Re: [PATCH] splitlru: BDI_CAP_SWAP_BACKED

[KOSAKI Motohiro <kosaki.motohiro@jp.fujitsu.com>]

> 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@veritas.com>

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.

Rik, What do you think about it?