Re: [PATCH] Radix-tree pagecache for 2.5

[Andrew Morton]

Linus Torvalds wrote:
> 
> On Thu, 31 Jan 2002, Rik van Riel wrote:
> >
> > It's still a question whether we'll want to use 128 as
> > the branch factor or another number ... but I'm sure
> > somebody will figure that out (and it can be changed
> > later, it's just one define).
> 
> Actually, I think the big question is whether somebody is willing to clean
> up and fix the "move_from_swap_cache()" issue with block_flushpage.
> 

It appears that move_from_swap_cache() is in good company:

1: shmem_unuse_inode() calls delete_from_swap_cache under
   spinlock, but delete_from_swap_cache() calls block_flushpage(),
   which can sleep.

2: shmem_getpage_locked() calls delete_from_swap_cache() calls
   block_flushpage() under info->lock.

3: zap_pte_range holds mm->page_table_lock, and calls
   free_swap_and_cache() calls delete_from_swap_cache() calls
   block_flushpage().

block_flushpage() can only sleep in the lock_buffer() in
discard_buffer().   It so happens that all three callers
are always using block_flushpage() against a locked
swapcache page, and (correct me if I'm wrong), it's
not possible for those buffers to be locked.

So we got lucky.

A short-term fix is to put a BIG FAT COMMENT over block_flushpage.

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