linux-kernel - Re: [PATCH v3 31/35] lib: add memory allocations report in show

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Message-ID: <CAJuCfpFyrUizGbS+ZnMdp4-chg8q49xtZgFhejHoSi76Du1Ocg@mail.gmail.com>
Date: Wed, 21 Feb 2024 10:26:04 -0800
From: Suren Baghdasaryan <surenb@...gle.com>
To: Tetsuo Handa <penguin-kernel@...ove.sakura.ne.jp>
Cc: Vlastimil Babka <vbabka@...e.cz>, Kent Overstreet <kent.overstreet@...ux.dev>, 
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Subject: Re: [PATCH v3 31/35] lib: add memory allocations report in show_mem()

On Wed, Feb 21, 2024 at 5:22 AM Tetsuo Handa
<penguin-kernel@...ove.sakura.ne.jp> wrote:
>
> On 2024/02/21 3:27, Vlastimil Babka wrote:
> > I'm sure more such scenarios exist, Cc: Tetsuo who I recall was an expert on
> > this topic.
>
> "[PATCH v3 10/35] lib: code tagging framework" says that codetag_lock_module_list()
> calls down_read() (i.e. sleeping operation), and
> "[PATCH v3 31/35] lib: add memory allocations report in show_mem()" says that
> __show_mem() calls alloc_tags_show_mem_report() after kmalloc(GFP_ATOMIC) (i.e.
> non-sleeping operation) but alloc_tags_show_mem_report() calls down_read() via
> codetag_lock_module_list() !?
>
> If __show_mem() might be called from atomic context (e.g. kmalloc(GFP_ATOMIC)),
> this will be a sleep in atomic bug.
> If __show_mem() might be called while semaphore is held for write,
> this will be a read-lock after write-lock deadlock bug.
>
> Not the matter of whether to allocate buffer statically or dynamically.
> Please don't hold a lock when trying to report memory usage.

Thanks for catching this, Tetsuo! Yes, we take the read-lock here to
ensure that the list of modules is stable. I'm thinking I can replace
the down_read() with down_read_trylock() and if we fail (there is a
race with module load/unload) we will skip generating this report. The
probability of racing with module load/unload while in OOM state I
think is quite low, so skipping this report should not cause much
information loss.

>
>