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Re: [PATCH] mm/vmscan: Do not block forever at shrink_inactive_list().

To: Dave Chinner <david@xxxxxxxxxxxxx>
Subject: Re: [PATCH] mm/vmscan: Do not block forever at shrink_inactive_list().
From: Andrew Morton <akpm@xxxxxxxxxxxxxxxxxxxx>
Date: Mon, 19 May 2014 22:59:15 -0700
Cc: Tetsuo Handa <penguin-kernel@xxxxxxxxxxxxxxxxxxx>, riel@xxxxxxxxxx, kosaki.motohiro@xxxxxxxxxxxxxx, fengguang.wu@xxxxxxxxx, kamezawa.hiroyu@xxxxxxxxxxxxxx, linux-kernel@xxxxxxxxxxxxxxx, xfs@xxxxxxxxxxx
Delivered-to: xfs@xxxxxxxxxxx
In-reply-to: <20140520004449.GE18954@dastard>
References: <201405192340.FCD48964.OFQHOOJLVSFFMt@xxxxxxxxxxxxxxxxxxx> <20140520004449.GE18954@dastard>
On Tue, 20 May 2014 10:44:49 +1000 Dave Chinner <david@xxxxxxxxxxxxx> wrote:

> @@ -258,14 +258,23 @@ xfs_bmapi_allocate_worker(
>       struct xfs_bmalloca     *args = container_of(work,
>                                               struct xfs_bmalloca, work);
>       unsigned long           pflags;
> +     unsigned long           new_pflags = PF_FSTRANS;
>  
> -     /* we are in a transaction context here */
> -     current_set_flags_nested(&pflags, PF_FSTRANS);
> +     /*
> +      * we are in a transaction context here, but may also be doing work
> +      * in kswapd context, and hence we may need to inherit that state
> +      * temporarily to ensure that we don't block waiting for memory reclaim
> +      * in any way.
> +      */
> +     if (args->kswapd)
> +             new_pflags |= PF_MEMALLOC | PF_SWAPWRITE | PF_KSWAPD;

So current_is_kswapd() returns true for a thread which is not kswapd. 
That's a bit smelly.

Should this thread really be incrementing KSWAPD_INODESTEAL instead of
PGINODESTEAL, for example?  current_is_kswapd() does a range of things,
only one(?) of which you actually want.

It would be cleaner to create a new PF_ flag to select just that
behavior.  That's a better model than telling the world "I am magic and
special".

But we're awfully close to running out of PF_ space and I don't know if
this ugly justifies consuming a flag.

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