linux-kernel - Re: [PATCH] mm: remove unintentional voluntary preemption in get_mmap_lock

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Date:   Sun, 20 Aug 2023 19:12:16 +0100
From:   Matthew Wilcox <willy@...radead.org>
To:     Mateusz Guzik <mjguzik@...il.com>
Cc:     torvalds@...ux-foundation.org, akpm@...ux-foundation.org,
        linux-kernel@...r.kernel.org, linux-mm@...ck.org
Subject: Re: [PATCH] mm: remove unintentional voluntary preemption in
 get_mmap_lock_carefully

On Sun, Aug 20, 2023 at 12:43:03PM +0200, Mateusz Guzik wrote:
> Found by checking off-CPU time during kernel build (like so:
> "offcputime-bpfcc -Ku"), sample backtrace:
>     finish_task_switch.isra.0
>     __schedule
>     __cond_resched
>     lock_mm_and_find_vma
>     do_user_addr_fault
>     exc_page_fault
>     asm_exc_page_fault
>     -                sh (4502)

Now I'm awake, this backtrace really surprises me.  Do we not check
need_resched on entry?  It seems terribly unlikely that need_resched
gets set between entry and getting to this point, so I guess we must
not.

I suggest the version of the patch which puts might_sleep() before the
mmap_read_trylock() is the right one to apply.  It's basically what
we've done forever, except that now we'll be rescheduling without the
mmap lock held, which just seems like an overall win.