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    SubjectRe: [PATCH] mm for fs: add truncate_pagecache_range
    On Sun, Mar 25, 2012 at 01:26:10PM -0700, Hugh Dickins wrote:
    > On Fri, 23 Mar 2012, Andrew Morton wrote:
    > > On Fri, 23 Mar 2012 14:14:54 -0700 (PDT)
    > > Hugh Dickins <hughd@google.com> wrote:
    > > > On Fri, 23 Mar 2012, Andrew Morton wrote:
    > > > >
    > > > > --- a/mm/truncate.c~mm-for-fs-add-truncate_pagecache_range-fix
    > > > > +++ a/mm/truncate.c
    > > > > @@ -639,6 +639,9 @@ int vmtruncate_range(struct inode *inode
    > > > > * with on-disk format, and the filesystem would not have to deal with
    > > > > * situations such as writepage being called for a page that has already
    > > > > * had its underlying blocks deallocated.
    > > > > + *
    > > > > + * Must be called with inode->i_mapping->i_mutex held.
    > > >
    > > > You catch me offguard: I forget whether that's an absolute requirement or
    > > > just commonly the case. What do the other interfaces in truncate.c say ?-)
    > >
    > > i_mutex is generally required, to stabilise i_size.
    >
    > Sorry for being quarrelsome, but I do want to Nak your followup "fix".
    >
    > Building a test kernel quickly told me that inode->i_mapping->i_mutex
    > doesn't exist, of course it's inode->i_mutex.
    >
    > Then running the test kernel quickly told me that neither ext4 nor xfs
    > (I didn't try ocfs2) holds inode->i_mutex where holepunching calls
    > truncate_inode_pages_range().

    Just for completeness:

    ocfs2 holds i_mutex around the entire ocfs2_change_file_space() call,
    which can do hole punching and unwritten extent allocation (it is a
    clone of xfs_change_file_space()). xfs itself seems hold its own idea
    of a shared lock while doing the work and an exclusive lock around the
    transaction join. I'm not clear enough about xfs to say how this
    compares or even if I read it right.

    But ocfs2 uses an allocation sem to protect allocation changes,
    including i_size, so perhaps i_mutex isn't strictly necessary. I don't
    think we contend enough here to try hard to remove it :-)

    Joel

    --

    "The question of whether computers can think is just like the question
    of whether submarines can swim."
    - Edsger W. Dijkstra

    http://www.jlbec.org/
    jlbec@evilplan.org


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