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

[Minchan Kim]

On Thu, Mar 12, 2009 at 9:02 AM, Andrew Morton
<akpm@xxxxxxxxxxxxxxxxxxxx> wrote:
> On Wed, 11 Mar 2009 15:03:02 -0700
> Andrew Morton <akpm@xxxxxxxxxxxxxxxxxxxx> wrote:
>
>> > The problem is that the pages are not marked dirty. ÂAnything 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. ÂI _think_ we did, in which case we later broke it. ÂIf
>> 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. ÂHow 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. ÂLonger-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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