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

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Message-ID: <90777046-a420-b19f-1847-d353b9938131@vigovproductions.net>
Date:   Sun, 30 Apr 2023 23:41:35 +0100
From:   Michael Keyes <mgkeyes@...ovproductions.net>
To:     Tad <support@...tco.us>, liam.howlett@...cle.com
Cc:     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}

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).

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…

-- 
Michael