Re: How to reclaim inode pages on demand

[Andrew Morton <akpm@osdl.org>]

Mel Gorman <mel@csn.ul.ie> wrote:
>
> Hi,
> 
> I am working on a direct reclaim strategy to free up large blocks of
> contiguous pages. The part I have is working fine, but I am finding a
> hundreds of pages that are being used for inodes that I need to reclaim. I
> tried purging the inode lists using a variation of prune_icache() but it
> is not working out.
> 
> Given a struct page, that one knows is an inode, can anyone suggest the
> best way to find the inode using it and free it?

Simple answer: invalidate_mapping_pages(page->mapping, start, end).

Problem: races.  If you pick a random page up off the LRU and start playing
with its mapping, what stops that mapping from getting freed under your
feet?

If the caller has a ref on the inode (say, you came in via a syscall
against an fd) then fine.

But if you picked the page up off the LRU then to avoid super-rare oopses
you'll need to pin the address_space somehow.  One way is to

a) lock the page, which pins the address_space.

b) check that page->mapping is still non-NULL.  Bale if it is.

c) bump page->mapping->host->i_count

d) unlock the page

e) diddle away with the address_space.

f) iput() the inode.

But beware that iput() can do a *ton* of stuff, so you need to be not
holding any locks and to not be called from any fancy contexts.