Re: [RFC PATCH 54/61] afs: Wait on PG_fscache before modifying/releasing a page

[David Howells]

Matthew Wilcox <willy@xxxxxxxxxxxxx> wrote:

> > Won't that screw up ITER_MAPPING?  Does that mean that ITER_MAPPING isn't
> > viable?
> 
> Can you remind me why ITER_MAPPING needs:
> 
> "The caller must guarantee that the pages are all present and they must be
> locked using PG_locked, PG_writeback or PG_fscache to prevent them from
> going away or being migrated whilst they're being accessed."
> 
> An elevated refcount prevents migration, and it also prevents the pages
> from being freed.  It doesn't prevent them from being truncated out of
> the file, but it does ensure the pages aren't reallocated.

ITER_MAPPING relies on the mapping to maintain the pointers to the pages so
that it can find them rather than being like ITER_BVEC where there's a
separate list.

Truncate removes the pages from the mapping - at which point ITER_MAPPING can
no longer find them.

David