On Wed, 01 Dec 2010 15:05:38 -0500 Trond Myklebust <Trond.Myklebust@xxxxxxxxxx> wrote: > On Wed, 2010-12-01 at 11:23 -0800, Hugh Dickins wrote: > > On Wed, 1 Dec 2010, Trond Myklebust wrote: > > > On Wed, 2010-12-01 at 08:17 -0800, Linus Torvalds wrote: > > > > include/linux/fs.h | 1 + > > > > mm/vmscan.c | 3 +++ > > > > 2 files changed, 4 insertions(+), 0 deletions(-) > > > > > > > > diff --git a/include/linux/fs.h b/include/linux/fs.h > > > > index c9e06cc..090f0ea 100644 > > > > --- a/include/linux/fs.h > > > > +++ b/include/linux/fs.h > > > > @@ -602,6 +602,7 @@ struct address_space_operations { > > > > sector_t (*bmap)(struct address_space *, sector_t); > > > > void (*invalidatepage) (struct page *, unsigned long); > > > > int (*releasepage) (struct page *, gfp_t); > > > > + void (*freepage)(struct page *); > > > > ssize_t (*direct_IO)(int, struct kiocb *, const struct iovec > > > > *iov, > > > > loff_t offset, unsigned long nr_segs); > > > > int (*get_xip_mem)(struct address_space *, pgoff_t, int, > > > > diff --git a/mm/vmscan.c b/mm/vmscan.c > > > > index d31d7ce..1accb01 100644 > > > > --- a/mm/vmscan.c > > > > +++ b/mm/vmscan.c > > > > @@ -499,6 +499,9 @@ static int __remove_mapping(struct address_space > > > > *mapping, struct page *page) > > > > mem_cgroup_uncharge_cache_page(page); > > > > } > > > > > > > > + if (mapping->a_ops->freepage) > > > > + mapping->a_ops->freepage(page); > > > > > > Hmm... Looking again at the problem, it appears that the same callback > > > needs to be added to truncate_complete_page() and > > > invalidate_complete_page2(). Otherwise we end up in a situation where > > > the page can sometimes be removed from the page cache without calling > > > freepage(). > > > > > > > + > > > > return 1; > > > > > > > > cannot_free: > > > > Yes, I was wondering quite how we would define this ->freepage thing, > > if it gets called from one place that removes from page cache and not > > from others. > > > > Another minor problem with it: it would probably need to take the > > struct address_space *mapping as arg as well as struct page *page: > > because by this time page->mapping has been reset to NULL. > > > > But I'm not at all keen on adding a calllback in this very special > > frozen-to-0-references place: please let's not do it without an okay > > from Nick Piggin (now Cc'ed). > > > > I agree completely with what Linus said originally about how the > > page cannot be freed while there's a reference to it, and it should > > be possible to work this without your additional page locks. > > > > Your ->releasepage should be able to judge whether the page is likely > > (not certain) to be freed - page_count 3? 1 for the page cache, 1 for > > the page_private reference, 1 for vmscan's reference, I think. Then > > it can mark !PageUptodate and proceed with freeing the stuff you had > > allocated, undo page_has_private and its reference, and return 1 (or > > return 0 if it decides to hold on to the page). > > That is very brittle. I'd prefer not to have to scan linux-mm every week > in order to find out if someone changed the page_count. > > However, while reading Documentation/filesystems/vfs.txt (in order to > add documentation for freepage) I was surprised to read that the > ->releasepage() is itself supposed to be allowed to actually remove the > page from the address space if it so desires. That doesn't sound right. It came from Neil in 2006. Neil, what were you thinking there? Did you find such a ->releasepage()? > Looking at the actual code in shrink_page_list() and friends I can't see > how that can possibly fail to break things, but if it were true, then > that might enable us to call remove_mapping() in order to safely free > the page before it gets cleared. > -- To unsubscribe from this list: send the line "unsubscribe linux-nfs" in the body of a message to majordomo@xxxxxxxxxxxxxxx More majordomo info at http://vger.kernel.org/majordomo-info.html