linux-kernel - Re: [PATCH v2] mm/mmap: Regression fix for unmapped_area{

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Message-ID: <20230515143625.jaipn7exqdr3ylzd@revolver>
Date:   Mon, 15 May 2023 10:36:25 -0400
From:   "Liam R. Howlett" <Liam.Howlett@...cle.com>
To:     Juhyung Park <qkrwngud825@...il.com>
Cc:     Michael Keyes <mgkeyes@...ovproductions.net>,
        Tad <support@...tco.us>, akpm@...ux-foundation.org,
        linux-kernel@...r.kernel.org, linux-mm@...ck.org,
        rick.p.edgecombe@...el.com
Subject: Re: [PATCH v2] mm/mmap: Regression fix for unmapped_area{_topdown}

* Juhyung Park <qkrwngud825@...il.com> [230515 04:57]:
> Hi Liam,
> 
> It's a bit hard to follow this particular issue on v6.1 as there are many
> email threads related to this.
> 
> I just wanted to ask if whether this is fixed on mainline and v6.1 stable
> yet.

Not yet.  It is in mm-unstable at this time.

> 
> If there's a new thread tackling this issue, I'd appreciate it if you can
> link it here.

https://lore.kernel.org/linux-mm/20230505145829.74574-1-zhangpeng.00@bytedance.com/


> 
> Thanks,
> regards
> 
> On 5/2/23 23:09, Liam R. Howlett wrote:
> > ...Adding Rick to the Cc this time.
> > 
> > * Liam R. Howlett <Liam.Howlett@...cle.com> [230502 10:08]:
> > > * Michael Keyes <mgkeyes@...ovproductions.net> [230430 18:41]:
> > > > On 29.04.23 15:32, Tad wrote:
> > > > > This reintroduces the issue described in
> > > > > https://lore.kernel.org/linux-mm/cb8dc31a-fef2-1d09-f133-e9f7b9f9e77a@sony.com/
> > > > Yes, I also ran into this (even though I'd somehow missed it the
> > > > previous time).
> > > 
> > > Rick Edgecombe reported something similar [1].
> > > 
> > > This is probably to do with my stack guard checks I recently added.
> > > 
> > > > 
> > > > Apparently the issue arises at mm/mmap.c:1582, where low_limit is set to
> > > > vm_end_gap(tmp). Occasionally, this returns a 64-bit address (e.g.
> > > > 0x7fedea581000), which is obviously greater than high_limit for a 32-bit
> > > > mmap, and causes the next call to mas_empty_area() to fail.
> > > > 
> > > > I'm not sure why vm_end_gap(tmp) occasionally returns a 64-bit address,
> > > > or if the best solution is to just check for this and skip the retry if
> > > > it occurs…
> > > > 
> > > 
> > > Thanks for the debugging.  I will look into it.
> > > 
> > > I am currently trying to revise how the iterators, prev/next deal with
> > > shifting outside the requested limits.  I suspect it's something to do
> > > with hitting the limit and what someone would assume the next operation
> > > means.
> > > 
> > > [1] https://lore.kernel.org/linux-mm/32f156ba80010fd97dbaf0a0cdfc84366608624d.camel@intel.com/
> > 
> >