Re: [PATCH v2] mm/mmap: Regression fix for unmapped_area{_topdown}

[Liam R. Howlett]

...Adding Rick to the Cc this time.

* Liam R. Howlett <Liam.Howlett@xxxxxxxxxx> [230502 10:08]:
> * Michael Keyes <mgkeyes@xxxxxxxxxxxxxxxxxxxx> [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@xxxxxxxx/
> > 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@xxxxxxxxx/