linux-kernel - 4.19-stable: Re: [PATCH 2/3] drm/i915: Break up error capture compression loops with cond

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Message-ID: <20201109102437.GA15835@amd>
Date:   Mon, 9 Nov 2020 11:24:37 +0100
From:   Pavel Machek <pavel@....cz>
To:     Chris Wilson <chris@...is-wilson.co.uk>,
        kernel list <linux-kernel@...r.kernel.org>
Cc:     intel-gfx@...ts.freedesktop.org,
        Mika Kuoppala <mika.kuoppala@...ux.intel.com>,
        stable@...r.kernel.org
Subject: 4.19-stable: Re: [PATCH 2/3] drm/i915: Break up error capture
 compression loops with cond_resched()

Hi!

> As the error capture will compress user buffers as directed to by the
> user, it can take an arbitrary amount of time and space. Break up the
> compression loops with a call to cond_resched(), that will allow other
> processes to schedule (avoiding the soft lockups) and also serve as a
> warning should we try to make this loop atomic in the future.
> 
> Signed-off-by: Chris Wilson <chris@...is-wilson.co.uk>
> Cc: Mika Kuoppala <mika.kuoppala@...ux.intel.com>
> Cc: stable@...r.kernel.org
> Reviewed-by: Mika Kuoppala <mika.kuoppala@...ux.intel.com>

This was queued for 4.19-stable, but is very likely wrong.

> @@ -397,6 +399,7 @@ static int compress_page(struct i915_vma_compress *c,
>  	if (!(wc && i915_memcpy_from_wc(ptr, src, PAGE_SIZE)))
>  		memcpy(ptr, src, PAGE_SIZE);
>  	dst->pages[dst->page_count++] = ptr;
> +	cond_resched();
>  
>  	return 0;
>  }

4.19 compress_page begins with

static int compress_page(struct compress *c,
...
        page = __get_free_page(GFP_ATOMIC | __GFP_NOWARN);

and likely may not sleep. That changed with commit
a42f45a2a85998453078, but that one is not present in 4.19..

I believe we don't need this in stable: dumping of error file will not
take so long to trigger softlockup detectors... and if userland access
blocked, we would be able to reschedule, anyway.

Best regards,
								Pavel
-- 
http://www.livejournal.com/~pavelmachek

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