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

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Message-ID: <c5bd4224-8c97-4854-a0d6-253fcd8bd92b@I-love.SAKURA.ne.jp>
Date: Wed, 21 Feb 2024 22:21:04 +0900
From: Tetsuo Handa <penguin-kernel@...ove.SAKURA.ne.jp>
To: Vlastimil Babka <vbabka@...e.cz>, Suren Baghdasaryan <surenb@...gle.com>,
        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 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.