linux-kernel - Re: isolate_lru

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Message-ID: <5679709B.8030908@suse.cz>
Date:	Tue, 22 Dec 2015 16:47:39 +0100
From:	Vlastimil Babka <vbabka@...e.cz>
To:	Michal Hocko <mhocko@...nel.org>,
	"Kirill A. Shutemov" <kirill@...temov.name>
Cc:	Andrew Morton <akpm@...ux-foundation.org>,
	Minchan Kim <minchan@...nel.org>, linux-mm@...ck.org,
	LKML <linux-kernel@...r.kernel.org>
Subject: Re: isolate_lru_page on !head pages

On 12/15/2015 05:59 PM, Michal Hocko wrote:
>>
>> head page is what linked into LRU, but not nessesary the way we obtain the
>> page to check. If we check PageLRU(pte_page(*pte)) it should produce the
>> right result.
>
> I am not following you here. Any pfn walk could get to a tail page and
> if we happen to do e.g. isolate_lru_page we have to remember that we
> should always treat compound page differently. This is subtle.

I think the problem is that isolate_lru_page() is not the only reason 
for calling PageLRU(). And the other use cases have different 
expectations, to either way (PF_ANY or PF_HEAD) you pick for PageLRU(), 
somebody will have to be careful. IMHO usually it's pfn scanners who 
have to be careful for many reasons...

> Anyway I
> am far from understading other parts of the refcount rework so I will
> spend time studying the code as soon as the time permits. In the
> meantime I agree that VM_BUG_ON_PAGE(PageTail(page), page) would be
> useful to catch all the fallouts.

+1

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