->bi_iter.bi_size + len) >> 9) > queue_max_hw_sectors(q))
                return 0;

*before* checking for the overflow condition.

So it could cause that bio_try_merge_pc_page() to be done despite the
overflow, and happily that path ends up having the bio_full() test
later anyway, but it does look a bit worrisome.

There's also __bio_add_page(), which does have a WARN_ON_ONCE(), but
then goes on and does the bi_size update regardless. Hmm.. It does
look like all callers either check bio_full() before, or do it with a
newly allocated bio.

             Linus

  reply	other threads:[~2019-11-15 21:17 UTC|newest]

Thread overview: 4+ messages / expand[flat|nested]  mbox.gz  Atom feed  top
2019-11-15 19:40 [GIT PULL] Fixes for 5.4-rc8/final Jens Axboe
2019-11-15 21:17 ` Linus Torvalds [this message]
2019-11-15 21:21   ` Jens Axboe
2019-11-15 22:20 ` pr-tracker-bot

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