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    SubjectRe: [PATCH v2 3/3] NFS: Fix a memory leak in nfs_readdir
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    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.

    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.

    Cheers
    Trond

    --
    Trond Myklebust
    Linux NFS client maintainer

    NetApp
    Trond.Myklebust@netapp.com
    www.netapp.com



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