linux-kernel - Re: [PATCH 02/30] mm: gfp_to_alloc

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Date:	Tue, 12 Aug 2008 19:33:16 +1000
From:	Neil Brown <neilb@...e.de>
To:	Peter Zijlstra <a.p.zijlstra@...llo.nl>
Cc:	Linus Torvalds <torvalds@...ux-foundation.org>,
	Andrew Morton <akpm@...ux-foundation.org>,
	linux-kernel@...r.kernel.org, linux-mm@...ck.org,
	netdev@...r.kernel.org, trond.myklebust@....uio.no,
	Daniel Lezcano <dlezcano@...ibm.com>,
	Pekka Enberg <penberg@...helsinki.fi>
Subject: Re: [PATCH 02/30] mm: gfp_to_alloc_flags()

On Tuesday August 12, a.p.zijlstra@...llo.nl wrote:
> On Tue, 2008-08-12 at 15:01 +1000, Neil Brown wrote:
> > Did I miss something?
> > If I did, maybe more text in the changelog entry (or the comment)
> > would help.
> 
> Ok, so the old code did:
> 
>   if (((p->flags & PF_MEMALLOC) || ...) && !in_interrupt) {
>     ....
>     goto nopage;
>   }
> 
> which avoid anything that has PF_MEMALLOC set from entering into direct
> reclaim, right?
> 
> Now, the new code reads:
> 
>   if (alloc_flags & ALLOC_NO_WATERMARK) {
>   }
> 
> Which might be false, even though we have PF_MEMALLOC set -
> __GFP_NOMEMALLOC comes to mind.
> 
> So we have to stop that recursion from happening.
> 
> so we add:
> 
>   if (p->flags & PF_MEMALLOC)
>     goto nopage;
> 
> Now, if it were done before the !wait check, we'd have to consider
> atomic contexts, but as those are - as you rightly pointed out - handled
> by the !wait case, we can plainly do this check.
> 
> 

Oh yes, obvious when you explain it, thanks.

cat << END >> Changelog

As the test
-	if (((p->flags & PF_MEMALLOC) || unlikely(test_thread_flag(TIF_MEMDIE)))
-			&& !in_interrupt()) {
-		if (!(gfp_mask & __GFP_NOMEMALLOC)) {
has been replaced with a slightly strong
+	if (alloc_flags & ALLOC_NO_WATERMARKS) {

we need to ensure we don't recurse when PF_MEMALLOC is set

END

??

Thanks,
NeilBrown
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