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

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Message-ID: <20230820130856.j2wbfe4z6iem4fis@f>
Date:   Sun, 20 Aug 2023 15:08:56 +0200
From:   Mateusz Guzik <mjguzik@...il.com>
To:     Linus Torvalds <torvalds@...ux-foundation.org>
Cc:     Matthew Wilcox <willy@...radead.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 02:59:07PM +0200, Linus Torvalds wrote:
> On Sun, 20 Aug 2023 at 14:47, Linus Torvalds
> <torvalds@...ux-foundation.org> wrote:
> >
> > But without that odd ifdef, I think it's fine.
> 
> Another option might be to just move the might_sleep() to the top, and
> do it unconditionally. If the trylock fails, the overhead of possibly
> doing a cond_resched() is kind of moot.
> 

I wanted to do it, but then I found this comment:

 * For example, if we have a kernel bug that causes a page
 * fault, we don't want to just use mmap_read_lock() to get
 * the mm lock, because that would deadlock if the bug were
 * to happen while we're holding the mm lock for writing.

I figured scheduling away while on the way to OOPS/similar is not the
best thing to happen.

> IOW, the main problem here is not that it causes a scheduling point
> (if the kernel isn't preemptable), it seems to be just that we
> unnecessarily schedule in a place with the mm lock is held, so it
> unnecessarily causes possible lock contention for writers.
> 
> With the per-vma locking catching most cases, does any of this even matter?
> 
> Mateusz - on that note: I'm wondering what made you see this as a
> problem. The case you quote doesn't actually seem to be threaded, so
> the vm lock contention shouldn't actually matter there.
> 
> Does it schedule away? Sure. But only if "needs_resched" is set, so it
> doesn't seem to be a *bad* thing per se.
> 
> It might just be that this particular scheduling point ends up being a
> common one on that load, and with those kernel config options (ie
> PREEMPT_VOLUNTARY)?
> 

I did not cause a slowdown for me and I did not say it did.

I am saying down_read + going off CPU, should it happen in a
multithreaded program, is going to delay any down_write issued by other
threads. And that going off CPU here was clearly not intended.

As I noted in another e-mail this is just a side thing I spotted while
looking at other stuff. I don't find it important enough to discuss it
any further, so as far as I am concerned you are most welcome to take
any of the 2 patches, write your own or or leave the code be.

[I am going to post other stuff later which *I am* going to argue to
push for ;>]