linux-kernel - Re: [PATCH 1/3] mmap: Fix vma_iterator in error path of vma

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Message-ID: <20230927172649.tivg3yxtmzo3fzjb@revolver>
Date:   Wed, 27 Sep 2023 13:26:49 -0400
From:   "Liam R. Howlett" <Liam.Howlett@...cle.com>
To:     Andrew Morton <akpm@...ux-foundation.org>
Cc:     maple-tree@...ts.infradead.org, linux-mm@...ck.org,
        linux-kernel@...r.kernel.org, Jann Horn <jannh@...gle.com>,
        Lorenzo Stoakes <lstoakes@...il.com>,
        Vlastimil Babka <vbabka@...e.cz>,
        Suren Baghdasaryan <surenb@...gle.com>,
        Matthew Wilcox <willy@...radead.org>, stable@...r.kernel.org
Subject: Re: [PATCH 1/3] mmap: Fix vma_iterator in error path of vma_merge()

* Andrew Morton <akpm@...ux-foundation.org> [230927 13:14]:
> On Wed, 27 Sep 2023 12:07:44 -0400 "Liam R. Howlett" <Liam.Howlett@...cle.com> wrote:
> 
> > When merging of the previous VMA fails after the vma iterator has been
> > moved to the previous entry, the vma iterator must be advanced to ensure
> > the caller takes the correct action on the next vma iterator event.  Fix
> > this by adding a vma_next() call to the error path.
> > 
> > Users may experience higher CPU usage, most likely in very low memory
> > situations.
> 
> Looking through this thread:
> 
> > Closes: https://lore.kernel.org/linux-mm/CAG48ez12VN1JAOtTNMY+Y2YnsU45yL5giS-Qn=ejtiHpgJAbdQ@mail.gmail.com/
> 
> I'm seeing no indication that the effect is CPU consumption?  Jann is
> excpecting bogus oom-killing?

His testing injected a bogus oom, but since the vma iterator may get
stuck in a "I can merge! oh, I'm out of memory" loop due to the
vma_merge() called with the same VMA in this loop, I would expect it to
be increased CPU usage when almost out of memory until a task is killed.
I don't think he expected a bogus OOM since we are using GFP_KERNEL
during mm/internal.h:vma_iter_prealloc() calls.