linux-kernel - Re: [syzbot] [mm?] BUG: Bad page map (7)

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Message-ID: <092a9bb2-727e-5849-fa4f-18535b998efc@intel.com>
Date:   Mon, 11 Sep 2023 13:22:51 -0700
From:   Dave Hansen <dave.hansen@...el.com>
To:     Matthew Wilcox <willy@...radead.org>
Cc:     Yin Fengwei <fengwei.yin@...el.com>,
        syzbot <syzbot+55cc72f8cc3a549119df@...kaller.appspotmail.com>,
        akpm@...ux-foundation.org, linux-kernel@...r.kernel.org,
        linux-mm@...ck.org, syzkaller-bugs@...glegroups.com
Subject: Re: [syzbot] [mm?] BUG: Bad page map (7)

On 9/11/23 12:12, Matthew Wilcox wrote:
> On Mon, Sep 11, 2023 at 09:55:37AM -0700, Dave Hansen wrote:
>> On 9/11/23 09:44, Matthew Wilcox wrote:
>>> After fixing your two typos, this assembles to 176 bytes more code than
>>> my version.  Not sure that's great.
>> Maybe I'm a fool, but 176 bytes of text bloat isn't scaring me off too
>> much.  I'd much rather have that than another window into x86 goofiness
>> to maintain.
>>
>> Does that 176 bytes translate into meaningful performance, or is it just
>> a bunch of register bit twiddling that the CPU will sail through?
> I'm ... not sure how to tell.  It's 1120 bytes vs 944 bytes and crawling
> through that much x86 assembly isn't my idea of a great time.  I can
> send you objdump -dr for all three options if you like?  Maybe there's
> a quick way to compare them that I've never known about.

Working patches would be great if you're got 'em handy, plus your
.config and generally what compiler you're on.

I'll see if there's anything silly happening that's causing the
generated code to blow up.