Re: [PATCH] splitlru: BDI_CAP_SWAP_BACKED

[KAMEZAWA Hiroyuki]

On Sun, 29 Jun 2008 17:48:59 +0900
KOSAKI Motohiro <kosaki.motohiro@xxxxxxxxxxxxxx> wrote:

> > The split-lru patches put file and swap-backed pages on different lrus.
> > shmem/tmpfs pages are awkward because they are swap-backed file pages.
> > Since it's difficult to change lru midstream, they are treated as swap-
> > backed throughout, with SetPageSwapBacked on allocation in shmem_getpage.
> > 
> > However, splice read (used by loop and sendfile) and readahead* allocate
> > pages first, add_to_page_cache_lru, and then call into the filesystem
> > through ->readpage.  Under memory pressure, the shmem pages arrive at
> > add_to_swap_cache and hit its BUG_ON(!PageSwapBacked(page)).
> > 
> > I've not yet found a better way to handle this than a "capability"
> > flag in shmem_backing_dev_info, tested by add_to_page_cache_lru.
> > And solely because it would look suspicious without it, set that
> > BDI_CAP_SWAP_BACKED in swap_backing_dev_info also.
> > 
> > * readahead on shmem/tmpfs?  I'd always thought ra_pages 0 prevented
> > that; but in fact readahead(2), fadvise(POSIX_FADV_WILLNEED) and
> > madvise(MADV_WILLNEED) all force_page_cache_readahead and get there.
> > 
> > Signed-off-by: Hugh Dickins <hugh@xxxxxxxxxxx>
> 
> great.
> 
> I'm not sure about this patch is perfect.
> but it seems makes sense and very good code.
> 
> So, I'll testing this patch for a while.
> 

Hmm, how about adding LRU only for shmem/tmpfs ? nonsense ? ;)
It's just a file if not mapped but it's swap backed.


Thanks,
-Kame

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