linux-kernel - Re: [PATCH] NOMMU: Pages allocated to a ramfs inode's pagecache may get wrongly discarded

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Date:	Thu, 12 Mar 2009 09:35:05 +0900
From:	Minchan Kim <minchan.kim@...il.com>
To:	Andrew Morton <akpm@...ux-foundation.org>
Cc:	dhowells@...hat.com, torvalds@...ux-foundation.org,
	peterz@...radead.org, Enrik.Berkhan@...com,
	uclinux-dev@...inux.org, linux-kernel@...r.kernel.org,
	linux-mm@...ck.org, Johannes Weiner <hannes@...xchg.org>,
	Minchan Kim <minchan.kim@...il.com>,
	Rik van Riel <riel@...riel.com>,
	Lee Schermerhorn <lee.schermerhorn@...com>,
	KOSAKI Motohiro <kosaki.motohiro@...fujitsu.com>
Subject: Re: [PATCH] NOMMU: Pages allocated to a ramfs inode's pagecache may 
	get wrongly discarded

On Thu, Mar 12, 2009 at 9:02 AM, Andrew Morton
<akpm@...ux-foundation.org> wrote:
> On Wed, 11 Mar 2009 15:03:02 -0700
> Andrew Morton <akpm@...ux-foundation.org> wrote:
>
>> > The problem is that the pages are not marked dirty.  Anything that creates data
>> > in an MMU-based ramfs will cause the pages holding that data will cause the
>> > set_page_dirty() aop to be called.
>> >
>> > For the NOMMU-based mmap, set_page_dirty() may be called by write(), but it
>> > won't be called by page-writing faults on writable mmaps, and it isn't called
>> > by ramfs_nommu_expand_for_mapping() when a file is being truncated from nothing
>> > to allocate a contiguous run.
>> >
>> > The solution is to mark the pages dirty at the point of allocation by
>> > the truncation code.
>>
>> Page reclaim shouldn't be even attempting to reclaim or write back
>> ramfs pagecache pages - reclaim can't possibly do anything with these
>> pages!
>>
>> Arguably those pages shouldn't be on the LRU at all, but we haven't
>> done that yet.
>>
>> Now, my problem is that I can't 100% be sure that we _ever_ implemented
>> this properly.  I _think_ we did, in which case we later broke it.  If
>> we've always been (stupidly) trying to pageout these pages then OK, I
>> guess your patch is a suitable 2.6.29 stopgap.
>
> OK, I can't find any code anywhere in which we excluded ramfs pages
> from consideration by page reclaim.  How dumb.


The ramfs  considers it in just CONFIG_UNEVICTABLE_LRU case
It that case, ramfs_get_inode calls mapping_set_unevictable.
So,  page reclaim can exclude ramfs pages by page_evictable.
It's problem .


> So I guess that for now the proposed patch is suitable.  Longer-term we
> should bale early in shrink_page_list(), or not add these pages to the
> LRU at all.

In future, we have to improve this.

> --
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-- 
Kinds regards,
Minchan Kim
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