linux-kernel - Re: [PATCH] mm: change may_enter

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Date:	Fri, 13 Nov 2015 16:36:15 +0100
From:	Michal Hocko <mhocko@...nel.org>
To:	yalin wang <yalin.wang2010@...il.com>
Cc:	Vlastimil Babka <vbabka@...e.cz>, akpm@...ux-foundation.org,
	vdavydov@...allels.com, hannes@...xchg.org,
	mgorman@...hsingularity.net, tj@...nel.org, linux-mm@...ck.org,
	linux-kernel@...r.kernel.org
Subject: Re: [PATCH] mm: change may_enter_fs check condition

On Fri 13-11-15 13:01:16, Vlastimil Babka wrote:
> On 11/13/2015 12:47 PM, yalin wang wrote:
> >Add page_is_file_cache() for __GFP_FS check,
> >otherwise, a Pageswapcache() && PageDirty() page can always be write
> >back if the gfp flag is __GFP_FS, this is not the expected behavior.
> 
> I'm not sure I understand your point correctly *), but you seem to imply
> that there would be an allocation that has __GFP_FS but doesn't have
> __GFP_IO? Are there such allocations and does it make sense?

No it doesn't. There is a natural layering here and __GFP_FS allocations
should contain __GFP_IO.

The patch as is makes only little sense to me. Are you seeing any issue
which this is trying to fix?

> *) It helps to state which problem you actually observed and are trying to
> fix. Or was this found by code inspection? In that case describe the
> theoretical problem, as "expected behavior" isn't always understood by
> everyone the same.
> 
> >Signed-off-by: yalin wang <yalin.wang2010@...il.com>
> >---
> >  mm/vmscan.c | 2 +-
> >  1 file changed, 1 insertion(+), 1 deletion(-)
> >
> >diff --git a/mm/vmscan.c b/mm/vmscan.c
> >index bd2918e..f8fc8c1 100644
> >--- a/mm/vmscan.c
> >+++ b/mm/vmscan.c
> >@@ -930,7 +930,7 @@ static unsigned long shrink_page_list(struct list_head *page_list,
> >  		if (page_mapped(page) || PageSwapCache(page))
> >  			sc->nr_scanned++;
> >
> >-		may_enter_fs = (sc->gfp_mask & __GFP_FS) ||
> >+		may_enter_fs = (page_is_file_cache(page) && (sc->gfp_mask & __GFP_FS)) ||
> >  			(PageSwapCache(page) && (sc->gfp_mask & __GFP_IO));
> >
> >  		/*
> >

-- 
Michal Hocko
SUSE Labs
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